diff --git a/chasm/lib/scheduler/backfiller.go b/chasm/lib/scheduler/backfiller.go
index 9693b999892..e132596730d 100644
--- a/chasm/lib/scheduler/backfiller.go
+++ b/chasm/lib/scheduler/backfiller.go
@@ -1,6 +1,7 @@
 package scheduler
 
 import (
+	"fmt"
 	"time"
 
 	schedulespb "go.temporal.io/server/api/schedule/v1"
@@ -45,6 +46,7 @@ func addBackfiller(
 		scheduler.Backfillers = make(chasm.Map[string, *Backfiller])
 	}
 	scheduler.Backfillers[id] = chasm.NewComponentField(ctx, backfiller)
+	scheduler.EventLog.Get(ctx).LogEvent(ctx, fmt.Sprintf("added backfiller: %s", id))
 
 	return backfiller
 }
@@ -60,6 +62,8 @@ func newBackfillerWithState(ctx chasm.MutableContext, state *schedulerpb.Backfil
 
 // scheduleTask schedules a BackfillerTask at the given time.
 func (b *Backfiller) scheduleTask(ctx chasm.MutableContext, scheduledTime time.Time) {
+	b.EventLog.Get(ctx).LogEvent(ctx,
+		fmt.Sprintf("scheduled backfillerTask for %s", scheduledTime.Format(time.RFC3339)))
 	ctx.AddTask(b, chasm.TaskAttributes{
 		ScheduledTime: scheduledTime,
 	}, &schedulerpb.BackfillerTask{})
diff --git a/chasm/lib/scheduler/backfiller_tasks.go b/chasm/lib/scheduler/backfiller_tasks.go
index fa5380b1fa3..525c595db44 100644
--- a/chasm/lib/scheduler/backfiller_tasks.go
+++ b/chasm/lib/scheduler/backfiller_tasks.go
@@ -45,15 +45,12 @@ func NewBackfillerTaskHandler(opts BackfillerTaskHandlerOptions) *BackfillerTask
 }
 
 func (b *BackfillerTaskHandler) Validate(
-	ctx chasm.Context,
+	_ chasm.Context,
 	backfiller *Backfiller,
 	attrs chasm.TaskAttributes,
 	_ *schedulerpb.BackfillerTask,
 ) (bool, error) {
-	return validateTaskHighWaterMark(
-		backfiller.GetLastProcessedTime(),
-		attrs.ScheduledTime,
-	)
+	return validateTaskHighWaterMark(backfiller.GetLastProcessedTime(), attrs.ScheduledTime)
 }
 
 func (b *BackfillerTaskHandler) Execute(
@@ -66,9 +63,10 @@ func (b *BackfillerTaskHandler) Execute(
 
 	scheduler := backfiller.Scheduler.Get(ctx)
 	logger := newTaggedLogger(b.baseLogger, scheduler)
-
 	invoker := scheduler.Invoker.Get(ctx)
 
+	backfiller.EventLog.Get(ctx).LogEvent(ctx, "backfillerTask executed")
+
 	// If the buffer is already full, don't move the watermark at all, just back off
 	// and retry.
 	tweakables := b.config.Tweakables(scheduler.Namespace)
@@ -110,6 +108,10 @@ func (b *BackfillerTaskHandler) Execute(
 		logger.Debug("backfill complete, deleting Backfiller",
 			tag.String("backfill-id", backfiller.GetBackfillId()))
 		delete(scheduler.Backfillers, backfiller.GetBackfillId())
+
+		// Revive the Generator so it can re-evaluate idle/close eligibility now
+		// that this backfiller is gone.
+		scheduler.Generator.Get(ctx).Generate(ctx)
 		return nil
 	}
 
diff --git a/chasm/lib/scheduler/config.go b/chasm/lib/scheduler/config.go
index df40ab4739b..3d57c24bee3 100644
--- a/chasm/lib/scheduler/config.go
+++ b/chasm/lib/scheduler/config.go
@@ -3,6 +3,7 @@ package scheduler
 import (
 	"time"
 
+	"go.temporal.io/server/chasm"
 	"go.temporal.io/server/common/backoff"
 	"go.temporal.io/server/common/dynamicconfig"
 )
@@ -28,6 +29,30 @@ type (
 	}
 )
 
+// tweakablesCtxKey keys the namespace-filtered Tweakables accessor that scheduler
+// components read via the CHASM context (registered in Library.Components).
+type tweakablesCtxKeyType struct{}
+
+var tweakablesCtxKey = tweakablesCtxKeyType{}
+
+// tweakablesFromContext returns the scheduler Tweakables for the context's namespace,
+// falling back to DefaultTweakables when no config is registered.
+func tweakablesFromContext(ctx chasm.Context) Tweakables {
+	if fn, ok := ctx.Value(tweakablesCtxKey).(dynamicconfig.TypedPropertyFnWithNamespaceFilter[Tweakables]); ok && fn != nil {
+		return fn(ctx.NamespaceEntry().Name().String())
+	}
+	return DefaultTweakables
+}
+
+// contextValues builds the CHASM context values exposed to scheduler components.
+func (c *Config) contextValues() map[any]any {
+	var tweakables dynamicconfig.TypedPropertyFnWithNamespaceFilter[Tweakables]
+	if c != nil {
+		tweakables = c.Tweakables
+	}
+	return map[any]any{tweakablesCtxKey: tweakables}
+}
+
 var (
 	CurrentTweakables = dynamicconfig.NewNamespaceTypedSetting(
 		"scheduler.tweakables",
diff --git a/chasm/lib/scheduler/eventlog.go b/chasm/lib/scheduler/eventlog.go
index 8a97d996cc1..91e35007aa7 100644
--- a/chasm/lib/scheduler/eventlog.go
+++ b/chasm/lib/scheduler/eventlog.go
@@ -30,10 +30,13 @@ func (e *EventLog) LifecycleState(ctx chasm.Context) chasm.LifecycleState {
 	return chasm.LifecycleStateRunning
 }
 
-// LogEvent appends an event with the given message. Messages longer than
-// maxMessageLen bytes are truncated at a UTF-8 rune boundary; once the log
-// has more than maxEntries entries, the earliest entries are dropped.
-func (e *EventLog) LogEvent(ctx chasm.MutableContext, msg string, maxEntries, maxMessageLen int) {
+// LogEvent appends an event with the given message. Messages longer than the
+// configured maximum length are truncated at a UTF-8 rune boundary; once the
+// log exceeds the configured maximum entries, the earliest entries are dropped.
+func (e *EventLog) LogEvent(ctx chasm.MutableContext, msg string) {
+	tw := tweakablesFromContext(ctx)
+	maxEntries, maxMessageLen := tw.EventLogMaxEntries, tw.EventLogMaxMessageLen
+
 	if len(msg) > maxMessageLen {
 		// Back off to the nearest UTF-8 rune boundary so we don't split a
 		// multibyte rune.
diff --git a/chasm/lib/scheduler/eventlog_test.go b/chasm/lib/scheduler/eventlog_test.go
index 64397077018..c6913e8f1e7 100644
--- a/chasm/lib/scheduler/eventlog_test.go
+++ b/chasm/lib/scheduler/eventlog_test.go
@@ -9,12 +9,14 @@ import (
 	"github.com/stretchr/testify/require"
 	enumspb "go.temporal.io/api/enums/v1"
 	schedulepb "go.temporal.io/api/schedule/v1"
+	"go.temporal.io/server/chasm"
+	"go.temporal.io/server/chasm/lib/scheduler"
 	"google.golang.org/protobuf/types/known/timestamppb"
 )
 
-const (
-	testEventLogMaxEntries    = 30
-	testEventLogMaxMessageLen = 1000
+var (
+	testEventLogMaxEntries    = scheduler.DefaultTweakables.EventLogMaxEntries
+	testEventLogMaxMessageLen = scheduler.DefaultTweakables.EventLogMaxMessageLen
 )
 
 func TestEventLog_Accumulates(t *testing.T) {
@@ -24,7 +26,7 @@ func TestEventLog_Accumulates(t *testing.T) {
 
 	messages := []string{"first", "second", "third"}
 	for _, m := range messages {
-		eventLog.LogEvent(ctx, m, testEventLogMaxEntries, testEventLogMaxMessageLen)
+		eventLog.LogEvent(ctx, m)
 	}
 
 	require.Len(t, eventLog.Events, len(messages))
@@ -40,7 +42,7 @@ func TestEventLog_TruncatesLongMessages(t *testing.T) {
 	eventLog.Events = nil
 
 	long := strings.Repeat("x", testEventLogMaxMessageLen+50)
-	eventLog.LogEvent(ctx, long, testEventLogMaxEntries, testEventLogMaxMessageLen)
+	eventLog.LogEvent(ctx, long)
 
 	require.Len(t, eventLog.Events, 1)
 	require.Len(t, eventLog.Events[0].Message, testEventLogMaxMessageLen)
@@ -55,7 +57,7 @@ func TestEventLog_DropsEarliestWhenFull(t *testing.T) {
 	const overflow = 5
 	total := testEventLogMaxEntries + overflow
 	for i := range total {
-		eventLog.LogEvent(ctx, fmt.Sprintf("event-%d", i), testEventLogMaxEntries, testEventLogMaxMessageLen)
+		eventLog.LogEvent(ctx, fmt.Sprintf("event-%d", i))
 	}
 
 	require.Len(t, eventLog.Events, testEventLogMaxEntries)
@@ -65,6 +67,24 @@ func TestEventLog_DropsEarliestWhenFull(t *testing.T) {
 	require.Equal(t, fmt.Sprintf("event-%d", total-1), eventLog.Events[len(eventLog.Events)-1].Message)
 }
 
+// TestEventLog_NoConfigFallsBackToDefaults checks that LogEvent applies the
+// default retention limits instead of panicking when no config is reachable via
+// the context, as in tdbg's registration-only setup.
+func TestEventLog_NoConfigFallsBackToDefaults(t *testing.T) {
+	ctx := &chasm.MockMutableContext{}
+	eventLog := scheduler.NewEventLog(ctx)
+
+	require.NotPanics(t, func() {
+		eventLog.LogEvent(ctx, "logged without a registered config")
+	})
+	require.Len(t, eventLog.Events, 1)
+
+	long := strings.Repeat("x", scheduler.DefaultTweakables.EventLogMaxMessageLen+50)
+	eventLog.LogEvent(ctx, long)
+	require.Len(t, eventLog.Events, 2)
+	require.Len(t, eventLog.Events[1].Message, scheduler.DefaultTweakables.EventLogMaxMessageLen)
+}
+
 func TestEventLog_EachComponentHasOwn(t *testing.T) {
 	sched, ctx, _ := setupSchedulerForTest(t)
 
@@ -86,7 +106,7 @@ func TestEventLog_EachComponentHasOwn(t *testing.T) {
 	sched.Generator.Get(ctx).EventLog.Get(ctx).Events = nil
 	backfiller.EventLog.Get(ctx).Events = nil
 
-	sched.EventLog.Get(ctx).LogEvent(ctx, "scheduler-event", testEventLogMaxEntries, testEventLogMaxMessageLen)
+	sched.EventLog.Get(ctx).LogEvent(ctx, "scheduler-event")
 	require.Len(t, sched.EventLog.Get(ctx).Events, 1)
 	require.Empty(t, sched.Generator.Get(ctx).EventLog.Get(ctx).Events)
 	require.Empty(t, backfiller.EventLog.Get(ctx).Events)
diff --git a/chasm/lib/scheduler/export_test.go b/chasm/lib/scheduler/export_test.go
index 0995ae1a056..f8522fc53d3 100644
--- a/chasm/lib/scheduler/export_test.go
+++ b/chasm/lib/scheduler/export_test.go
@@ -37,3 +37,16 @@ func (s *Scheduler) RecordCompletedAction(
 func (i *Invoker) RunningWorkflowID(requestID string) string {
 	return i.runningWorkflowID(requestID)
 }
+
+// RecordExecuteResult exposes recordExecuteResult so tests can pin the
+// per-RequestId idempotency guard against concurrent ExecuteTasks.
+func (i *Invoker) RecordExecuteResult(
+	ctx chasm.MutableContext,
+	completed []*schedulespb.BufferedStart,
+	retryable []*schedulespb.BufferedStart,
+) (newlyStarted, droppedDuplicates int) {
+	return i.recordExecuteResult(ctx, &executeResult{
+		CompletedStarts: completed,
+		RetryableStarts: retryable,
+	})
+}
diff --git a/chasm/lib/scheduler/gen/schedulerpb/v1/message.pb.go b/chasm/lib/scheduler/gen/schedulerpb/v1/message.pb.go
index ac27ad2e34b..81f66a4e995 100644
--- a/chasm/lib/scheduler/gen/schedulerpb/v1/message.pb.go
+++ b/chasm/lib/scheduler/gen/schedulerpb/v1/message.pb.go
@@ -49,8 +49,13 @@ type SchedulerState struct {
 	// Set when a migration to workflow-backed scheduler (V1) is pending.
 	// Unpause operations are blocked while this is set.
 	WorkflowMigration *WorkflowMigrationState `protobuf:"bytes,11,opt,name=workflow_migration,json=workflowMigration,proto3" json:"workflow_migration,omitempty"`
-	unknownFields     protoimpl.UnknownFields
-	sizeCache         protoimpl.SizeCache
+	// The time at which the idle timer is scheduled to fire and close this
+	// schedule. Set when the schedule goes idle; cleared when it has work again.
+	// Surfaced as the ScheduleIdleCloseTime search attribute for stuck-schedule
+	// detection.
+	IdleCloseTime *timestamppb.Timestamp `protobuf:"bytes,12,opt,name=idle_close_time,json=idleCloseTime,proto3" json:"idle_close_time,omitempty"`
+	unknownFields protoimpl.UnknownFields
+	sizeCache     protoimpl.SizeCache
 }
 
 func (x *SchedulerState) Reset() {
@@ -146,6 +151,13 @@ func (x *SchedulerState) GetWorkflowMigration() *WorkflowMigrationState {
 	return nil
 }
 
+func (x *SchedulerState) GetIdleCloseTime() *timestamppb.Timestamp {
+	if x != nil {
+		return x.IdleCloseTime
+	}
+	return nil
+}
+
 // WorkflowMigrationState tracks the state of an in-progress V2-to-V1 migration.
 type WorkflowMigrationState struct {
 	state protoimpl.MessageState `protogen:"open.v1"`
@@ -700,7 +712,7 @@ var File_temporal_server_chasm_lib_scheduler_proto_v1_message_proto protoreflect
 
 const file_temporal_server_chasm_lib_scheduler_proto_v1_message_proto_rawDesc = "" +
 	"\n" +
-	":temporal/server/chasm/lib/scheduler/proto/v1/message.proto\x12,temporal.server.chasm.lib.scheduler.proto.v1\x1a\x1fgoogle/protobuf/timestamp.proto\x1a$temporal/api/common/v1/message.proto\x1a%temporal/api/failure/v1/message.proto\x1a&temporal/api/schedule/v1/message.proto\x1a-temporal/server/api/schedule/v1/message.proto\"\xbe\x03\n" +
+	":temporal/server/chasm/lib/scheduler/proto/v1/message.proto\x12,temporal.server.chasm.lib.scheduler.proto.v1\x1a\x1fgoogle/protobuf/timestamp.proto\x1a$temporal/api/common/v1/message.proto\x1a%temporal/api/failure/v1/message.proto\x1a&temporal/api/schedule/v1/message.proto\x1a-temporal/server/api/schedule/v1/message.proto\"\x82\x04\n" +
 	"\x0eSchedulerState\x12>\n" +
 	"\bschedule\x18\x02 \x01(\v2\".temporal.api.schedule.v1.ScheduleR\bschedule\x12:\n" +
 	"\x04info\x18\x03 \x01(\v2&.temporal.api.schedule.v1.ScheduleInfoR\x04info\x12\x1c\n" +
@@ -712,7 +724,8 @@ const file_temporal_server_chasm_lib_scheduler_proto_v1_message_proto_rawDesc =
 	"\x06closed\x18\t \x01(\bR\x06closed\x12\x1a\n" +
 	"\bsentinel\x18\n" +
 	" \x01(\bR\bsentinel\x12s\n" +
-	"\x12workflow_migration\x18\v \x01(\v2D.temporal.server.chasm.lib.scheduler.proto.v1.WorkflowMigrationStateR\x11workflowMigration\"z\n" +
+	"\x12workflow_migration\x18\v \x01(\v2D.temporal.server.chasm.lib.scheduler.proto.v1.WorkflowMigrationStateR\x11workflowMigration\x12B\n" +
+	"\x0fidle_close_time\x18\f \x01(\v2\x1a.google.protobuf.TimestampR\ridleCloseTime\"z\n" +
 	"\x16WorkflowMigrationState\x120\n" +
 	"\x14pre_migration_paused\x18\x01 \x01(\bR\x12preMigrationPaused\x12.\n" +
 	"\x13pre_migration_notes\x18\x02 \x01(\tR\x11preMigrationNotes\"\xa8\x01\n" +
@@ -798,34 +811,35 @@ var file_temporal_server_chasm_lib_scheduler_proto_v1_message_proto_depIdxs = []
 	12, // 0: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerState.schedule:type_name -> temporal.api.schedule.v1.Schedule
 	13, // 1: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerState.info:type_name -> temporal.api.schedule.v1.ScheduleInfo
 	1,  // 2: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerState.workflow_migration:type_name -> temporal.server.chasm.lib.scheduler.proto.v1.WorkflowMigrationState
-	14, // 3: temporal.server.chasm.lib.scheduler.proto.v1.GeneratorState.last_processed_time:type_name -> google.protobuf.Timestamp
-	14, // 4: temporal.server.chasm.lib.scheduler.proto.v1.GeneratorState.future_action_times:type_name -> google.protobuf.Timestamp
-	15, // 5: temporal.server.chasm.lib.scheduler.proto.v1.InvokerState.buffered_starts:type_name -> temporal.server.api.schedule.v1.BufferedStart
-	16, // 6: temporal.server.chasm.lib.scheduler.proto.v1.InvokerState.cancel_workflows:type_name -> temporal.api.common.v1.WorkflowExecution
-	16, // 7: temporal.server.chasm.lib.scheduler.proto.v1.InvokerState.terminate_workflows:type_name -> temporal.api.common.v1.WorkflowExecution
-	14, // 8: temporal.server.chasm.lib.scheduler.proto.v1.InvokerState.last_processed_time:type_name -> google.protobuf.Timestamp
-	17, // 9: temporal.server.chasm.lib.scheduler.proto.v1.BackfillerState.backfill_request:type_name -> temporal.api.schedule.v1.BackfillRequest
-	18, // 10: temporal.server.chasm.lib.scheduler.proto.v1.BackfillerState.trigger_request:type_name -> temporal.api.schedule.v1.TriggerImmediatelyRequest
-	14, // 11: temporal.server.chasm.lib.scheduler.proto.v1.BackfillerState.last_processed_time:type_name -> google.protobuf.Timestamp
-	19, // 12: temporal.server.chasm.lib.scheduler.proto.v1.LastCompletionResult.success:type_name -> temporal.api.common.v1.Payload
-	20, // 13: temporal.server.chasm.lib.scheduler.proto.v1.LastCompletionResult.failure:type_name -> temporal.api.failure.v1.Failure
-	0,  // 14: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.scheduler_state:type_name -> temporal.server.chasm.lib.scheduler.proto.v1.SchedulerState
-	2,  // 15: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.generator_state:type_name -> temporal.server.chasm.lib.scheduler.proto.v1.GeneratorState
-	3,  // 16: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.invoker_state:type_name -> temporal.server.chasm.lib.scheduler.proto.v1.InvokerState
-	9,  // 17: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.backfillers:type_name -> temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.BackfillersEntry
-	5,  // 18: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.last_completion_result:type_name -> temporal.server.chasm.lib.scheduler.proto.v1.LastCompletionResult
-	10, // 19: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.search_attributes:type_name -> temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.SearchAttributesEntry
-	11, // 20: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.memo:type_name -> temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.MemoEntry
-	8,  // 21: temporal.server.chasm.lib.scheduler.proto.v1.EventLog.events:type_name -> temporal.server.chasm.lib.scheduler.proto.v1.Event
-	14, // 22: temporal.server.chasm.lib.scheduler.proto.v1.Event.time:type_name -> google.protobuf.Timestamp
-	4,  // 23: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.BackfillersEntry.value:type_name -> temporal.server.chasm.lib.scheduler.proto.v1.BackfillerState
-	19, // 24: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.SearchAttributesEntry.value:type_name -> temporal.api.common.v1.Payload
-	19, // 25: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.MemoEntry.value:type_name -> temporal.api.common.v1.Payload
-	26, // [26:26] is the sub-list for method output_type
-	26, // [26:26] is the sub-list for method input_type
-	26, // [26:26] is the sub-list for extension type_name
-	26, // [26:26] is the sub-list for extension extendee
-	0,  // [0:26] is the sub-list for field type_name
+	14, // 3: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerState.idle_close_time:type_name -> google.protobuf.Timestamp
+	14, // 4: temporal.server.chasm.lib.scheduler.proto.v1.GeneratorState.last_processed_time:type_name -> google.protobuf.Timestamp
+	14, // 5: temporal.server.chasm.lib.scheduler.proto.v1.GeneratorState.future_action_times:type_name -> google.protobuf.Timestamp
+	15, // 6: temporal.server.chasm.lib.scheduler.proto.v1.InvokerState.buffered_starts:type_name -> temporal.server.api.schedule.v1.BufferedStart
+	16, // 7: temporal.server.chasm.lib.scheduler.proto.v1.InvokerState.cancel_workflows:type_name -> temporal.api.common.v1.WorkflowExecution
+	16, // 8: temporal.server.chasm.lib.scheduler.proto.v1.InvokerState.terminate_workflows:type_name -> temporal.api.common.v1.WorkflowExecution
+	14, // 9: temporal.server.chasm.lib.scheduler.proto.v1.InvokerState.last_processed_time:type_name -> google.protobuf.Timestamp
+	17, // 10: temporal.server.chasm.lib.scheduler.proto.v1.BackfillerState.backfill_request:type_name -> temporal.api.schedule.v1.BackfillRequest
+	18, // 11: temporal.server.chasm.lib.scheduler.proto.v1.BackfillerState.trigger_request:type_name -> temporal.api.schedule.v1.TriggerImmediatelyRequest
+	14, // 12: temporal.server.chasm.lib.scheduler.proto.v1.BackfillerState.last_processed_time:type_name -> google.protobuf.Timestamp
+	19, // 13: temporal.server.chasm.lib.scheduler.proto.v1.LastCompletionResult.success:type_name -> temporal.api.common.v1.Payload
+	20, // 14: temporal.server.chasm.lib.scheduler.proto.v1.LastCompletionResult.failure:type_name -> temporal.api.failure.v1.Failure
+	0,  // 15: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.scheduler_state:type_name -> temporal.server.chasm.lib.scheduler.proto.v1.SchedulerState
+	2,  // 16: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.generator_state:type_name -> temporal.server.chasm.lib.scheduler.proto.v1.GeneratorState
+	3,  // 17: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.invoker_state:type_name -> temporal.server.chasm.lib.scheduler.proto.v1.InvokerState
+	9,  // 18: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.backfillers:type_name -> temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.BackfillersEntry
+	5,  // 19: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.last_completion_result:type_name -> temporal.server.chasm.lib.scheduler.proto.v1.LastCompletionResult
+	10, // 20: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.search_attributes:type_name -> temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.SearchAttributesEntry
+	11, // 21: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.memo:type_name -> temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.MemoEntry
+	8,  // 22: temporal.server.chasm.lib.scheduler.proto.v1.EventLog.events:type_name -> temporal.server.chasm.lib.scheduler.proto.v1.Event
+	14, // 23: temporal.server.chasm.lib.scheduler.proto.v1.Event.time:type_name -> google.protobuf.Timestamp
+	4,  // 24: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.BackfillersEntry.value:type_name -> temporal.server.chasm.lib.scheduler.proto.v1.BackfillerState
+	19, // 25: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.SearchAttributesEntry.value:type_name -> temporal.api.common.v1.Payload
+	19, // 26: temporal.server.chasm.lib.scheduler.proto.v1.SchedulerMigrationState.MemoEntry.value:type_name -> temporal.api.common.v1.Payload
+	27, // [27:27] is the sub-list for method output_type
+	27, // [27:27] is the sub-list for method input_type
+	27, // [27:27] is the sub-list for extension type_name
+	27, // [27:27] is the sub-list for extension extendee
+	0,  // [0:27] is the sub-list for field type_name
 }
 
 func init() { file_temporal_server_chasm_lib_scheduler_proto_v1_message_proto_init() }
diff --git a/chasm/lib/scheduler/generator.go b/chasm/lib/scheduler/generator.go
index 5f771e49152..5b49162d0ec 100644
--- a/chasm/lib/scheduler/generator.go
+++ b/chasm/lib/scheduler/generator.go
@@ -1,6 +1,7 @@
 package scheduler
 
 import (
+	"fmt"
 	"time"
 
 	"go.temporal.io/server/chasm"
@@ -49,6 +50,8 @@ func (g *Generator) Generate(ctx chasm.MutableContext) {
 
 // scheduleTask schedules a GeneratorTask at the given time.
 func (g *Generator) scheduleTask(ctx chasm.MutableContext, scheduledTime time.Time) {
+	g.EventLog.Get(ctx).LogEvent(ctx,
+		fmt.Sprintf("scheduled generatorTask for %s", scheduledTime.Format(time.RFC3339)))
 	ctx.AddTask(g, chasm.TaskAttributes{
 		ScheduledTime: scheduledTime,
 	}, &schedulerpb.GeneratorTask{})
diff --git a/chasm/lib/scheduler/generator_tasks.go b/chasm/lib/scheduler/generator_tasks.go
index 2644f59ca05..222851e49e7 100644
--- a/chasm/lib/scheduler/generator_tasks.go
+++ b/chasm/lib/scheduler/generator_tasks.go
@@ -2,6 +2,7 @@ package scheduler
 
 import (
 	"fmt"
+	"time"
 
 	"go.temporal.io/server/chasm"
 	"go.temporal.io/server/chasm/lib/scheduler/gen/schedulerpb/v1"
@@ -59,6 +60,8 @@ func (g *GeneratorTaskHandler) Execute(
 
 	now := ctx.Now(generator)
 
+	generator.EventLog.Get(ctx).LogEvent(ctx, "generatorTask executed")
+
 	// If we have no last processed time, this is a new schedule.
 	if generator.LastProcessedTime == nil {
 		createdAt := timestamppb.New(now)
@@ -84,6 +87,8 @@ func (g *GeneratorTaskHandler) Execute(
 		t2 = t1
 	}
 
+	// Generate BufferedStarts and determine the next HWM. Actions are skipped when
+	// they can't be taken (paused, or limited and without any remaining actions).
 	result, err := g.SpecProcessor.ProcessTimeRange(
 		scheduler,
 		t1, t2,
@@ -101,6 +106,8 @@ func (g *GeneratorTaskHandler) Execute(
 
 	// Emit metrics and update state for any dropped actions.
 	if result.DroppedCount > 0 {
+		generator.EventLog.Get(ctx).LogEvent(ctx,
+			fmt.Sprintf("buffer overrun, dropped %d actions", result.DroppedCount))
 		logger.Warn("Buffer overrun, dropping actions",
 			tag.Int64("dropped-count", result.DroppedCount))
 		metricsHandler.Counter(metrics.ScheduleBufferOverruns.Name()).Record(result.DroppedCount)
@@ -116,30 +123,47 @@ func (g *GeneratorTaskHandler) Execute(
 	generator.LastProcessedTime = timestamppb.New(result.LastActionTime)
 	generator.UpdateFutureActionTimes(ctx, g.specBuilder)
 
-	// Check if the schedule has gone idle.
+	// Schedule the next timer task. Three outcomes are possible:
+	// - isIdle: the schedule is done; arm the idle task to close it.
+	// - NextWakeupTime is set: arm the next generator tick.
+	// - Neither: Hold open without a task. This requires both that
+	//   isHeldOpen is true (paused or backfill pending) AND that no spec
+	//   wakeup is available, e.g. a paused manual-only schedule. IdleTime=0
+	//   also lands here. An external trigger (Patch.Unpause, Update, or a
+	//   BackfillerTask's completion-time Generate call) revives us.
 	idleTimeTotal := g.config.Tweakables(scheduler.Namespace).IdleTime
 	idleExpiration, isIdle := scheduler.getIdleExpiration(ctx, idleTimeTotal, result.NextWakeupTime)
 	if isIdle {
 		// Schedule is complete, no need for another buffer task. We keep the schedule's
-		// backing mutable state explicitly open for a the idle period, during which the
+		// backing mutable state explicitly open for the idle period, during which the
 		// customer can describe/modify/restart the schedule.
 		//
 		// Once the idle timer expires, we close the component.
+		generator.EventLog.Get(ctx).LogEvent(ctx,
+			fmt.Sprintf("scheduled idle task for %s", idleExpiration.Format(time.RFC3339)))
 		ctx.AddTask(scheduler, chasm.TaskAttributes{
 			ScheduledTime: idleExpiration,
 		}, &schedulerpb.SchedulerIdleTask{
 			IdleTimeTotal: durationpb.New(idleTimeTotal),
 		})
-		return nil
-	}
+		// Record the idle-close deadline so it can be surfaced as the
+		// ScheduleIdleCloseTime search attribute for stuck-schedule detection.
+		scheduler.IdleCloseTime = timestamppb.New(idleExpiration)
 
-	// No more tasks if we're paused.
-	if scheduler.Schedule.State.Paused {
 		return nil
 	}
 
-	// Another buffering task is added if we aren't completely out of actions or paused.
-	generator.scheduleTask(ctx, result.NextWakeupTime)
+	// Not idle: the schedule has work again (or is being held open), so it's
+	// no longer pending an idle close.
+	scheduler.IdleCloseTime = nil
+
+	if !result.NextWakeupTime.IsZero() {
+		// Keep the generator task perpetually scheduled. When paused, the next
+		// fire will simply advance the HWM without appending actions (handled in
+		// ProcessTimeRange).
+		generator.scheduleTask(ctx, result.NextWakeupTime)
+	}
+	// else: hold open without a task; see the comment block above.
 
 	return nil
 }
@@ -148,8 +172,7 @@ func (g *GeneratorTaskHandler) logSchedule(ctx chasm.MutableContext, logger log.
 	spec := jsonStringer{sched.Schedule.Spec}
 	policies := jsonStringer{sched.Schedule.Policies}
 
-	tw := g.config.Tweakables(sched.Namespace)
-	generator.EventLog.Get(ctx).LogEvent(ctx, fmt.Sprintf("%s:\nSpec: %s\nPolicies: %s\n", msg, spec, policies), tw.EventLogMaxEntries, tw.EventLogMaxMessageLen)
+	generator.EventLog.Get(ctx).LogEvent(ctx, fmt.Sprintf("%s:\nSpec: %s\nPolicies: %s\n", msg, spec, policies))
 	logger.Info(msg,
 		tag.Stringer("spec", spec),
 		tag.Stringer("policies", policies))
diff --git a/chasm/lib/scheduler/generator_tasks_test.go b/chasm/lib/scheduler/generator_tasks_test.go
index 21f18a527cc..117126e5fe1 100644
--- a/chasm/lib/scheduler/generator_tasks_test.go
+++ b/chasm/lib/scheduler/generator_tasks_test.go
@@ -134,6 +134,47 @@ func TestGeneratorTask_UpdateFutureActionTimes_LimitedActions(t *testing.T) {
 	require.Len(t, generator.FutureActionTimes, 2)
 }
 
+func TestGeneratorTask_Idle_SetsIdleCloseTime(t *testing.T) {
+	env := newTestEnv(t)
+	handler := newGeneratorHandler(env)
+
+	ctx := env.MutableContext()
+	sched := env.Scheduler
+	generator := sched.Generator.Get(ctx)
+
+	// Exhaust the schedule's actions so it goes idle (no more work to do).
+	sched.Schedule.State.LimitedActions = true
+	sched.Schedule.State.RemainingActions = 0
+
+	err := handler.Execute(ctx, generator, chasm.TaskAttributes{}, &schedulerpb.GeneratorTask{})
+	require.NoError(t, err)
+
+	// The idle-close deadline must be recorded so it can be surfaced as the
+	// ScheduleIdleCloseTime search attribute.
+	require.NotNil(t, sched.IdleCloseTime, "expected IdleCloseTime to be set once idle")
+	require.True(t, sched.IdleCloseTime.AsTime().After(ctx.Now(generator)),
+		"expected idle-close deadline in the future, got %v", sched.IdleCloseTime.AsTime())
+}
+
+func TestGeneratorTask_NonIdle_ClearsIdleCloseTime(t *testing.T) {
+	env := newTestEnv(t)
+	handler := newGeneratorHandler(env)
+
+	ctx := env.MutableContext()
+	sched := env.Scheduler
+	generator := sched.Generator.Get(ctx)
+
+	// Simulate a stale deadline left over from a prior idle period.
+	sched.IdleCloseTime = timestamppb.New(ctx.Now(generator).Add(7 * 24 * time.Hour))
+
+	// A normal execution generates future actions, so the schedule is not idle.
+	err := handler.Execute(ctx, generator, chasm.TaskAttributes{}, &schedulerpb.GeneratorTask{})
+	require.NoError(t, err)
+
+	require.NotEmpty(t, generator.FutureActionTimes)
+	require.Nil(t, sched.IdleCloseTime, "expected IdleCloseTime to be cleared when the schedule has work")
+}
+
 func TestGeneratorTask_UpdateFutureActionTimes_SkipsBeforeUpdateTime(t *testing.T) {
 	env := newTestEnv(t)
 	handler := newGeneratorHandler(env)
diff --git a/chasm/lib/scheduler/helper_test.go b/chasm/lib/scheduler/helper_test.go
index f28ff945cec..32a4e52dc5f 100644
--- a/chasm/lib/scheduler/helper_test.go
+++ b/chasm/lib/scheduler/helper_test.go
@@ -88,9 +88,12 @@ func newTestLibrary(logger log.Logger, specProcessor scheduler.SpecProcessor) *s
 		SpecProcessor:  specProcessor,
 	}
 	return scheduler.NewLibrary(
+		config,
 		nil,
 		scheduler.NewSchedulerIdleTaskHandler(scheduler.SchedulerIdleTaskHandlerOptions{
-			Config: config,
+			Config:         config,
+			MetricsHandler: metrics.NoopMetricsHandler,
+			BaseLogger:     logger,
 		}),
 		scheduler.NewSchedulerCallbacksTaskHandler(scheduler.SchedulerCallbacksTaskHandlerOptions{
 			Config: config,
@@ -209,6 +212,7 @@ func newTestEnv(t *testing.T, opts ...testEnvOption) *testEnv {
 		HandleGetCurrentVersion:   func() int64 { return 1 },
 		HandleGetWorkflowKey:      tv.Any().WorkflowKey,
 		HandleIsWorkflow:          func() bool { return false },
+		HandleGetNamespaceEntry:   tv.Namespace,
 		HandleCurrentVersionedTransition: func() *persistencespb.VersionedTransition {
 			return &persistencespb.VersionedTransition{
 				NamespaceFailoverVersion: 1,
@@ -346,6 +350,7 @@ func setupTestInfra(t *testing.T, specProcessor scheduler.SpecProcessor) *testIn
 	nodeBackend.HandleGetCurrentVersion = func() int64 { return 1 }
 	nodeBackend.HandleGetWorkflowKey = tv.Any().WorkflowKey
 	nodeBackend.HandleIsWorkflow = func() bool { return false }
+	nodeBackend.HandleGetNamespaceEntry = tv.Namespace
 	nodeBackend.HandleCurrentVersionedTransition = func() *persistencespb.VersionedTransition {
 		return &persistencespb.VersionedTransition{
 			NamespaceFailoverVersion: 1,
diff --git a/chasm/lib/scheduler/invoker.go b/chasm/lib/scheduler/invoker.go
index 39090c53140..2a12684ecdb 100644
--- a/chasm/lib/scheduler/invoker.go
+++ b/chasm/lib/scheduler/invoker.go
@@ -1,6 +1,7 @@
 package scheduler
 
 import (
+	"fmt"
 	"slices"
 	"time"
 
@@ -49,6 +50,9 @@ func newInvokerWithState(ctx chasm.MutableContext, state *schedulerpb.InvokerSta
 // immediately kicking off a processing task.
 func (i *Invoker) EnqueueBufferedStarts(ctx chasm.MutableContext, starts []*schedulespb.BufferedStart) {
 	i.BufferedStarts = append(i.BufferedStarts, starts...)
+	if len(starts) > 0 {
+		i.EventLog.Get(ctx).LogEvent(ctx, fmt.Sprintf("enqueued %d buffered start(s)", len(starts)))
+	}
 	i.addTasks(ctx)
 }
 
@@ -61,10 +65,12 @@ type processBufferResult struct {
 	discardStarts []*schedulespb.BufferedStart
 
 	// Number of buffered starts dropped due to overlap policy during processing.
-	overlapSkipped int64
+	overlapSkipped         int64
+	overlapSkippedByPolicy map[enumspb.ScheduleOverlapPolicy]int64
 
-	// Nunmber of buffered starts dropped from missing the catchup window.
-	missedCatchupWindow int64
+	// Number of buffered starts dropped from missing the catchup window,
+	// bucketed by whether a running action contributed to the miss.
+	missedCatchupByActionRunning map[bool]int64
 }
 
 // recordProcessBufferResult updates the Invoker's internal state based on result, as well as the
@@ -81,6 +87,8 @@ func (i *Invoker) recordProcessBufferResult(ctx chasm.MutableContext, result *pr
 
 	// Drop discarded starts, and update requested starts for execution.
 	var starts []*schedulespb.BufferedStart
+	readiedStarts := 0
+	deferredStarts := 0
 	for _, start := range i.GetBufferedStarts() {
 		if discards[start.RequestId] {
 			continue
@@ -89,31 +97,33 @@ func (i *Invoker) recordProcessBufferResult(ctx chasm.MutableContext, result *pr
 		// Starts ready for execution are set to their first attempt.
 		if ready[start.RequestId] && start.Attempt < 1 {
 			start.Attempt = 1
+			readiedStarts++
 		} else if start.Attempt == 0 {
 			// Start was processed but deferred (e.g., BUFFER_ONE policy with running workflow).
-			// Mark as deferred (-1) to distinguish from newly-enqueued starts. This prevents
-			// processingDeadline() from scheduling an immediate task for deferred starts.
+			// Mark as deferred (-1) to distinguish from newly-enqueued starts so addTasks
+			// won't schedule an immediate ProcessBuffer task for them - they wait on
+			// recordCompletedAction to re-enable.
 			start.Attempt = -1
+			deferredStarts++
 		}
 
 		starts = append(starts, start)
 	}
 
+	if readiedStarts > 0 || deferredStarts > 0 {
+		i.EventLog.Get(ctx).LogEvent(ctx,
+			fmt.Sprintf("recordProcessBufferResult readied %d starts, deferred %d starts", readiedStarts, deferredStarts))
+	}
+
 	// Update internal state.
 	i.BufferedStarts = starts
 	i.CancelWorkflows = append(i.GetCancelWorkflows(), result.cancelWorkflows...)
 	i.TerminateWorkflows = append(i.GetTerminateWorkflows(), result.terminateWorkflows...)
 	i.LastProcessedTime = timestamppb.New(ctx.Now(i))
 
-	// Only schedule new tasks if this processBuffer call actually did something.
-	// This prevents duplicate task scheduling when multiple ProcessBuffer tasks
-	// run in the same transaction (e.g., from multiple backfillers).
-	if len(result.startWorkflows) > 0 ||
-		len(result.discardStarts) > 0 ||
-		len(result.cancelWorkflows) > 0 ||
-		len(result.terminateWorkflows) > 0 {
-		i.addTasks(ctx)
-	}
+	// Re-arm tasks if this call changed state, or if the LastProcessedTime advance
+	// just unblocked backed-off starts.
+	i.addTasks(ctx)
 }
 
 type executeResult struct {
@@ -143,8 +153,11 @@ func (e *executeResult) Append(o executeResult) executeResult {
 }
 
 // recordExecuteResult updates the Invoker's internal state with the results of a
-// completed InvokerExecuteTask. Tasks to continue execution are added, if needed.
-func (i *Invoker) recordExecuteResult(ctx chasm.MutableContext, result *executeResult) {
+// completed InvokerExecuteTask. It returns the number of *new* actions recorded
+// (starts that transitioned from "no RunId" to "has RunId" in this call) and
+// the number of completed results that were dropped because they were previously
+// recorded.
+func (i *Invoker) recordExecuteResult(ctx chasm.MutableContext, result *executeResult) (newlyStarted, droppedDuplicates int) {
 	completed := make(map[string]*schedulespb.BufferedStart) // request ID -> BufferedStart with RunId/StartTime
 	failed := make(map[string]bool)                          // request ID -> is present
 	retryable := make(map[string]*schedulespb.BufferedStart) // request ID -> *BufferedStart
@@ -168,31 +181,50 @@ func (i *Invoker) recordExecuteResult(ctx chasm.MutableContext, result *executeR
 	}
 
 	// Remove failed (non-retryable) starts from the buffer.
+	removedStarts := 0
+	retriedStarts := 0
 	i.BufferedStarts = slices.DeleteFunc(i.GetBufferedStarts(), func(start *schedulespb.BufferedStart) bool {
+		removedStarts++
 		return failed[start.RequestId]
 	})
 	i.CancelWorkflows = slices.DeleteFunc(i.GetCancelWorkflows(), func(we *commonpb.WorkflowExecution) bool {
+		removedStarts++
 		return canceled[we.RunId]
 	})
 	i.TerminateWorkflows = slices.DeleteFunc(i.GetTerminateWorkflows(), func(we *commonpb.WorkflowExecution) bool {
+		removedStarts++
 		return terminated[we.RunId]
 	})
 
-	// Update BufferedStarts with results.
+	// Update BufferedStarts with results, dropping duplicates.
 	for _, start := range i.GetBufferedStarts() {
+		if start.RunId != "" {
+			if _, isDuplicate := completed[start.RequestId]; isDuplicate {
+				droppedDuplicates++
+			}
+			continue
+		}
 		if completedStart, ok := completed[start.RequestId]; ok {
 			start.RunId = completedStart.GetRunId()
 			start.StartTime = completedStart.GetStartTime()
 			start.HasCallback = true
+			newlyStarted++
 		}
 		if retry, ok := retryable[start.RequestId]; ok {
 			start.Attempt++
 			start.BackoffTime = retry.GetBackoffTime()
+			retriedStarts++
 		}
 	}
 
-	// Add tasks if other actions are backing off or still pending execution.
+	i.EventLog.Get(ctx).LogEvent(ctx,
+		fmt.Sprintf("recordExecuteResult kicked off %d starts, removed %d starts, retried %d starts",
+			newlyStarted,
+			removedStarts,
+			retriedStarts))
+
 	i.addTasks(ctx)
+	return newlyStarted, droppedDuplicates
 }
 
 // runningWorkflowID returns the workflow ID associated with the given
@@ -216,6 +248,8 @@ func (i *Invoker) recordCompletedAction(
 	completed *schedulespb.CompletedResult,
 	requestID string,
 ) (scheduleTime time.Time) {
+	i.EventLog.Get(ctx).LogEvent(ctx, fmt.Sprintf("recording completed action: %s", requestID))
+
 	// Find the BufferedStart and mark it as completed.
 	for _, start := range i.BufferedStarts {
 		if start.GetRequestId() == requestID {
@@ -236,8 +270,10 @@ func (i *Invoker) recordCompletedAction(
 
 	// Update DesiredTime on the first pending start for metrics. DesiredTime is used
 	// to drive action latency between buffered starts (the time it takes between
-	// completing one start and kicking off the next). We set that on the first start
-	// pending execution.
+	// completing one start and kicking off the next). It also signals in processBuffer
+	// that this start was blocked behind a running action: if DesiredTime (the previous
+	// action's CloseTime) is past the start's catchup deadline, the previous action's
+	// duration caused the miss.
 	idx := slices.IndexFunc(i.BufferedStarts, func(start *schedulespb.BufferedStart) bool {
 		return start.Attempt == 0
 	})
@@ -258,40 +294,63 @@ func (i *Invoker) recordCompletedAction(
 // addTasks adds both ProcessBuffer and Execute tasks as needed. It should be
 // called when completing processing/executing tasks, to drive backoff/retry.
 func (i *Invoker) addTasks(ctx chasm.MutableContext) {
-	totalStarts := len(i.GetBufferedStarts())
-	eligibleStarts := len(i.getEligibleBufferedStarts())
-
-	// Add a ProcessBuffer pure task whenever there are BufferedStarts that are
-	// backing off, or are still pending initial processing.
-	if (totalStarts - eligibleStarts) > 0 {
+	// If we have Attempt = 0 starts, generate a ProcessBufferTask immediately. If we
+	// have starts that are backing off, add a timer task for the earliest backoff time.
+	if i.hasUnprocessedStarts() {
+		i.EventLog.Get(ctx).LogEvent(ctx, "scheduled processBufferTask immediately")
+		ctx.AddTask(i, chasm.TaskAttributes{
+			ScheduledTime: chasm.TaskScheduledTimeImmediate,
+		}, &schedulerpb.InvokerProcessBufferTask{})
+	} else if deadline := i.nextBackoffDeadline(); !deadline.IsZero() {
+		i.EventLog.Get(ctx).LogEvent(ctx,
+			fmt.Sprintf("scheduled processBufferTask for %s", deadline.Format(time.RFC3339)))
 		ctx.AddTask(i, chasm.TaskAttributes{
-			ScheduledTime: i.processingDeadline(),
+			ScheduledTime: deadline,
 		}, &schedulerpb.InvokerProcessBufferTask{})
 	}
 
-	// Add an Execute side effect task whenever there are any eligible actions
-	// pending execution.
-	if len(i.GetCancelWorkflows()) > 0 || len(i.GetTerminateWorkflows()) > 0 || eligibleStarts > 0 {
+	// Execute drains work that's ready now: pending cancels/terminates, and
+	// starts that are past their backoff.
+	if len(i.GetCancelWorkflows()) > 0 ||
+		len(i.GetTerminateWorkflows()) > 0 ||
+		len(i.getEligibleBufferedStarts()) > 0 {
+		i.EventLog.Get(ctx).LogEvent(ctx, "scheduled executeTask")
 		ctx.AddTask(i, chasm.TaskAttributes{}, &schedulerpb.InvokerExecuteTask{})
 	}
 }
 
-// processingDeadline returns the earliest possible time that the BufferedStarts
-// queue should be processed, taking into account starts that have not yet been
-// attempted, as well as those that are pending backoff to retry. If the buffer
-// is empty, the return value will be Time's zero value.
-func (i *Invoker) processingDeadline() time.Time {
-	var deadline time.Time
+// hasUnprocessedStarts reports whether any BufferedStart is still awaiting its
+// initial ProcessBuffer pass (Attempt == 0).
+func (i *Invoker) hasUnprocessedStarts() bool {
 	for _, start := range i.GetBufferedStarts() {
 		if start.GetAttempt() == 0 {
-			// Return zero time to schedule an immediate task for unprocessed starts.
-			return chasm.TaskScheduledTimeImmediate
+			return true
 		}
+	}
+	return false
+}
+
+// nextBackoffDeadline returns the earliest BackoffTime among starts that are
+// retrying, or the zero time if none are.
+func (i *Invoker) nextBackoffDeadline() time.Time {
+	var deadline time.Time
+	lastProcessedTime := i.LastProcessedTime.AsTime()
+	for _, start := range i.GetBufferedStarts() {
 		backoff := start.GetBackoffTime().AsTime()
+		// We only care about starts that are retrying.
+		if start.GetAttempt() <= 0 ||
+			start.GetRunId() != "" ||
+			start.GetCompleted() != nil ||
+			// Backed-off starts will be selected by getEligibleBufferedStarts and kick off
+			// an Execute task, instead.
+			start.BackoffTime.AsTime().Before(lastProcessedTime) {
+			continue
+		}
 		if deadline.IsZero() || backoff.Before(deadline) {
 			deadline = backoff
 		}
 	}
+
 	return deadline
 }
 
@@ -299,10 +358,11 @@ func (i *Invoker) processingDeadline() time.Time {
 // execution (Attempt > 0), haven't been started yet (no RunId), and aren't
 // presently backing off, based on last processed time.
 func (i *Invoker) getEligibleBufferedStarts() []*schedulespb.BufferedStart {
+	lastProcessed := i.GetLastProcessedTime().AsTime()
 	return util.FilterSlice(i.GetBufferedStarts(), func(start *schedulespb.BufferedStart) bool {
 		return start.Attempt > 0 &&
 			start.GetRunId() == "" &&
-			start.BackoffTime.AsTime().Before(i.GetLastProcessedTime().AsTime())
+			!start.GetBackoffTime().AsTime().After(lastProcessed)
 	})
 }
 
diff --git a/chasm/lib/scheduler/invoker_execute_task_test.go b/chasm/lib/scheduler/invoker_execute_task_test.go
index 34732f6b5a3..f1456d9a1ce 100644
--- a/chasm/lib/scheduler/invoker_execute_task_test.go
+++ b/chasm/lib/scheduler/invoker_execute_task_test.go
@@ -3,6 +3,7 @@ package scheduler_test
 import (
 	"fmt"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/require"
 	commonpb "go.temporal.io/api/common/v1"
@@ -403,6 +404,168 @@ func TestExecuteTask_ExceedsMaxActionsPerExecution(t *testing.T) {
 	})
 }
 
+// Two concurrent ExecuteTasks can both clone the invoker before either
+// commits, both fire StartWorkflow for the same RequestId, and both observe
+// success (server dedupes by RequestId). The losing commit must not
+// double-count, stomp the winner's RunId/StartTime/HasCallback, or rewind
+// Attempt/BackoffTime on the already-running entry.
+func TestExecuteTask_RecordResultIdempotentOnRace(t *testing.T) {
+	env := newTestEnv(t)
+	ctx := env.MutableContext()
+	invoker := env.Scheduler.Invoker.Get(ctx)
+
+	startTime := timestamppb.New(env.TimeSource.Now())
+	winning := "winning-run"
+	invoker.BufferedStarts = []*schedulespb.BufferedStart{{
+		NominalTime: startTime,
+		ActualTime:  startTime,
+		DesiredTime: startTime,
+		RequestId:   "req",
+		WorkflowId:  "wf",
+		Attempt:     1,
+		RunId:       winning,
+		StartTime:   startTime,
+		HasCallback: true,
+	}}
+	invoker.LastProcessedTime = timestamppb.New(env.TimeSource.Now())
+
+	loserStartTime := timestamppb.New(env.TimeSource.Now().Add(time.Second))
+	loser := []*schedulespb.BufferedStart{{
+		RequestId: "req",
+		RunId:     "loser-run",
+		StartTime: loserStartTime,
+	}}
+
+	newlyStarted, droppedDuplicates := invoker.RecordExecuteResult(ctx, loser, nil)
+	require.Equal(t, 0, newlyStarted, "duplicate RunId-set start must not be counted")
+	require.Equal(t, 1, droppedDuplicates, "the dropped completion must be reported for observability")
+	live := invoker.BufferedStarts[0]
+	require.Equal(t, winning, live.RunId, "live RunId must not be stomped")
+	require.Equal(t, startTime.AsTime(), live.StartTime.AsTime(), "live StartTime must not be stomped")
+	require.True(t, live.HasCallback, "live HasCallback must not be cleared")
+
+	// First-mover case: a CompletedStart for a fresh RequestId increments
+	// newlyStarted and writes through to the live entry.
+	invoker.BufferedStarts = append(invoker.BufferedStarts, &schedulespb.BufferedStart{
+		NominalTime: startTime,
+		ActualTime:  startTime,
+		DesiredTime: startTime,
+		RequestId:   "req2",
+		WorkflowId:  "wf2",
+		Attempt:     1,
+	})
+	first := []*schedulespb.BufferedStart{{
+		RequestId: "req2",
+		RunId:     "first-run",
+		StartTime: startTime,
+	}}
+	newlyStarted, droppedDuplicates = invoker.RecordExecuteResult(ctx, first, nil)
+	require.Equal(t, 1, newlyStarted, "first-time RunId assignment must be counted")
+	require.Equal(t, 0, droppedDuplicates, "no duplicate was dropped")
+	freshlyStarted := invoker.BufferedStarts[1]
+	require.Equal(t, "first-run", freshlyStarted.RunId)
+	require.Equal(t, startTime.AsTime(), freshlyStarted.StartTime.AsTime())
+	require.True(t, freshlyStarted.HasCallback, "first-time RunId assignment must set HasCallback")
+}
+
+// A RetryableStart for a request whose live BufferedStart already has RunId
+// set must not bump Attempt or set BackoffTime - the same RunId guard that
+// protects the completed branch must also protect the retryable branch,
+// otherwise stale retry metadata persists on an already-running entry.
+func TestExecuteTask_RecordResultIdempotentOnRetryableRace(t *testing.T) {
+	env := newTestEnv(t)
+	ctx := env.MutableContext()
+	invoker := env.Scheduler.Invoker.Get(ctx)
+
+	startTime := timestamppb.New(env.TimeSource.Now())
+	invoker.BufferedStarts = []*schedulespb.BufferedStart{{
+		NominalTime: startTime,
+		ActualTime:  startTime,
+		DesiredTime: startTime,
+		RequestId:   "req",
+		WorkflowId:  "wf",
+		Attempt:     1,
+		RunId:       "winning-run",
+		StartTime:   startTime,
+		HasCallback: true,
+	}}
+	invoker.LastProcessedTime = timestamppb.New(env.TimeSource.Now())
+
+	// A losing concurrent Execute saw the start as eligible, its RPC failed
+	// retryably, and applyBackoff produced a RetryableStart entry.
+	loserBackoff := timestamppb.New(env.TimeSource.Now().Add(time.Minute))
+	retryable := []*schedulespb.BufferedStart{{
+		RequestId:   "req",
+		BackoffTime: loserBackoff,
+	}}
+
+	newlyStarted, droppedDuplicates := invoker.RecordExecuteResult(ctx, nil, retryable)
+	require.Equal(t, 0, newlyStarted)
+	require.Equal(t, 0, droppedDuplicates, "retryable drops aren't counted as duplicates - only completed-side drops are")
+	live := invoker.BufferedStarts[0]
+	require.Equal(t, int64(1), live.Attempt, "Attempt must not be incremented on a started entry")
+	require.Nil(t, live.BackoffTime, "BackoffTime must not be set on a started entry")
+}
+
+// A start whose BackoffTime exactly equals LastProcessedTime must be eligible.
+// Regression for the strict-Before check, which excluded equal-time entries
+// and stranded retries that landed precisely at the HWM boundary.
+func TestExecuteTask_Validate_BackoffEqualToLPTIsEligible(t *testing.T) {
+	env := newInvokerExecuteTestEnv(t)
+	ctx := env.MutableContext()
+	invoker := env.Scheduler.Invoker.Get(ctx)
+
+	now := env.TimeSource.Now()
+	invoker.LastProcessedTime = timestamppb.New(now)
+	invoker.BufferedStarts = []*schedulespb.BufferedStart{{
+		RequestId:   "boundary",
+		Attempt:     2,
+		BackoffTime: timestamppb.New(now),
+	}}
+
+	valid, err := env.handler.Validate(ctx, invoker, chasm.TaskAttributes{}, &schedulerpb.InvokerExecuteTask{})
+	require.NoError(t, err)
+	require.True(t, valid, "BackoffTime == LastProcessedTime must be eligible (<=, not strict <)")
+}
+
+// BackoffTime must be derived from the framework clock (chasm.Context.Now),
+// not wall-clock time.Now. Test by advancing TimeSource so it diverges from
+// the wall clock, then asserting BackoffTime falls within the framework
+// clock's frame.
+func TestExecuteTask_BackoffUsesFrameworkClock(t *testing.T) {
+	env := newInvokerExecuteTestEnv(t)
+	frameworkNow := env.TimeSource.Now().Add(24 * time.Hour)
+	env.TimeSource.Update(frameworkNow)
+
+	startTime := timestamppb.New(env.TimeSource.Now())
+	env.mockFrontendClient.EXPECT().
+		StartWorkflowExecution(gomock.Any(), gomock.Any()).
+		Times(1).
+		Return(nil, serviceerror.NewDeadlineExceeded("transient"))
+
+	runExecuteTestCase(t, env, &executeTestCase{
+		InitialBufferedStarts: []*schedulespb.BufferedStart{{
+			NominalTime:   startTime,
+			ActualTime:    startTime,
+			DesiredTime:   startTime,
+			RequestId:     "retry",
+			OverlapPolicy: enumspb.SCHEDULE_OVERLAP_POLICY_ALLOW_ALL,
+			Attempt:       1,
+		}},
+		ExpectedBufferedStarts: 1,
+		ValidateInvoker: func(t *testing.T, invoker *scheduler.Invoker, env *invokerExecuteTestEnv) {
+			backoff := invoker.BufferedStarts[0].BackoffTime.AsTime()
+			require.True(t, backoff.After(frameworkNow),
+				"BackoffTime %v must be after framework clock %v", backoff, frameworkNow)
+			// time.Now() + any plausible delay is far before frameworkNow (TimeSource
+			// was advanced by 24h). If BackoffTime were derived from wall clock,
+			// this assertion would fail.
+			require.True(t, backoff.After(time.Now().Add(time.Hour)),
+				"BackoffTime %v looks derived from wall clock, not framework clock", backoff)
+		},
+	})
+}
+
 type startWorkflowExecutionRequestIDMatcher struct {
 	RequestID string
 }
diff --git a/chasm/lib/scheduler/invoker_process_buffer_task_test.go b/chasm/lib/scheduler/invoker_process_buffer_task_test.go
index 9178a44667d..acfee763c53 100644
--- a/chasm/lib/scheduler/invoker_process_buffer_task_test.go
+++ b/chasm/lib/scheduler/invoker_process_buffer_task_test.go
@@ -13,9 +13,76 @@ import (
 	"go.temporal.io/server/chasm/lib/scheduler/gen/schedulerpb/v1"
 	"go.temporal.io/server/common/metrics"
 	"go.temporal.io/server/common/util"
+	"go.temporal.io/server/service/history/tasks"
 	"google.golang.org/protobuf/types/known/timestamppb"
 )
 
+// A buffer of only deferred starts (Attempt=-1) must NOT emit a
+// ProcessBufferTask. Deferred starts wait on completion events, not on a
+// wall-clock deadline, so emitting an immediate ProcessBufferTask would
+// spin-loop on a no-op processBuffer call.
+func TestInvoker_AddTasks_AllDeferredEmitsNothing(t *testing.T) {
+	env := newTestEnv(t)
+	ctx := env.MutableContext()
+	invoker := env.Scheduler.Invoker.Get(ctx)
+
+	now := env.TimeSource.Now()
+	invoker.LastProcessedTime = timestamppb.New(now)
+	invoker.BufferedStarts = []*schedulespb.BufferedStart{{
+		RequestId:  "deferred",
+		WorkflowId: "wf-deferred",
+		Attempt:    -1,
+		ActualTime: timestamppb.New(now),
+	}}
+
+	env.NodeBackend.TasksByCategory = nil
+	invoker.EnqueueBufferedStarts(ctx, nil)
+	require.NoError(t, env.CloseTransaction())
+
+	require.False(t, env.HasTask(&tasks.ChasmTask{}, chasm.TaskScheduledTimeImmediate),
+		"all-deferred buffer must not emit any side-effect task")
+	require.False(t, env.HasTask(&tasks.ChasmTaskPure{}, chasm.TaskScheduledTimeImmediate),
+		"all-deferred buffer must not emit an immediate ProcessBufferTask")
+}
+
+// After ProcessBuffer fires for a backed-off start whose BackoffTime has just
+// elapsed, the HWM advance must re-arm processing so the retry actually runs.
+// Regression for the addTasks gate that previously suppressed re-arm when
+// processBuffer found nothing to process (no Attempt==0 starts).
+func TestProcessBufferTask_RearmsBackedOffRetry(t *testing.T) {
+	env := newTestEnv(t)
+	ctx := env.MutableContext()
+	invoker := env.Scheduler.Invoker.Get(ctx)
+
+	now := env.TimeSource.Now()
+	// Set LPT to the past so the start (with BackoffTime at now) becomes
+	// eligible only after the HWM advance in this run.
+	invoker.LastProcessedTime = timestamppb.New(now.Add(-time.Minute))
+	invoker.BufferedStarts = []*schedulespb.BufferedStart{{
+		NominalTime:   timestamppb.New(now),
+		ActualTime:    timestamppb.New(now),
+		DesiredTime:   timestamppb.New(now),
+		RequestId:     "retry-ready",
+		OverlapPolicy: enumspb.SCHEDULE_OVERLAP_POLICY_ALLOW_ALL,
+		Attempt:       2,
+		BackoffTime:   timestamppb.New(now),
+	}}
+
+	err := newProcessBufferHandler(env).Execute(ctx, invoker, chasm.TaskAttributes{}, &schedulerpb.InvokerProcessBufferTask{})
+	require.NoError(t, err)
+	require.NoError(t, env.CloseTransaction())
+
+	// Precondition for re-arm: the HWM advance in this Execute must have made
+	// the previously-backing-off start eligible (BackoffTime <= LPT).
+	require.False(t, invoker.BufferedStarts[0].BackoffTime.AsTime().After(invoker.LastProcessedTime.AsTime()),
+		"setup invariant: start must be eligible after HWM advance")
+
+	// addTasks must have emitted an immediate side-effect ChasmTask carrying
+	// the InvokerExecuteTask payload so the retry actually fires.
+	require.True(t, env.HasTask(&tasks.ChasmTask{}, chasm.TaskScheduledTimeImmediate),
+		"expected an immediate side-effect task to re-arm the backed-off retry")
+}
+
 func newProcessBufferHandler(env *testEnv) *scheduler.InvokerProcessBufferTaskHandler {
 	return scheduler.NewInvokerProcessBufferTaskHandler(scheduler.InvokerTaskHandlerOptions{
 		Config:         defaultConfig(),
@@ -315,6 +382,38 @@ func TestProcessBufferTask_NeedsTerminate(t *testing.T) {
 	})
 }
 
+// Past-catchup automated starts must drop WITHOUT consuming a LimitedActions
+// slot. Regression for the order-of-checks bug where useScheduledAction(true)
+// fired before the catchup-window check, decrementing RemainingActions for
+// starts that never ran.
+//
+// Not reachable via the public API today: backfill/TriggerImmediately starts
+// are Manual=true (see backfiller_tasks.go) and bypass the catchup-window
+// check. The bug only manifests for automated Generator starts that age past
+// their catchup window while processing is stalled.
+func TestProcessBufferTask_MissedCatchupPreservesRemainingActions(t *testing.T) {
+	env := newTestEnv(t)
+	env.Scheduler.Schedule.State.LimitedActions = true
+	env.Scheduler.Schedule.State.RemainingActions = 3
+
+	now := env.TimeSource.Now()
+	startTime := timestamppb.New(now.Add(-defaultCatchupWindow * 2))
+	runProcessBufferTestCase(t, env, &processBufferTestCase{
+		InitialBufferedStarts: []*schedulespb.BufferedStart{{
+			NominalTime:   startTime,
+			ActualTime:    startTime,
+			DesiredTime:   startTime,
+			Manual:        false,
+			RequestId:     "expired",
+			OverlapPolicy: enumspb.SCHEDULE_OVERLAP_POLICY_ALLOW_ALL,
+		}},
+		ExpectedBufferedStarts:      0,
+		ExpectedMissedCatchupWindow: 1,
+	})
+	require.Equal(t, int64(3), env.Scheduler.Schedule.State.RemainingActions,
+		"RemainingActions must not be consumed by a start that was dropped for missing the catchup window")
+}
+
 // A buffered start with an overlap policy to cancel other workflows is processed.
 func TestProcessBufferTask_NeedsCancel(t *testing.T) {
 	env := newTestEnv(t)
diff --git a/chasm/lib/scheduler/invoker_tasks.go b/chasm/lib/scheduler/invoker_tasks.go
index 19d5bc17309..4069bc3cfc8 100644
--- a/chasm/lib/scheduler/invoker_tasks.go
+++ b/chasm/lib/scheduler/invoker_tasks.go
@@ -10,7 +10,6 @@ import (
 
 	commonpb "go.temporal.io/api/common/v1"
 	enumspb "go.temporal.io/api/enums/v1"
-	schedulepb "go.temporal.io/api/schedule/v1"
 	"go.temporal.io/api/serviceerror"
 	"go.temporal.io/api/workflowservice/v1"
 	"go.temporal.io/server/api/historyservice/v1"
@@ -120,10 +119,10 @@ func (h *InvokerExecuteTaskHandler) Validate(
 	// If another execute task already happened to kick everything off, we don't need
 	// this one.
 	eligibleStarts := invoker.getEligibleBufferedStarts()
-	valid := len(invoker.GetTerminateWorkflows())+
-		len(invoker.GetCancelWorkflows())+
-		len(eligibleStarts) > 0
-	return valid, nil
+	if len(invoker.GetTerminateWorkflows())+len(invoker.GetCancelWorkflows())+len(eligibleStarts) > 0 {
+		return true, nil
+	}
+	return false, nil
 }
 
 func (h *InvokerExecuteTaskHandler) Execute(
@@ -138,6 +137,7 @@ func (h *InvokerExecuteTaskHandler) Execute(
 	var scheduler *Scheduler
 	var lastCompletionState *schedulerpb.LastCompletionResult
 	var callback *commonpb.Callback
+	var now time.Time
 
 	// Read and deep copy returned components, since we'll continue to access them
 	// outside of this function (outside of the MS lock).
@@ -166,6 +166,10 @@ func (h *InvokerExecuteTaskHandler) Execute(
 			}
 			callback = common.CloneProto(cb)
 
+			// Captured here for applyBackoff (runs outside this closure): BackoffTime
+			// must be framework-clock to match LastProcessedTime and task deadlines.
+			now = ctx.Now(i)
+
 			return struct{}{}, nil
 		},
 		nil,
@@ -186,8 +190,7 @@ func (h *InvokerExecuteTaskHandler) Execute(
 	ictx := h.newInvokerTaskHandlerContext(ctx, scheduler)
 	result = result.Append(h.terminateWorkflows(ictx, logger, metricsHandler, scheduler, invoker.GetTerminateWorkflows()))
 	result = result.Append(h.cancelWorkflows(ictx, logger, metricsHandler, scheduler, invoker.GetCancelWorkflows()))
-	sres, startResults := h.startWorkflows(ictx, logger, metricsHandler, scheduler, invoker, lastCompletionState, callback)
-	result = result.Append(sres)
+	result = result.Append(h.startWorkflows(ictx, logger, metricsHandler, scheduler, invoker, lastCompletionState, callback, now))
 
 	// Record action results on the Invoker (internal state), as well as the
 	// Scheduler (user-facing metrics).
@@ -196,10 +199,10 @@ func (h *InvokerExecuteTaskHandler) Execute(
 		invokerRef,
 		func(i *Invoker, ctx chasm.MutableContext, _ any) (chasm.NoValue, error) {
 			s := i.Scheduler.Get(ctx)
-
-			i.recordExecuteResult(ctx, &result)
-			s.recordActionResult(&schedulerActionResult{actionCount: int64(len(startResults))})
-
+			// Use newlyStarted (not len(result.CompletedStarts)) so a concurrent
+			// ExecuteTask's duplicate StartWorkflow can't inflate ActionCount.
+			newlyStarted, _ := i.recordExecuteResult(ctx, &result)
+			s.recordActionResult(&schedulerActionResult{actionCount: int64(newlyStarted)})
 			return nil, nil
 		},
 		nil,
@@ -306,7 +309,8 @@ func (h *InvokerExecuteTaskHandler) startWorkflows(
 	invoker *Invoker,
 	lastCompletionState *schedulerpb.LastCompletionResult,
 	callback *commonpb.Callback,
-) (result executeResult, startResults []*schedulepb.ScheduleActionResult) {
+	now time.Time,
+) (result executeResult) {
 	metricsWithTag := metricsHandler.WithTags(
 		metrics.StringTag(metrics.ScheduleActionTypeTag, metrics.ScheduleActionStartWorkflow))
 
@@ -335,7 +339,7 @@ func (h *InvokerExecuteTaskHandler) startWorkflows(
 		// Run all starts concurrently.
 		newCtx := ctx.Clone()
 		wg.Go(func() {
-			startResult, err := h.startWorkflow(newCtx, metricsHandler, scheduler, start, lastCompletionState, callback)
+			err := h.startWorkflow(newCtx, metricsHandler, scheduler, start, lastCompletionState, callback)
 
 			resultMutex.Lock()
 			defer resultMutex.Unlock()
@@ -350,8 +354,7 @@ func (h *InvokerExecuteTaskHandler) startWorkflows(
 				}
 
 				if isRetryableError(err) {
-					// Apply backoff to start and retry.
-					h.applyBackoff(start, err)
+					h.applyBackoff(start, err, now)
 					result.RetryableStarts = append(result.RetryableStarts, start)
 				} else {
 					// Drop the start from the buffer.
@@ -363,7 +366,6 @@ func (h *InvokerExecuteTaskHandler) startWorkflows(
 
 			metricsWithTag.Counter(metrics.ScheduleActionSuccess.Name()).Record(1)
 			result.CompletedStarts = append(result.CompletedStarts, start)
-			startResults = append(startResults, startResult)
 		})
 	}
 
@@ -372,15 +374,12 @@ func (h *InvokerExecuteTaskHandler) startWorkflows(
 }
 
 func (h *InvokerProcessBufferTaskHandler) Validate(
-	ctx chasm.Context,
+	_ chasm.Context,
 	invoker *Invoker,
 	attrs chasm.TaskAttributes,
 	_ *schedulerpb.InvokerProcessBufferTask,
 ) (bool, error) {
-	return validateTaskHighWaterMark(
-		invoker.GetLastProcessedTime(),
-		attrs.ScheduledTime,
-	)
+	return validateTaskHighWaterMark(invoker.GetLastProcessedTime(), attrs.ScheduledTime)
 }
 
 func (h *InvokerProcessBufferTaskHandler) Execute(
@@ -391,6 +390,8 @@ func (h *InvokerProcessBufferTaskHandler) Execute(
 ) error {
 	scheduler := invoker.Scheduler.Get(ctx)
 
+	invoker.EventLog.Get(ctx).LogEvent(ctx, "processBufferTask executed")
+
 	// Make sure we have something to start.
 	executionInfo := scheduler.Schedule.GetAction().GetStartWorkflow()
 	if executionInfo == nil {
@@ -401,10 +402,25 @@ func (h *InvokerProcessBufferTaskHandler) Execute(
 	result := h.processBuffer(ctx, invoker, scheduler)
 
 	// Update Scheduler metadata.
+	var totalMissedCatchup int64
+	for _, count := range result.missedCatchupByActionRunning {
+		totalMissedCatchup += count
+	}
 	scheduler.recordActionResult(&schedulerActionResult{
 		overlapSkipped:      result.overlapSkipped,
-		missedCatchupWindow: result.missedCatchupWindow,
+		missedCatchupWindow: totalMissedCatchup,
 	})
+	for overlapPolicy, count := range result.overlapSkippedByPolicy {
+		newTaggedMetricsHandler(h.metricsHandler, scheduler).WithTags(
+			metrics.StringTag(metrics.ScheduleOverlapPolicyTag, overlapPolicy.String()),
+		).Counter(metrics.ScheduleOverlapSkipped.Name()).Record(count)
+	}
+	for actionRunning, count := range result.missedCatchupByActionRunning {
+		newTaggedMetricsHandler(h.metricsHandler, scheduler).WithTags(
+			metrics.StringTag(metrics.ScheduleMissedReasonTag, metrics.ScheduleMissedReasonBufferExpired),
+			metrics.StringTag(metrics.ScheduleActionRunningTag, fmt.Sprintf("%t", actionRunning)),
+		).Counter(metrics.ScheduleMissedCatchupWindow.Name()).Record(count)
+	}
 
 	// Update internal state and create new tasks.
 	invoker.recordProcessBufferResult(ctx, &result)
@@ -422,6 +438,7 @@ func (h *InvokerProcessBufferTaskHandler) processBuffer(
 ) (result processBufferResult) {
 	runningWorkflows := invoker.runningWorkflowExecutions()
 	isRunning := len(runningWorkflows) > 0
+	result.missedCatchupByActionRunning = make(map[bool]int64)
 
 	// Processing completely ignores any BufferedStart that's already executing/backing off.
 	pendingBufferedStarts := util.FilterSlice(invoker.GetBufferedStarts(), func(start *schedulespb.BufferedStart) bool {
@@ -446,24 +463,39 @@ func (h *InvokerProcessBufferTaskHandler) processBuffer(
 
 	// Update result metrics.
 	result.overlapSkipped = action.OverlapSkipped
+	result.overlapSkippedByPolicy = action.OverlapSkippedByPolicy
 
-	// Add starting workflows to result, trim others.
+	// Add starting workflows to result, trim others. Catchup-window expiry is
+	// checked before useScheduledAction so that a start past its catchup
+	// window doesn't consume a LimitedActions slot.
 	for _, start := range readyStarts {
-		// Ensure we can take more actions. Manual actions are always allowed.
-		if !start.Manual && !scheduler.useScheduledAction(true) {
-			// Drop buffered automated actions while paused.
+		deadline := h.startWorkflowDeadline(ctx, scheduler, start)
+		if ctx.Now(invoker).After(deadline) {
+			// Action was buffered in time but expired before execution
+			// (e.g., due to overlap deferral, retries, or system delay).
+			// Only emit the metric if the schedule would have run this
+			// start -- skip paused or action-exhausted schedules.
+			if start.Manual || scheduler.useScheduledAction(false) {
+				// Determine if a running action contributed: either one is still
+				// running, or the previous action's CloseTime (stored in DesiredTime)
+				// was already past this start's deadline.
+				// Note: if no prior action completed, DesiredTime is zero-valued,
+				// so After(deadline) is false, correctly yielding actionRunning=false.
+				actionRunning := isRunning ||
+					start.GetDesiredTime().AsTime().After(deadline)
+				result.missedCatchupByActionRunning[actionRunning]++
+			}
 			result.discardStarts = append(result.discardStarts, start)
 			continue
 		}
 
-		if ctx.Now(invoker).After(h.startWorkflowDeadline(ctx, scheduler, start)) {
-			// Drop expired starts.
-			result.missedCatchupWindow++
+		// Ensure we can take more actions. Manual actions are always allowed.
+		if !start.Manual && !scheduler.useScheduledAction(true) {
+			// Drop buffered automated actions while paused or out of actions.
 			result.discardStarts = append(result.discardStarts, start)
 			continue
 		}
 
-		// Append for immediate execution.
 		keepStarts[start.GetRequestId()] = struct{}{}
 		result.startWorkflows = append(result.startWorkflows, start)
 	}
@@ -484,22 +516,22 @@ func (h *InvokerProcessBufferTaskHandler) processBuffer(
 }
 
 // applyBackoff updates start's BackoffTime based on err and the retry policy.
-func (h *InvokerExecuteTaskHandler) applyBackoff(start *schedulespb.BufferedStart, err error) {
+// `now` is the framework clock captured at task start - using time.Now would
+// diverge from the LastProcessedTime/eligibility comparison in tests.
+func (h *InvokerExecuteTaskHandler) applyBackoff(start *schedulespb.BufferedStart, err error, now time.Time) {
 	if err == nil {
 		return
 	}
 
 	var delay time.Duration
 	if rateLimitDelay, ok := isRateLimitedError(err); ok {
-		// If we have the rate limiter's delay, use that.
 		delay = rateLimitDelay
 	} else {
-		// Otherwise, use the backoff policy. Elapsed time is left at 0 because we bound
-		// on number of attempts.
+		// Elapsed time is left at 0 because we bound on number of attempts.
 		delay = h.config.RetryPolicy().ComputeNextDelay(0, int(start.Attempt), nil)
 	}
 
-	start.BackoffTime = timestamppb.New(time.Now().Add(delay))
+	start.BackoffTime = timestamppb.New(now.Add(delay))
 }
 
 // startWorkflowDeadline returns the latest time at which a buffered workflow
@@ -536,21 +568,21 @@ func (h *InvokerExecuteTaskHandler) startWorkflow(
 	start *schedulespb.BufferedStart,
 	lastCompletionState *schedulerpb.LastCompletionResult,
 	callback *commonpb.Callback,
-) (*schedulepb.ScheduleActionResult, error) {
+) error {
 	requestSpec := scheduler.GetSchedule().GetAction().GetStartWorkflow()
 
 	if start.Attempt >= InvokerMaxStartAttempts {
-		return nil, errRetryLimitExceeded
+		return errRetryLimitExceeded
 	}
 
 	// Get rate limiter permission once per buffered start, on the first attempt only.
 	if start.Attempt == 1 {
 		delay, err := h.getRateLimiterPermission()
 		if err != nil {
-			return nil, err
+			return err
 		}
 		if delay > 0 {
-			return nil, newRateLimitedError(delay)
+			return newRateLimitedError(delay)
 		}
 	}
 
@@ -590,7 +622,7 @@ func (h *InvokerExecuteTaskHandler) startWorkflow(
 
 	result, err := h.frontendClient.StartWorkflowExecution(ctx, request)
 	if err != nil {
-		return nil, err
+		return err
 	}
 	actualStartTime := time.Now()
 
@@ -607,17 +639,12 @@ func (h *InvokerExecuteTaskHandler) startWorkflow(
 		metricsHandler.
 			Timer(metrics.ScheduleActionDelay.Name()).
 			Record(actualStartTime.Sub(desiredTime.AsTime()))
+		// Record total delay from original schedule time, including any overlap policy wait.
+		metricsHandler.
+			Timer(metrics.ScheduleActionE2EDelay.Name()).
+			Record(actualStartTime.Sub(start.ActualTime.AsTime()))
 	}
-
-	return &schedulepb.ScheduleActionResult{
-		ScheduleTime: start.ActualTime,
-		ActualTime:   timestamppb.New(actualStartTime),
-		StartWorkflowResult: &commonpb.WorkflowExecution{
-			WorkflowId: start.WorkflowId,
-			RunId:      result.RunId,
-		},
-		StartWorkflowStatus: result.Status, // usually should be RUNNING
-	}, nil
+	return nil
 }
 
 func (h *InvokerExecuteTaskHandler) terminateWorkflow(
diff --git a/chasm/lib/scheduler/library.go b/chasm/lib/scheduler/library.go
index cd575ba8242..e200b52cb12 100644
--- a/chasm/lib/scheduler/library.go
+++ b/chasm/lib/scheduler/library.go
@@ -10,6 +10,7 @@ type (
 	Library struct {
 		chasm.UnimplementedLibrary
 
+		config  *Config
 		handler *handler
 
 		SchedulerIdleTaskHandler        *SchedulerIdleTaskHandler
@@ -29,6 +30,7 @@ func NewNilLibrary() *Library {
 }
 
 func NewLibrary(
+	config *Config,
 	handler *handler,
 	SchedulerIdleTaskHandler *SchedulerIdleTaskHandler,
 	SchedulerCallbacksTaskHandler *SchedulerCallbacksTaskHandler,
@@ -39,6 +41,7 @@ func NewLibrary(
 	MigrateToWorkflowTaskHandler *SchedulerMigrateToWorkflowTaskHandler,
 ) *Library {
 	return &Library{
+		config:                          config,
 		handler:                         handler,
 		SchedulerIdleTaskHandler:        SchedulerIdleTaskHandler,
 		SchedulerCallbacksTaskHandler:   SchedulerCallbacksTaskHandler,
@@ -62,7 +65,11 @@ func (l *Library) Components() []*chasm.RegistrableComponent {
 			chasm.WithSearchAttributes(
 				executionStatusSearchAttribute,
 				scheduleNextActionTimeSearchAttribute,
+				scheduleIdleCloseTimeSearchAttribute,
 			),
+			// Exposes Tweakables to scheduler components via the CHASM context
+			// (see tweakablesFromContext).
+			chasm.WithContextValues(l.config.contextValues()),
 		),
 		chasm.NewRegistrableComponent[*Generator]("generator"),
 		chasm.NewRegistrableComponent[*Invoker]("invoker"),
diff --git a/chasm/lib/scheduler/proto/v1/message.proto b/chasm/lib/scheduler/proto/v1/message.proto
index 8591f802b2a..1663daa23ea 100644
--- a/chasm/lib/scheduler/proto/v1/message.proto
+++ b/chasm/lib/scheduler/proto/v1/message.proto
@@ -36,6 +36,12 @@ message SchedulerState {
   // Set when a migration to workflow-backed scheduler (V1) is pending.
   // Unpause operations are blocked while this is set.
   WorkflowMigrationState workflow_migration = 11;
+
+  // The time at which the idle timer is scheduled to fire and close this
+  // schedule. Set when the schedule goes idle; cleared when it has work again.
+  // Surfaced as the ScheduleIdleCloseTime search attribute for stuck-schedule
+  // detection.
+  google.protobuf.Timestamp idle_close_time = 12;
 }
 
 // WorkflowMigrationState tracks the state of an in-progress V2-to-V1 migration.
diff --git a/chasm/lib/scheduler/scheduler.go b/chasm/lib/scheduler/scheduler.go
index 72bf4150da4..b042b029248 100644
--- a/chasm/lib/scheduler/scheduler.go
+++ b/chasm/lib/scheduler/scheduler.go
@@ -19,6 +19,7 @@ import (
 	"go.temporal.io/server/chasm/lib/scheduler/gen/schedulerpb/v1"
 	"go.temporal.io/server/common"
 	"go.temporal.io/server/common/contextutil"
+	"go.temporal.io/server/common/metrics"
 	"go.temporal.io/server/common/payload"
 	"go.temporal.io/server/common/primitives/timestamp"
 	"go.temporal.io/server/common/util"
@@ -69,10 +70,12 @@ var (
 )
 
 const ScheduleNextActionTimeName = "ScheduleNextActionTime"
+const ScheduleIdleCloseTimeName = "ScheduleIdleCloseTime"
 
 var (
 	executionStatusSearchAttribute        = chasm.NewSearchAttributeKeyword("ExecutionStatus", chasm.SearchAttributeFieldLowCardinalityKeyword01)
 	scheduleNextActionTimeSearchAttribute = chasm.NewSearchAttributeDateTime(ScheduleNextActionTimeName, chasm.SearchAttributeFieldDateTime01)
+	scheduleIdleCloseTimeSearchAttribute  = chasm.NewSearchAttributeDateTime(ScheduleIdleCloseTimeName, chasm.SearchAttributeFieldDateTime02)
 )
 
 var initialSerializedConflictToken = serializeConflictToken(scheduler.InitialConflictToken)
@@ -166,6 +169,7 @@ func NewSentinel(
 			Info:          &schedulepb.ScheduleInfo{},
 		},
 		cacheConflictToken: scheduler.InitialConflictToken,
+		EventLog:           chasm.NewComponentField(ctx, NewEventLog(ctx)),
 	}
 	now := ctx.Now(s)
 	s.Info.CreateTime = timestamppb.New(now)
@@ -463,60 +467,64 @@ func (s *Scheduler) updateConflictToken() {
 // value is used for calculating the retention time (how long an idle schedule
 // lives after becoming idle).
 func (s *Scheduler) getLastEventTime(ctx chasm.Context) time.Time {
-	invoker := s.Invoker.Get(ctx)
-	recentActions := invoker.recentActions()
-
 	latest := util.MaxTime(
 		s.Info.GetCreateTime().AsTime(),
 		s.Info.GetUpdateTime().AsTime(),
 	)
-	if len(recentActions) > 0 {
-		latest = util.MaxTime(latest, recentActions[len(recentActions)-1].GetActualTime().AsTime())
+
+	// The recentActions list is unsorted.
+	for _, a := range s.Invoker.Get(ctx).recentActions() {
+		latest = util.MaxTime(latest, a.GetActualTime().AsTime())
 	}
+
 	return latest
 }
 
-// getIdleExpiration returns an idle close time and the boolean value of 'true'
-// for when a schedule is idle (pending soft delete).
+// isHeldOpen reports whether the schedule must stay open regardless of having
+// no current work:
+// - Paused: customer expects to resume.
+// - Pending backfill: must drain (any pending backfill blocks idle).
+// - Sentinels are exempt .
+//
+// Manual-only schedules (empty spec, but active) idle out like any other -
+// matching V1's RetentionTime semantics, where lastEventTime is advanced by
+// manual triggers via recentActions.
+func (s *Scheduler) isHeldOpen() bool {
+	if s.IsSentinel() {
+		return false
+	}
+	return s.Schedule.GetState().GetPaused() ||
+		s.hasMoreBackfills()
+}
+
+// idleDeadline returns when the schedule becomes eligible to close. Sentinels
+// anchor on CreateTime - they have no Invoker (and thus no recentActions).
+func (s *Scheduler) idleDeadline(ctx chasm.Context, idleTime time.Duration) time.Time {
+	if s.IsSentinel() {
+		return s.Info.GetCreateTime().AsTime().Add(idleTime)
+	}
+	return s.getLastEventTime(ctx).Add(idleTime)
+}
+
+// getIdleExpiration returns an idle close time and whether the schedule is
+// idle. Generator-only: nextWakeup must reflect the spec's next pending wakeup
+// so the predicate can skip arming idle when work remains. Validate calls
+// isHeldOpen and idleDeadline directly.
 func (s *Scheduler) getIdleExpiration(
 	ctx chasm.Context,
 	idleTime time.Duration,
 	nextWakeup time.Time,
 ) (time.Time, bool) {
-	// The idle timer to close off the component is started only for schedules with
-	// no more work to do. Paused schedules are held open indefinitely.
 	if idleTime == 0 ||
-		s.GetSchedule().GetState().GetPaused() ||
-		(!nextWakeup.IsZero() && s.useScheduledAction(false)) ||
-		s.hasMoreAllowAllBackfills(ctx) {
+		s.isHeldOpen() ||
+		(!nextWakeup.IsZero() && s.useScheduledAction(false)) {
 		return time.Time{}, false
 	}
+	return s.idleDeadline(ctx, idleTime), true
+}
 
-	if s.IsSentinel() {
-		return s.Info.GetCreateTime().AsTime().Add(idleTime), true
-	}
-	return s.getLastEventTime(ctx).Add(idleTime), true
-}
-
-func (s *Scheduler) hasMoreAllowAllBackfills(ctx chasm.Context) bool {
-	for _, field := range s.Backfillers {
-		backfiller := field.Get(ctx)
-		var policy enumspb.ScheduleOverlapPolicy
-		switch request := backfiller.GetRequest().(type) {
-		case *schedulerpb.BackfillerState_BackfillRequest:
-			policy = request.BackfillRequest.OverlapPolicy
-		case *schedulerpb.BackfillerState_TriggerRequest:
-			policy = request.TriggerRequest.OverlapPolicy
-		default:
-			return false
-		}
-
-		if enumspb.SCHEDULE_OVERLAP_POLICY_ALLOW_ALL == s.resolveOverlapPolicy(policy) {
-			return true
-		}
-	}
-
-	return false
+func (s *Scheduler) hasMoreBackfills() bool {
+	return len(s.Backfillers) > 0
 }
 
 type schedulerActionResult struct {
@@ -561,6 +569,16 @@ func (s *Scheduler) HandleNexusCompletion(
 		return nil
 	}
 
+	// Record how long it took for the callback to arrive after the action completed.
+	// Use ctx.Now instead of time.Since to use a consistent time source across nodes,
+	// and clamp to zero in case of clock skew.
+	if closeTime := info.GetCloseTime().AsTime(); !closeTime.IsZero() {
+		latency := max(0, ctx.Now(s).Sub(closeTime))
+		newTaggedMetricsHandler(ctx.MetricsHandler(), s).
+			Timer(metrics.ScheduleCallbackLatency.Name()).
+			Record(latency)
+	}
+
 	// Handle last completed/failed status and payloads.
 	//
 	// TODO - also record payload sizes once we have metrics wired into CHASM context.
@@ -781,6 +799,8 @@ func (s *Scheduler) MigrateToWorkflow(
 	s.Schedule.State.Paused = true
 	s.Schedule.State.Notes = "paused for migration to workflow-backed scheduler"
 
+	s.EventLog.Get(ctx).LogEvent(ctx, "started migration to V1")
+
 	// Schedule a side-effect task to export state and start the V1 workflow.
 	ctx.AddTask(s, chasm.TaskAttributes{}, &schedulerpb.SchedulerMigrateToWorkflowTask{})
 
@@ -837,10 +857,10 @@ func (s *Scheduler) Update(
 
 	s.Info.UpdateTime = timestamppb.New(ctx.Now(s))
 	s.updateConflictToken()
+	s.EventLog.Get(ctx).LogEvent(ctx, "updated via API")
 
 	// Since the spec may have been updated, kick off the generator.
-	generator := s.Generator.Get(ctx)
-	generator.Generate(ctx)
+	s.Generator.Get(ctx).Generate(ctx)
 
 	return &schedulerpb.UpdateScheduleResponse{
 		FrontendResponse: &workflowservice.UpdateScheduleResponse{},
@@ -864,6 +884,7 @@ func (s *Scheduler) Patch(
 	if req.FrontendRequest.Patch.Pause != "" {
 		s.Schedule.State.Paused = true
 		s.Schedule.State.Notes = req.FrontendRequest.Patch.Pause
+		s.EventLog.Get(ctx).LogEvent(ctx, fmt.Sprintf("paused via API: %s", req.FrontendRequest.Patch.Pause))
 	}
 	if req.FrontendRequest.Patch.Unpause != "" {
 		if s.WorkflowMigration != nil {
@@ -871,7 +892,7 @@ func (s *Scheduler) Patch(
 		}
 		s.Schedule.State.Paused = false
 		s.Schedule.State.Notes = req.FrontendRequest.Patch.Unpause
-		s.Generator.Get(ctx).Generate(ctx)
+		s.EventLog.Get(ctx).LogEvent(ctx, fmt.Sprintf("unpaused via API: %s", req.FrontendRequest.Patch.Unpause))
 	}
 
 	if err := s.handlePatch(ctx, req.FrontendRequest.Patch); err != nil {
@@ -881,6 +902,9 @@ func (s *Scheduler) Patch(
 	s.Info.UpdateTime = timestamppb.New(ctx.Now(s))
 	s.updateConflictToken()
 
+	// Since the paused state may have been updated, kick off the generator.
+	s.Generator.Get(ctx).Generate(ctx)
+
 	return &schedulerpb.PatchScheduleResponse{
 		FrontendResponse: &workflowservice.PatchScheduleResponse{},
 	}, nil
@@ -922,6 +946,9 @@ func (s *Scheduler) SearchAttributes(ctx chasm.Context) []chasm.SearchAttributeK
 		if gen := s.Generator.Get(ctx); len(gen.FutureActionTimes) > 0 {
 			out = append(out, scheduleNextActionTimeSearchAttribute.Value(gen.FutureActionTimes[0].AsTime()))
 		}
+		if s.IdleCloseTime != nil {
+			out = append(out, scheduleIdleCloseTimeSearchAttribute.Value(s.IdleCloseTime.AsTime()))
+		}
 	}
 	return out
 }
diff --git a/chasm/lib/scheduler/scheduler_idle_tasks_test.go b/chasm/lib/scheduler/scheduler_idle_tasks_test.go
index 075e838b719..b63fb71d8a5 100644
--- a/chasm/lib/scheduler/scheduler_idle_tasks_test.go
+++ b/chasm/lib/scheduler/scheduler_idle_tasks_test.go
@@ -8,18 +8,33 @@ import (
 	"go.temporal.io/server/chasm"
 	"go.temporal.io/server/chasm/lib/scheduler"
 	"go.temporal.io/server/chasm/lib/scheduler/gen/schedulerpb/v1"
+	"go.temporal.io/server/common/log"
+	"go.temporal.io/server/common/metrics"
 	"google.golang.org/protobuf/types/known/durationpb"
 	"google.golang.org/protobuf/types/known/timestamppb"
 )
 
+func newIdleHandler(idleTime time.Duration) *scheduler.SchedulerIdleTaskHandler {
+	return scheduler.NewSchedulerIdleTaskHandler(scheduler.SchedulerIdleTaskHandlerOptions{
+		Config: &scheduler.Config{
+			Tweakables: func(_ string) scheduler.Tweakables {
+				t := scheduler.DefaultTweakables
+				t.IdleTime = idleTime
+				return t
+			},
+		},
+		MetricsHandler: metrics.NoopMetricsHandler,
+		BaseLogger:     log.NewTestLogger(),
+	})
+}
+
 type idleValidateTestCase struct {
-	configIdleTime           time.Duration
-	taskIdleTimeTotal        time.Duration
-	scheduledTime            time.Time
-	schedulerClosed          bool
-	idleMatchesScheduledTime bool
-	setupScheduler           func(*scheduler.Scheduler, chasm.Context)
-	expectedValid            bool
+	configIdleTime    time.Duration
+	taskIdleTimeTotal time.Duration
+	scheduledTime     time.Time
+	schedulerClosed   bool
+	setupScheduler    func(*scheduler.Scheduler, chasm.Context)
+	expectedValid     bool
 }
 
 func runIdleValidateTestCase(t *testing.T, env *testEnv, c *idleValidateTestCase) {
@@ -27,60 +42,36 @@ func runIdleValidateTestCase(t *testing.T, env *testEnv, c *idleValidateTestCase
 	sched := env.Scheduler
 
 	sched.Closed = c.schedulerClosed
-
 	if c.setupScheduler != nil {
 		c.setupScheduler(sched, ctx)
 	}
 
-	config := &scheduler.Config{
-		Tweakables: func(_ string) scheduler.Tweakables {
-			tweakables := scheduler.DefaultTweakables
-			tweakables.IdleTime = c.configIdleTime
-			return tweakables
-		},
-	}
-
-	handler := scheduler.NewSchedulerIdleTaskHandler(scheduler.SchedulerIdleTaskHandlerOptions{
-		Config: config,
-	})
-
-	task := &schedulerpb.SchedulerIdleTask{
-		IdleTimeTotal: durationpb.New(c.taskIdleTimeTotal),
-	}
-
-	scheduledTime := c.scheduledTime
-	if c.idleMatchesScheduledTime {
-		lastEventTime := scheduledTime.Add(-c.configIdleTime)
-		sched.Info.UpdateTime = timestamppb.New(lastEventTime)
-		sched.Info.CreateTime = timestamppb.New(lastEventTime)
-	}
-
-	taskAttrs := chasm.TaskAttributes{
-		ScheduledTime: scheduledTime,
-	}
+	handler := newIdleHandler(c.configIdleTime)
+	task := &schedulerpb.SchedulerIdleTask{IdleTimeTotal: durationpb.New(c.taskIdleTimeTotal)}
+	taskAttrs := chasm.TaskAttributes{ScheduledTime: c.scheduledTime}
 
 	isValid, err := handler.Validate(ctx, sched, taskAttrs, task)
 	require.NoError(t, err)
 	require.Equal(t, c.expectedValid, isValid)
 }
 
+// anchorLastEventTo backdates Info.UpdateTime/CreateTime so that
+// idleDeadline = anchor + idleTime; pairs with scheduledTime = anchor + idleTime
+// to make Validate's expiration check resolve to "stable".
+func anchorLastEventTo(sched *scheduler.Scheduler, anchor time.Time) {
+	sched.Info.UpdateTime = timestamppb.New(anchor)
+	sched.Info.CreateTime = timestamppb.New(anchor)
+}
+
 func TestIdleTask_Execute(t *testing.T) {
 	env := newTestEnv(t)
 	ctx := env.MutableContext()
 	sched := env.Scheduler
 
-	handler := scheduler.NewSchedulerIdleTaskHandler(scheduler.SchedulerIdleTaskHandlerOptions{
-		Config: defaultConfig(),
-	})
-
-	// Verify scheduler starts open.
+	handler := newIdleHandler(10 * time.Minute)
 	require.False(t, sched.Closed)
-
-	// Execute the idle task.
 	err := handler.Execute(ctx, sched, chasm.TaskAttributes{}, &schedulerpb.SchedulerIdleTask{})
 	require.NoError(t, err)
-
-	// Verify scheduler is now closed.
 	require.True(t, sched.Closed)
 }
 
@@ -91,8 +82,7 @@ func TestIdleTask_Validate_SchedulerNotIdle(t *testing.T) {
 		configIdleTime:    10 * time.Minute,
 		taskIdleTimeTotal: 10 * time.Minute,
 		scheduledTime:     now,
-		setupScheduler: func(sched *scheduler.Scheduler, ctx chasm.Context) {
-			// Make scheduler not idle by setting it as paused.
+		setupScheduler: func(sched *scheduler.Scheduler, _ chasm.Context) {
 			sched.Schedule.State.Paused = true
 		},
 		expectedValid: false,
@@ -103,11 +93,13 @@ func TestIdleTask_Validate_ValidIdleTask(t *testing.T) {
 	env := newTestEnv(t)
 	now := env.TimeSource.Now()
 	runIdleValidateTestCase(t, env, &idleValidateTestCase{
-		configIdleTime:           10 * time.Minute,
-		taskIdleTimeTotal:        10 * time.Minute,
-		scheduledTime:            now,
-		idleMatchesScheduledTime: true,
-		expectedValid:            true,
+		configIdleTime:    10 * time.Minute,
+		taskIdleTimeTotal: 10 * time.Minute,
+		scheduledTime:     now,
+		setupScheduler: func(sched *scheduler.Scheduler, _ chasm.Context) {
+			anchorLastEventTo(sched, now.Add(-10*time.Minute))
+		},
+		expectedValid: true,
 	})
 }
 
@@ -115,11 +107,13 @@ func TestIdleTask_Validate_SchedulerAlreadyClosed(t *testing.T) {
 	env := newTestEnv(t)
 	now := env.TimeSource.Now()
 	runIdleValidateTestCase(t, env, &idleValidateTestCase{
-		configIdleTime:           10 * time.Minute,
-		taskIdleTimeTotal:        10 * time.Minute,
-		scheduledTime:            now,
-		schedulerClosed:          true,
-		idleMatchesScheduledTime: true,
-		expectedValid:            false, // Should return !scheduler.Closed (false when closed).
+		configIdleTime:    10 * time.Minute,
+		taskIdleTimeTotal: 10 * time.Minute,
+		scheduledTime:     now,
+		schedulerClosed:   true,
+		setupScheduler: func(sched *scheduler.Scheduler, _ chasm.Context) {
+			anchorLastEventTo(sched, now.Add(-10*time.Minute))
+		},
+		expectedValid: false,
 	})
 }
diff --git a/chasm/lib/scheduler/scheduler_tasks.go b/chasm/lib/scheduler/scheduler_tasks.go
index 69dcbc1676e..e8688f231de 100644
--- a/chasm/lib/scheduler/scheduler_tasks.go
+++ b/chasm/lib/scheduler/scheduler_tasks.go
@@ -4,7 +4,6 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"time"
 
 	commonpb "go.temporal.io/api/common/v1"
 	enumspb "go.temporal.io/api/enums/v1"
@@ -16,6 +15,9 @@ import (
 	"go.temporal.io/server/chasm"
 	"go.temporal.io/server/chasm/lib/scheduler/gen/schedulerpb/v1"
 	"go.temporal.io/server/common"
+	"go.temporal.io/server/common/log"
+	"go.temporal.io/server/common/log/tag"
+	"go.temporal.io/server/common/metrics"
 	"go.temporal.io/server/common/resource"
 	"go.uber.org/fx"
 	"google.golang.org/protobuf/types/known/timestamppb"
@@ -24,17 +26,23 @@ import (
 type SchedulerIdleTaskHandlerOptions struct {
 	fx.In
 
-	Config *Config
+	Config         *Config
+	MetricsHandler metrics.Handler
+	BaseLogger     log.Logger
 }
 
 type SchedulerIdleTaskHandler struct {
 	chasm.PureTaskHandlerBase
-	config *Config
+	config         *Config
+	metricsHandler metrics.Handler
+	baseLogger     log.Logger
 }
 
 func NewSchedulerIdleTaskHandler(opts SchedulerIdleTaskHandlerOptions) *SchedulerIdleTaskHandler {
 	return &SchedulerIdleTaskHandler{
-		config: opts.Config,
+		config:         opts.Config,
+		metricsHandler: opts.MetricsHandler,
+		baseLogger:     opts.BaseLogger,
 	}
 }
 
@@ -44,34 +52,45 @@ func (r *SchedulerIdleTaskHandler) Execute(
 	_ chasm.TaskAttributes,
 	_ *schedulerpb.SchedulerIdleTask,
 ) error {
+	scheduler.EventLog.Get(ctx).LogEvent(ctx, "schedule closed from idle timer")
 	scheduler.Closed = true
 	return nil
 }
 
-// Validate returns true when all three conditions hold:
-//  1. The schedule is still idle — no new actions have been queued since this
-//     task was created (getIdleExpiration returns isIdle=true).
-//  2. The idle expiration has not shifted — getIdleExpiration recomputes
-//     getLastEventTime()+idleTimeTotal and it must equal the task's scheduled
-//     time. If a workflow start has advanced getLastEventTime() since the task
-//     was written, the expiration no longer matches and the task is dropped.
-//  3. The scheduler is not already closed (idempotency guard).
+// Validate returns true (fire) when the schedule should still close. False
+// drops for: already-closed (idempotency), held-open by state, or a deadline
+// that shifted later than ScheduledTime (the Generator will have re-armed at
+// the new deadline). It deliberately does not re-derive "is the spec
+// exhausted" - that's an arm-time concern.
 func (r *SchedulerIdleTaskHandler) Validate(
 	ctx chasm.Context,
 	scheduler *Scheduler,
 	taskAttrs chasm.TaskAttributes,
 	task *schedulerpb.SchedulerIdleTask,
 ) (bool, error) {
-	idleTimeTotal := task.IdleTimeTotal.AsDuration()
-	idleExpiration, isIdle := scheduler.getIdleExpiration(ctx, idleTimeTotal, time.Time{})
+	if scheduler.Closed {
+		return false, nil
+	}
+	if scheduler.isHeldOpen() {
+		return false, nil
+	}
 
-	// If the scheduler has since woken up, or its idle expiration time changed, this
-	// task must be obsolete.
-	if !isIdle || idleExpiration.Compare(taskAttrs.ScheduledTime) != 0 {
+	// Use After (not strict equality) so sub-precision drift doesn't drop tasks
+	// that should still fire.
+	idleExpiration := scheduler.idleDeadline(ctx, task.IdleTimeTotal.AsDuration())
+	if idleExpiration.After(taskAttrs.ScheduledTime) {
 		return false, nil
 	}
 
-	return !scheduler.Closed, nil
+	// Deadline moved earlier - shouldn't happen if getLastEventTime is monotonic.
+	// Fire (closing the schedule is the safe call) but log so a real regression
+	// surfaces.
+	if idleExpiration.Before(taskAttrs.ScheduledTime) {
+		newTaggedLogger(r.baseLogger, scheduler).Warn("idle deadline regressed",
+			tag.Timestamp(idleExpiration),
+			tag.NewTimeTag("scheduled-time", taskAttrs.ScheduledTime))
+	}
+	return true, nil
 }
 
 type SchedulerCallbacksTaskHandlerOptions struct {
@@ -171,6 +190,9 @@ func (r *SchedulerCallbacksTaskHandler) Execute(
 				}
 			}
 
+			s.EventLog.Get(ctx).LogEvent(ctx,
+				fmt.Sprintf("attached callbacks to %d already-running workflow(s)", len(results)))
+
 			// Now that running workflow state has been refreshed, scheduler tasks can be
 			// fired.
 			invoker.addTasks(ctx)
diff --git a/chasm/lib/scheduler/scheduler_test.go b/chasm/lib/scheduler/scheduler_test.go
index 811da88df7c..a2e17a43c38 100644
--- a/chasm/lib/scheduler/scheduler_test.go
+++ b/chasm/lib/scheduler/scheduler_test.go
@@ -412,6 +412,51 @@ func TestSearchAttributes_NextActionTime(t *testing.T) {
 	})
 }
 
+func TestSearchAttributes_IdleCloseTime(t *testing.T) {
+
+	t.Run("idle schedule emits IdleCloseTime", func(t *testing.T) {
+		sched, ctx, _ := setupSchedulerForTest(t)
+
+		closeTime := time.Now().Add(7 * 24 * time.Hour).UTC().Truncate(time.Second)
+		sched.IdleCloseTime = timestamppb.New(closeTime)
+
+		sas := sched.SearchAttributes(ctx)
+		val, ok := findSearchAttribute(t, sas, scheduler.ScheduleIdleCloseTimeName)
+		require.True(t, ok, "expected %s to be present", scheduler.ScheduleIdleCloseTimeName)
+		require.True(t, closeTime.Equal(val.(time.Time)), "want %v, got %v", closeTime, val)
+	})
+
+	t.Run("schedule with no idle deadline does not emit", func(t *testing.T) {
+		sched, ctx, _ := setupSchedulerForTest(t)
+
+		sched.IdleCloseTime = nil
+
+		sas := sched.SearchAttributes(ctx)
+		_, has := findSearchAttribute(t, sas, scheduler.ScheduleIdleCloseTimeName)
+		require.False(t, has, "expected %s to be absent when not idle", scheduler.ScheduleIdleCloseTimeName)
+	})
+
+	t.Run("closed does not emit IdleCloseTime", func(t *testing.T) {
+		sched, ctx, _ := setupSchedulerForTest(t)
+
+		sched.Closed = true
+		// A deadline left over from before close must not leak through.
+		sched.IdleCloseTime = timestamppb.New(time.Now().Add(7 * 24 * time.Hour))
+
+		sas := sched.SearchAttributes(ctx)
+		_, has := findSearchAttribute(t, sas, scheduler.ScheduleIdleCloseTimeName)
+		require.False(t, has, "expected %s to be absent once closed", scheduler.ScheduleIdleCloseTimeName)
+	})
+
+	t.Run("sentinel does not emit", func(t *testing.T) {
+		sentinel, ctx, _ := setupSentinelForTest(t)
+
+		sas := sentinel.SearchAttributes(ctx)
+		_, has := findSearchAttribute(t, sas, scheduler.ScheduleIdleCloseTimeName)
+		require.False(t, has)
+	})
+}
+
 func findSearchAttribute(t *testing.T, sas []chasm.SearchAttributeKeyValue, alias string) (any, bool) {
 	t.Helper()
 	for _, sa := range sas {
@@ -435,8 +480,9 @@ func TestSearchAttributes_RoundTripThroughCloseTransaction(t *testing.T) {
 	nextAction := time.Now().Add(time.Hour).UTC().Truncate(time.Second)
 	generator := sched.Generator.Get(ctx)
 	generator.FutureActionTimes = []*timestamppb.Timestamp{timestamppb.New(nextAction)}
+	sched.IdleCloseTime = timestamppb.New(time.Now().Add(7 * 24 * time.Hour))
 
 	require.NoError(t, node.SetRootComponent(sched))
 	_, err := node.CloseTransaction()
-	require.NoError(t, err, "CloseTransaction should accept TemporalScheduleNextActionTime")
+	require.NoError(t, err, "CloseTransaction should accept the scheduler search attributes")
 }
diff --git a/chasm/lib/scheduler/sentinel_test.go b/chasm/lib/scheduler/sentinel_test.go
index b5e31d77000..c00fd034fe3 100644
--- a/chasm/lib/scheduler/sentinel_test.go
+++ b/chasm/lib/scheduler/sentinel_test.go
@@ -2,7 +2,6 @@ package scheduler_test
 
 import (
 	"testing"
-	"time"
 
 	"github.com/stretchr/testify/require"
 	"go.temporal.io/server/chasm"
@@ -18,8 +17,7 @@ func TestNewSentinel(t *testing.T) {
 	require.NotNil(t, sentinel.Info.GetCreateTime())
 	require.False(t, sentinel.Info.CreateTime.AsTime().IsZero())
 
-	// Sentinels should have no Visibility component, which prevents them from
-	// appearing in ListSchedules results.
+	// Sentinels have no Visibility component, which keeps them out of ListSchedules.
 	_, ok := sentinel.Visibility.TryGet(ctx)
 	require.False(t, ok)
 }
@@ -27,13 +25,8 @@ func TestNewSentinel(t *testing.T) {
 func TestSentinelIdleTask_Validate_Valid(t *testing.T) {
 	sentinel, ctx, _ := setupSentinelForTest(t)
 
-	executor := scheduler.NewSchedulerIdleTaskHandler(scheduler.SchedulerIdleTaskHandlerOptions{
-		Config: defaultConfig(),
-	})
-
-	task := &schedulerpb.SchedulerIdleTask{
-		IdleTimeTotal: durationpb.New(scheduler.SentinelIdleTime),
-	}
+	executor := newIdleHandler(scheduler.SentinelIdleTime)
+	task := &schedulerpb.SchedulerIdleTask{IdleTimeTotal: durationpb.New(scheduler.SentinelIdleTime)}
 	taskAttrs := chasm.TaskAttributes{
 		ScheduledTime: sentinel.Info.CreateTime.AsTime().Add(scheduler.SentinelIdleTime),
 	}
@@ -47,13 +40,8 @@ func TestSentinelIdleTask_Validate_InvalidAfterClosed(t *testing.T) {
 	sentinel, ctx, _ := setupSentinelForTest(t)
 	sentinel.Closed = true
 
-	executor := scheduler.NewSchedulerIdleTaskHandler(scheduler.SchedulerIdleTaskHandlerOptions{
-		Config: defaultConfig(),
-	})
-
-	task := &schedulerpb.SchedulerIdleTask{
-		IdleTimeTotal: durationpb.New(scheduler.SentinelIdleTime),
-	}
+	executor := newIdleHandler(scheduler.SentinelIdleTime)
+	task := &schedulerpb.SchedulerIdleTask{IdleTimeTotal: durationpb.New(scheduler.SentinelIdleTime)}
 	taskAttrs := chasm.TaskAttributes{
 		ScheduledTime: sentinel.Info.CreateTime.AsTime().Add(scheduler.SentinelIdleTime),
 	}
@@ -63,18 +51,15 @@ func TestSentinelIdleTask_Validate_InvalidAfterClosed(t *testing.T) {
 	require.False(t, isValid)
 }
 
-func TestSentinelIdleTask_Validate_MismatchedScheduledTime(t *testing.T) {
+// Task armed before the natural sentinel deadline must drop: idleDeadline
+// (CreateTime + SentinelIdleTime) is after ScheduledTime (CreateTime).
+func TestSentinelIdleTask_Validate_ExpirationShiftedLater(t *testing.T) {
 	sentinel, ctx, _ := setupSentinelForTest(t)
 
-	executor := scheduler.NewSchedulerIdleTaskHandler(scheduler.SchedulerIdleTaskHandlerOptions{
-		Config: defaultConfig(),
-	})
-
-	task := &schedulerpb.SchedulerIdleTask{
-		IdleTimeTotal: durationpb.New(scheduler.SentinelIdleTime),
-	}
+	executor := newIdleHandler(scheduler.SentinelIdleTime)
+	task := &schedulerpb.SchedulerIdleTask{IdleTimeTotal: durationpb.New(scheduler.SentinelIdleTime)}
 	taskAttrs := chasm.TaskAttributes{
-		ScheduledTime: sentinel.Info.CreateTime.AsTime().Add(99 * time.Hour),
+		ScheduledTime: sentinel.Info.CreateTime.AsTime(),
 	}
 
 	isValid, err := executor.Validate(ctx, sentinel, taskAttrs, task)
@@ -85,10 +70,7 @@ func TestSentinelIdleTask_Validate_MismatchedScheduledTime(t *testing.T) {
 func TestSentinelIdleTask_Execute(t *testing.T) {
 	sentinel, ctx, _ := setupSentinelForTest(t)
 
-	executor := scheduler.NewSchedulerIdleTaskHandler(scheduler.SchedulerIdleTaskHandlerOptions{
-		Config: defaultConfig(),
-	})
-
+	executor := newIdleHandler(scheduler.SentinelIdleTime)
 	require.False(t, sentinel.Closed)
 	err := executor.Execute(ctx, sentinel, chasm.TaskAttributes{}, &schedulerpb.SchedulerIdleTask{})
 	require.NoError(t, err)
diff --git a/chasm/lib/scheduler/spec_processor.go b/chasm/lib/scheduler/spec_processor.go
index 61585781b80..f3b60d48e11 100644
--- a/chasm/lib/scheduler/spec_processor.go
+++ b/chasm/lib/scheduler/spec_processor.go
@@ -123,6 +123,7 @@ func (s *SpecProcessorImpl) ProcessTimeRange(
 	var err error
 	var bufferedStarts []*schedulespb.BufferedStart
 	var droppedCount int64
+	recordedGenerateLatency := false
 	limitReached := false
 	for next, err = s.NextTime(scheduler, start); err == nil && (!next.Next.IsZero() && !next.Next.After(end)); next, err = s.NextTime(scheduler, next.Next) {
 		lastAction = next.Next
@@ -138,21 +139,28 @@ func (s *SpecProcessorImpl) ProcessTimeRange(
 			continue
 		}
 
+		// Record generate latency only for the first action in the batch to
+		// avoid inflating the metric when catching up over a large time range.
+		if !manual && !recordedGenerateLatency {
+			metricsHandler.Timer(metrics.ScheduleGenerateLatency.Name()).
+				Record(end.Sub(next.Next))
+			recordedGenerateLatency = true
+		}
+
 		if !manual && end.Sub(next.Next) > catchupWindow {
 			s.logger.Info("Schedule missed catchup window",
 				tag.Time("now", end),
 				tag.Time("time", next.Next))
-			metricsHandler.Counter(metrics.ScheduleMissedCatchupWindow.Name()).Record(1)
+			// Action's nominal time was already past the catchup window when
+			// the generator processed the time range. It was never buffered.
+			metricsHandler.WithTags(
+				metrics.StringTag(metrics.ScheduleMissedReasonTag, metrics.ScheduleMissedReasonNotBuffered),
+			).Counter(metrics.ScheduleMissedCatchupWindow.Name()).Record(1)
 
 			scheduler.Info.MissedCatchupWindow++
 			continue
 		}
 
-		if !manual {
-			metricsHandler.Timer(metrics.ScheduleGenerateLatency.Name()).
-				Record(end.Sub(next.Next))
-		}
-
 		if limitReached {
 			droppedCount++
 			continue
diff --git a/common/dynamicconfig/constants.go b/common/dynamicconfig/constants.go
index ae75bb80e84..523c80f401e 100644
--- a/common/dynamicconfig/constants.go
+++ b/common/dynamicconfig/constants.go
@@ -604,6 +604,51 @@ is currently processing a task.
 		`BuildIdScavengerVisibilityRPS is the rate limit for visibility calls from the build id scavenger`,
 	)
 
+	ScheduleInvariantsScannerOverdueNextActionTimeEnabled = NewGlobalBoolSetting(
+		"worker.scheduleInvariantsScannerOverdueNextActionTimeEnabled",
+		false,
+		`ScheduleInvariantsScannerOverdueNextActionTimeEnabled enables flagging schedules whose
+TemporalScheduleNextActionTime lies further in the past than the tolerance.`,
+	)
+	ScheduleInvariantsScannerStuckOpenEnabled = NewGlobalBoolSetting(
+		"worker.scheduleInvariantsScannerStuckOpenEnabled",
+		false,
+		`ScheduleInvariantsScannerStuckOpenEnabled enables flagging schedules that appear stuck open
+long after their CloseTime.`,
+	)
+	ScheduleInvariantsScannerUnknownStateEnabled = NewGlobalBoolSetting(
+		"worker.scheduleInvariantsScannerUnknownStateEnabled",
+		false,
+		`ScheduleInvariantsScannerUnknownStateEnabled enables flagging running, unpaused schedules with
+no TemporalScheduleNextActionTime. Ship disabled until TemporalScheduleNextActionTime is known to be
+backfilled on legacy schedules.`,
+	)
+	ScheduleInvariantsScannerOverdueNextActionTimeTolerance = NewGlobalDurationSetting(
+		"worker.scheduleInvariantsScannerOverdueNextActionTimeTolerance",
+		10*time.Minute,
+		`ScheduleInvariantsScannerOverdueNextActionTimeTolerance is how far in the past
+TemporalScheduleNextActionTime must be before the schedule is flagged.`,
+	)
+	ScheduleInvariantsScannerVisibilityRPS = NewGlobalFloatSetting(
+		"worker.scheduleInvariantsScannerVisibilityRPS",
+		1.0,
+		`ScheduleInvariantsScannerVisibilityRPS rate-limits visibility calls from the
+schedule-invariants scanner.`,
+	)
+	ScheduleInvariantsScannerScanInterval = NewGlobalDurationSetting(
+		"worker.scheduleInvariantsScannerScanInterval",
+		15*time.Minute,
+		`ScheduleInvariantsScannerScanInterval is how often each schedule-invariants scanner
+activity kicks off a fresh scan pass.`,
+	)
+	ScheduleInvariantsScannerStuckOpenIdleTimeBufferMultiplier = NewGlobalIntSetting(
+		"worker.scheduleInvariantsScannerStuckOpenIdleTimeBufferMultiplier",
+		2,
+		`ScheduleInvariantsScannerStuckOpenIdleTimeBufferMultiplier multiplies the configured
+schedule IdleTime to set how far past a schedule's idle-close deadline it must be before the
+stuck-open scanner flags it.`,
+	)
+
 	// keys for frontend
 	FrontendAllowedExperiments = NewNamespaceTypedSetting(
 		"frontend.allowedExperiments",
@@ -2945,6 +2990,14 @@ Requires service restart to take effect.`,
 instead of the existing (V1) implementation.`,
 	)
 
+	CHASMSchedulerCreationRolloutPercent = NewNamespaceIntSetting(
+		"history.chasmSchedulerCreationRolloutPercent",
+		0,
+		`CHASMSchedulerCreationRolloutPercent is the per-namespace percentage of new schedules that will be
+created on the CHASM (V2) implementation. This setting is only consulted when EnableCHASMSchedulerCreation is true and
+is re-evaluated on every CreateSchedule RPC.`,
+	)
+
 	EnableCHASMSchedulerRouting = NewNamespaceBoolSetting(
 		"history.enableCHASMSchedulerRouting",
 		true,
@@ -2959,6 +3012,23 @@ first (with fallback to V1), excluding CreateSchedule.`,
 to the CHASM (V2) implementation on active scheduler workflows.`,
 	)
 
+	CHASMSchedulerMigrationRolloutPercent = NewNamespaceIntSetting(
+		"history.chasmSchedulerMigrationRolloutPercent",
+		0,
+		`CHASMSchedulerMigrationRolloutPercent is the per-namespace percentage of V1 schedules that will be
+migrated to the CHASM (V2) implementation This setting is only consulted when
+EnableCHASMSchedulerMigration is true. The decision is re-evaluated when a
+scheduler workflow starts or continues-as-new.`,
+	)
+
+	EnableCHASMSchedulerMigrationWithRunningWorkflows = NewNamespaceBoolSetting(
+		"history.enableCHASMSchedulerMigrationWithRunningWorkflows",
+		false,
+		`EnableCHASMSchedulerMigrationWithRunningWorkflows, when set to false, prevents schedules with
+running workflows from being migrated. This works around a known bug in 3P SDKs involving updating
+existing workflows to attach callbacks.`,
+	)
+
 	EnableCHASMSchedulerSentinels = NewNamespaceBoolSetting(
 		"history.enableCHASMSchedulerSentinels",
 		true,
diff --git a/common/dynamicconfig/rollout.go b/common/dynamicconfig/rollout.go
new file mode 100644
index 00000000000..09f8241de40
--- /dev/null
+++ b/common/dynamicconfig/rollout.go
@@ -0,0 +1,27 @@
+package dynamicconfig
+
+import (
+	"math"
+
+	"github.com/dgryski/go-farm"
+)
+
+// RolloutAccepts reports whether key falls within the given percentage [0,100]
+// using a stable hash, so dialing the percent up is monotonic: a key accepted
+// at percent P is accepted at every percent >= P.
+//
+// The hash algorithm and the bytes of key are a load-bearing rollout key:
+// changing either re-shuffles every cohort and breaks monotonicity for
+// in-flight rollouts. Callers are responsible for constructing key (including
+// any separator bytes between fields) and must not change that construction
+// once a rollout is in progress.
+func RolloutAccepts(key []byte, percent int) bool {
+	if percent >= 100 {
+		return true
+	}
+	if percent <= 0 {
+		return false
+	}
+	threshold := uint32(float64(percent) / 100.0 * float64(math.MaxUint32))
+	return farm.Fingerprint32(key) < threshold
+}
diff --git a/common/dynamicconfig/rollout_test.go b/common/dynamicconfig/rollout_test.go
new file mode 100644
index 00000000000..b34c3dcc6d1
--- /dev/null
+++ b/common/dynamicconfig/rollout_test.go
@@ -0,0 +1,41 @@
+package dynamicconfig
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func rolloutKey(namespace, id string) []byte {
+	return fmt.Appendf(nil, "%s\x00%s", namespace, id)
+}
+
+func TestRolloutAccepts_Boundaries(t *testing.T) {
+	t.Parallel()
+
+	key := rolloutKey("ns", "id")
+	require.True(t, RolloutAccepts(key, 100))
+	require.True(t, RolloutAccepts(key, 101)) // clamped
+	require.False(t, RolloutAccepts(key, 0))
+	require.False(t, RolloutAccepts(key, -5)) // clamped
+}
+
+func TestRolloutAccepts_Monotonic(t *testing.T) {
+	t.Parallel()
+
+	// An ID accepted at percent P must also be accepted at every percent >= P.
+	for i := range 500 {
+		key := rolloutKey("ns", fmt.Sprintf("id-%d", i))
+		firstAcceptedAt := -1
+		for pct := 0; pct <= 100; pct++ {
+			if RolloutAccepts(key, pct) {
+				if firstAcceptedAt < 0 {
+					firstAcceptedAt = pct
+				}
+			} else if firstAcceptedAt >= 0 {
+				t.Fatalf("id %d accepted at pct=%d but rejected at pct=%d", i, firstAcceptedAt, pct)
+			}
+		}
+	}
+}
diff --git a/common/metrics/metric_defs.go b/common/metrics/metric_defs.go
index b04c64cae23..dda31d0ed24 100644
--- a/common/metrics/metric_defs.go
+++ b/common/metrics/metric_defs.go
@@ -506,6 +506,8 @@ const (
 	VisibilityArchiverScope = "VisibilityArchiver"
 	// HistoryScavengerScope is scope used by all metrics emitted by worker.history.Scavenger module
 	HistoryScavengerScope = "HistoryScavenger"
+	// ScheduleInvariantsScannerScope is scope used by metrics emitted by the schedule-invariants scanner
+	ScheduleInvariantsScannerScope = "ScheduleInvariantsScanner"
 	// ArchiverDeleteHistoryActivityScope is scope used by all metrics emitted by archiver.DeleteHistoryActivity
 	ArchiverDeleteHistoryActivityScope = "ArchiverDeleteHistoryActivity"
 	// ArchiverUploadHistoryActivityScope is scope used by all metrics emitted by archiver.UploadHistoryActivity
@@ -617,6 +619,11 @@ const (
 	ScheduleBackendChasm                 = "chasm"
 	ScheduleBackendLegacy                = "legacy"
 	ScheduleBackendWorkflow              = "workflow"
+	ScheduleOverlapPolicyTag             = "schedule_overlap_policy"
+	ScheduleMissedReasonTag              = "reason"
+	ScheduleMissedReasonNotBuffered      = "not_buffered"
+	ScheduleMissedReasonBufferExpired    = "buffer_expired"
+	ScheduleActionRunningTag             = "action_running"
 	ScheduleMigrationDirectionTag        = "schedule_migration_direction"
 	ScheduleMigrationDirectionToChasm    = "to_chasm"
 	ScheduleMigrationDirectionToWorkflow = "to_workflow"
@@ -1324,17 +1331,21 @@ var (
 	HistoryWorkflowExecutionCacheLatency          = NewTimerDef("history_workflow_execution_cache_latency")
 	HistoryWorkflowExecutionCacheLockHoldDuration = NewTimerDef("history_workflow_execution_cache_lock_hold_duration")
 
-	VisibilityArchiverArchiveNonRetryableErrorCount = NewCounterDef("visibility_archiver_archive_non_retryable_error")
-	VisibilityArchiverArchiveTransientErrorCount    = NewCounterDef("visibility_archiver_archive_transient_error")
-	VisibilityArchiveSuccessCount                   = NewCounterDef("visibility_archiver_archive_success")
-	HistoryScavengerSuccessCount                    = NewCounterDef("scavenger_success")
-	HistoryScavengerErrorCount                      = NewCounterDef("scavenger_errors")
-	HistoryScavengerSkipCount                       = NewCounterDef("scavenger_skips")
-	ExecutionsOutstandingCount                      = NewGaugeDef("executions_outstanding")
-	ScavengerValidationRequestsCount                = NewCounterDef("scavenger_validation_requests")
-	ScavengerValidationFailuresCount                = NewCounterDef("scavenger_validation_failures")
-	ScavengerValidationSkipsCount                   = NewCounterDef("scavenger_validation_skips")
-	AddSearchAttributesFailuresCount                = NewCounterDef("add_search_attributes_failures")
+	VisibilityArchiverArchiveNonRetryableErrorCount     = NewCounterDef("visibility_archiver_archive_non_retryable_error")
+	VisibilityArchiverArchiveTransientErrorCount        = NewCounterDef("visibility_archiver_archive_transient_error")
+	VisibilityArchiveSuccessCount                       = NewCounterDef("visibility_archiver_archive_success")
+	HistoryScavengerSuccessCount                        = NewCounterDef("scavenger_success")
+	HistoryScavengerErrorCount                          = NewCounterDef("scavenger_errors")
+	HistoryScavengerSkipCount                           = NewCounterDef("scavenger_skips")
+	ScheduleInvariantsScannerOverdueNextActionTimeCount = NewCounterDef("schedule_invariants_scanner_overdue_next_action_time")
+	ScheduleInvariantsScannerStuckOpenCount             = NewCounterDef("schedule_invariants_scanner_stuck_open")
+	ScheduleInvariantsScannerUnknownStateCount          = NewCounterDef("schedule_invariants_scanner_unknown_state")
+	ScheduleInvariantsScannerErrorCount                 = NewCounterDef("schedule_invariants_scanner_errors")
+	ExecutionsOutstandingCount                          = NewGaugeDef("executions_outstanding")
+	ScavengerValidationRequestsCount                    = NewCounterDef("scavenger_validation_requests")
+	ScavengerValidationFailuresCount                    = NewCounterDef("scavenger_validation_failures")
+	ScavengerValidationSkipsCount                       = NewCounterDef("scavenger_validation_skips")
+	AddSearchAttributesFailuresCount                    = NewCounterDef("add_search_attributes_failures")
 
 	// Delete Namespace metrics.
 	ReclaimResourcesNamespaceDeleteSuccessCount = NewCounterDef(
@@ -1433,6 +1444,10 @@ var (
 		"schedule_action_delay",
 		WithDescription("Delay between when scheduled actions should/actually happen"),
 	)
+	ScheduleActionE2EDelay = NewTimerDef(
+		"schedule_action_e2e_delay",
+		WithDescription("End-to-end delay between the action's original schedule time and when it was actually started, including overlap policy wait"),
+	)
 	ScheduleGenerateLatency = NewTimerDef(
 		"schedule_generate_latency",
 		WithDescription("Delay between when a scheduled action was due and when the generator buffered it"),
@@ -1453,6 +1468,14 @@ var (
 		"schedule_migration_failed",
 		WithDescription("The number of times a schedule migration fails"),
 	)
+	ScheduleOverlapSkipped = NewCounterDef(
+		"schedule_overlap_skipped",
+		WithDescription("The number of schedule actions skipped due to overlap policy"),
+	)
+	ScheduleCallbackLatency = NewTimerDef(
+		"schedule_callback_latency",
+		WithDescription("Latency between a scheduled action completing and the scheduler receiving the completion callback"),
+	)
 
 	// Worker Versioning
 	WorkerDeploymentCreated                           = NewCounterDef("worker_deployment_created")
diff --git a/docs/architecture/schedules.md b/docs/architecture/schedules.md
index 34d923ad9e2..c65df4647d0 100644
--- a/docs/architecture/schedules.md
+++ b/docs/architecture/schedules.md
@@ -5,10 +5,10 @@ This page documents the internal architecture behind Temporal's [Schedules featu
 
 ### Introduction
 
-Scheduler is implemented as a [CHASM](https://github.com/temporalio/temporal/blob/main/docs/architecture/chasm.md) library, with all related implementation code located in [`chasm/lib/scheduler`](https://github.com/temporalio/temporal/tree/main/chasm/lib/scheduler). Every schedule is backed by an execution whose root component is a [`Scheduler`](https://github.com/temporalio/temporal/blob/main/chasm/lib/scheduler/scheduler.go#L35), which parents several other key components:
+Scheduler is implemented as a [CHASM](https://github.com/temporalio/temporal/blob/main/docs/architecture/chasm.md) library, with all related implementation code located in [`chasm/lib/scheduler`](https://github.com/temporalio/temporal/tree/main/chasm/lib/scheduler). Every schedule is backed by an execution whose root component is a [`Scheduler`](https://github.com/temporalio/temporal/blob/main/chasm/lib/scheduler/scheduler.go), which parents several other key components:
 
-* [**Generator**](https://github.com/temporalio/temporal/blob/main/chasm/lib/scheduler/generator.go#L11): buffers actions in accordance with the schedule's specification (one per `Scheduler`).
-* [**Invoker**](https://github.com/temporalio/temporal/blob/main/chasm/lib/scheduler/invoker.go#L16): executes buffered actions and handles retry logic (one per `Scheduler`).
+* [**Generator**](https://github.com/temporalio/temporal/blob/main/chasm/lib/scheduler/generator.go): buffers actions in accordance with the schedule's specification (one per `Scheduler`).
+* [**Invoker**](https://github.com/temporalio/temporal/blob/main/chasm/lib/scheduler/invoker.go): executes buffered actions and handles retry logic (one per `Scheduler`).
 * [**Backfiller**](https://github.com/temporalio/temporal/blob/main/chasm/lib/scheduler/backfiller.go): buffers actions from manually requested backfills and trigger requests (one `Backfiller` per pending request).
 
 Only the `Invoker` makes service calls with side effect tasks; the rest of the scheduler is driven through pure timer tasks. The `Scheduler` component itself implements required CHASM interfaces for lifecycle management and visibility, request-reply handler functions, and helper functions for mutating state common to the entire scheduler tree.
@@ -50,7 +50,11 @@ classDiagram
 
 ### Generator (Automated Actions)
 
-The Generator component buffers automated actions according to the schedule's specification. This is driven through `GeneratorTask`, a timer task that fires, and re-schedules itself, on the schedule's interval until the schedule is complete. Generator is solely responsible for buffering automated actions, not driving those actions to execution; that work is handed off to the `Invoker`. The following high-level sequence diagram indicates the Generator's contribution towards firing an action:
+The Generator component buffers automated actions according to the schedule's specification. This is driven through `GeneratorTask`, a timer task that fires, and re-schedules itself, on the schedule's interval until the schedule is complete. Generator is solely responsible for buffering automated actions, not driving those actions to execution; that work is handed off to the `Invoker`.
+
+The Generator runs continuously rather than stopping while paused: a paused fire still advances the high water mark but produces no `BufferedStart`s. This guarantees that `FutureActionTimes` remains accurate, including in visibility, even for paused schedules.
+
+The following high-level sequence diagram indicates the Generator's contribution towards firing an action:
 
 ```mermaid
 sequenceDiagram
@@ -84,7 +88,7 @@ sequenceDiagram
 #### Tasks
 `GeneratorTask`: Drives the automated actions loop. When a schedule is first created (or updated), a `GeneratorTask` is scheduled for immediate execution. `GeneratorTask` will reschedule itself for subsequent execution at the point in time where the next automated action is scheduled to fire. If the schedule has completed (by exceeding its lifetime, or action count), `GeneratorTask` won't reschedule itself, and will instead schedule the Scheduler's `IdleTask`. The idle task keeps the schedule open for a retention period before closing it, ensuring the schedule doesn't immediately disappear after completion.
 
-`GeneratorTask`'s primary responsibility is to call `EnqueueBufferedStarts` on the `Invoker`, handing off actions for execution (see also *Specification Processor* in this document):
+`GeneratorTask`'s primary responsibility is to call `EnqueueBufferedStarts` on the `Invoker`, handing off actions for execution (see also *Specification Processor* in this document). After processing, three terminal outcomes are possible: arm the idle task to close the schedule, reschedule for the next spec wakeup, or hold open without a task (paused with no spec wakeup, or a paused schedule with a pending backfiller - revived by external events like `Patch.Unpause` or `BackfillerTask` completion).
 
 ```mermaid
 flowchart TD
@@ -97,8 +101,9 @@ flowchart TD
     G[Update LastProcessedTime]
     H{Is scheduler idle?}
     I[Schedule IdleTask]
-    J{Is scheduler paused?}
+    J{Spec has next wakeup?}
     K[Reschedule GeneratorTask]
+    M[Hold open - no task]
     L{{Finish executing}}
 
     A --> B
@@ -113,21 +118,20 @@ flowchart TD
     H -->|Yes| I
     H -->|No| J
     I --> L
-    J -->|Yes| L
-    J -->|No| K
+    J -->|Yes| K
+    J -->|No| M
     K --> L
+    M --> L
 ```
 
 *Figure: `GeneratorTask`'s decision tree for buffering starts (trivial branches omitted).*
 
-Note that when the schedule is paused, `GeneratorTask` finishes without rescheduling itself. Whenever the schedule is updated (including being resumed from a paused state), a new `GeneratorTask` is scheduled for immediate execution, restarting the loop.
-
 #### Notes
 - At time of writing, `FutureActionTimes` is also maintained in Generator's state. This field is a cached view of speculated action times, and is returned as part of describe responses, but is not involved in actual start time calculations, action buffering, or even as assured guarantees of execution (e.g., a listed time may be dropped for having missed catchup window).
 
 ### Backfiller (Manual Actions)
 
-A Backfiller component is created for every backfill requested through the API (including "trigger immediately" requests). Similar to the `GeneratorTask`, the `BackfillerTask` is responsible for driving a backfill to completion by enqueueing buffered actions into the Invoker. Each backfiller acts as a distinct buffer per backfill request, and will avoid [monopolizing the Invoker's shared `BufferedStarts` field](https://github.com/temporalio/temporal/blob/658b80da2ed66355fe78650788ac33814ffd1561/chasm/lib/scheduler/backfiller_tasks.go#L233-L234), instead backing off and retrying while waiting for the buffer to drain. 
+A Backfiller component is created for every backfill requested through the API (including "trigger immediately" requests). Similar to the `GeneratorTask`, the `BackfillerTask` is responsible for driving a backfill to completion by enqueueing buffered actions into the Invoker. Each backfiller acts as a distinct buffer per backfill request, and will avoid monopolizing the Invoker's shared `BufferedStarts` field (see [`BackfillerTask.Execute`](https://github.com/temporalio/temporal/blob/main/chasm/lib/scheduler/backfiller_tasks.go) for the buffer-full backoff), instead backing off and retrying while waiting for the buffer to drain. 
 
 The Backfiller component fits into the scheduler sequence diagram similarly to the Generator:
 
@@ -193,7 +197,7 @@ The Invoker component is responsible for driving buffered starts to execution, m
 
 #### State
 - `BufferedStarts`: Invoker manages the scheduler's buffered starts/actions through their entire lifecycle (see *BufferedStart Lifecycle* below). The Invoker function `EnqueueBufferedStarts` is used to schedule an action for execution by adding it to this list, setting `Attempt` to `0`. Once added to the list, the `ProcessBufferTask` will evaluate the schedule's overlap policy, as well as other scheduler state, before bumping `Attempt` to `1`, which makes it eligible for `ExecuteTask` to drive to completion. Entries remain in `BufferedStarts` after starting (with `RunId` set) and after completion (with `Completed` set), providing computed views for running workflows and recent actions.
-	- Each `BufferedStart` entry is itself stateful, maintaining its own `Attempt` and `BackoffTime`. Only once `Attempt` is past `0` will it be eligible for execution (see [`getEligibleBufferedStarts`](https://github.com/temporalio/temporal/blob/85635674d7c55a8679b715b118aaf14c973266eb/chasm/lib/scheduler/invoker.go#L276)).
+	- Each `BufferedStart` entry is itself stateful, maintaining its own `Attempt` and `BackoffTime`. Only once `Attempt` is past `0` will it be eligible for execution (see `getEligibleBufferedStarts` in [`invoker.go`](https://github.com/temporalio/temporal/blob/main/chasm/lib/scheduler/invoker.go)).
 - `CancelWorkflows`: Workflows that will be cancelled due to overlap policy. The `ProcessBufferTask` adds to this list, and the `ExecuteTask` consumes the list.
 - `TerminateWorkflows`: Similar to `CancelWorkflows`.
 - `LastProcessedTime`: A high water mark used to evaluate when to fire tasks that are pending a retry after execution failure.
@@ -208,10 +212,9 @@ flowchart TD
     C{Need terminate or cancel?}
     D[Queue terminate/cancel requests]
     E{Starts ready for execution?}
-    F{Is action manual?}
-    G{Is schedule paused?}
-    H{Scheduled actions remaining?}
     I{Is start expired?}
+    F{Is action manual?}
+    N{Paused or no actions remaining?}
     J[Discard start]
     K[Mark for execution]
     L([Schedule ExecuteTask])
@@ -222,24 +225,22 @@ flowchart TD
     C -->|Yes| D
     C -->|No| E
     D --> E
-    E -->|Yes| F
+    E -->|Yes| I
     E -->|No| L
-    F -->|Yes| I
-    F -->|No| G
-    G -->|Yes| J
-    G -->|No| H
-    H -->|No| J
-    H -->|Yes| I
     I -->|Yes| J
-    I -->|No| K
+    I -->|No| F
+    F -->|Yes| K
+    F -->|No| N
+    N -->|Yes| J
+    N -->|No| K
     J --> E
     K --> E
     L --> M
 ```
 
-*Figure: `ProcessBufferTask`'s decision tree (trivial branches omitted).*
+*Figure: `ProcessBufferTask`'s decision tree (trivial branches omitted). The catchup-window check runs before consuming a `LimitedActions` slot - otherwise an expired start would burn a slot it never actually used.*
 
-`ExecuteTask`: Drives all [eligible buffered starts](https://github.com/temporalio/temporal/blob/85635674d7c55a8679b715b118aaf14c973266eb/chasm/lib/scheduler/invoker.go#L276) and pending cancel/terminate requests to execution by making service calls. `ExecuteTask` reschedules itself whenever a service call fails and must be retried, as well as when it wants to "checkpoint" a batch of completed executions by flushing to mutable state.
+`ExecuteTask`: Drives all eligible buffered starts (via `getEligibleBufferedStarts` in [`invoker.go`](https://github.com/temporalio/temporal/blob/main/chasm/lib/scheduler/invoker.go)) and pending cancel/terminate requests to execution by making service calls. On retryable failures, the start's `BackoffTime` is set and `addTasks` arms a future `ProcessBufferTask` at that backoff. There's no direct "reschedule ExecuteTask after backoff" path: `ProcessBufferTask` fires at the backoff deadline, re-promotes the eligible start, then emits a fresh `ExecuteTask`.
 
 ```mermaid
 flowchart TD
@@ -256,8 +257,8 @@ flowchart TD
     K[Apply backoff for retry]
     L[Drop start]
     M([Record execute results])
-    N{Starts pending retry?}
-    O([Reschedule ExecuteTask])
+    N{Any buffered starts left?}
+    O([addTasks: ProcessBuffer at backoff, or Execute immediate])
     P{{Finish executing}}
 
     A --> B
@@ -341,7 +342,7 @@ The `BufferedStarts` queue allows scheduler to compute the following `Info` bloc
 
 ### Completion Callbacks
 
-In order to determine when an action started by a schedule has completed, Scheduler makes use of Nexus completion callbacks. When the Invoker starts an action, a Nexus callback token [is generated](https://github.com/temporalio/temporal/blob/main/chasm/nexus_completion.go#L26), and set as part of the [`StartWorkflowExecution` request](https://github.com/temporalio/temporal/blob/main/chasm/lib/scheduler/invoker_tasks.go#L551). Once the action has completed, the callback mechanism will eventually trigger the [scheduler's completion handler](https://github.com/temporalio/temporal/blob/main/chasm/lib/scheduler/scheduler.go#L419).
+In order to determine when an action started by a schedule has completed, Scheduler makes use of Nexus completion callbacks. When the Invoker starts an action, a Nexus callback token is generated (see `GenerateNexusCallback` in [`nexus_completion.go`](https://github.com/temporalio/temporal/blob/main/chasm/nexus_completion.go)), and set as part of the `StartWorkflowExecution` request (see `startWorkflow` in [`invoker_tasks.go`](https://github.com/temporalio/temporal/blob/main/chasm/lib/scheduler/invoker_tasks.go)). Once the action has completed, the callback mechanism will eventually trigger the scheduler's completion handler (see `HandleNexusCompletion` in [`scheduler.go`](https://github.com/temporalio/temporal/blob/main/chasm/lib/scheduler/scheduler.go)).
 
 The completion handler has several responsibilities:
 - Records the completed action's payloads (success/failure) to the scheduler's `LastCompletionResult`.
@@ -350,9 +351,21 @@ The completion handler has several responsibilities:
 - Records the action's close time, to measure the latency between "last action completed" and "next action started" when multiple starts are buffered sequentially and pending execution.
 - Applies retention to keep only the most recent completed actions in `BufferedStarts`.
 - Schedules another `ProcessBufferTask`, as a completion may make a buffered start eligible for execution.
+- Schedules another `GeneratorTask`, to handle a potentially updated idle close time.
 
 Nexus completion callbacks are the only way that Scheduler can find out about an action's completion (there is no additional synchronous path). When the Invoker's `ProcessBufferTask` evaluates running workflow status, the computed view of running workflows (derived from `BufferedStarts` entries with `RunId` set but not completed) is used instead of making additional describe calls. 
 
+### Idle Task (Closing)
+
+`SchedulerIdleTask` is a pure task armed by the `Generator` when it determines the schedule has no more work to do (no future spec wakeups, no pending backfills, no remaining limited actions, and not paused). When the task fires, it sets `Scheduler.Closed = true`, ending the schedule's lifetime. Between arming and firing the task waits `IdleTime` (a tweakable, default 7 days), which is the retention window during which a customer can still describe/modify the schedule.
+
+`Validate` enforces three drop conditions, each tagged with a `reason` for the `ScheduleIdleTaskInvalidated` counter:
+- `closed` - the schedule is already closed (idempotency guard).
+- `held_open` - the schedule transitioned into a state that disqualifies idle close: paused or has a pending backfiller.
+- `expiration_shift` - the recomputed idle deadline (anchor + `IdleTime`) has moved later than the task's `ScheduledTime`, meaning the Generator has since armed a new task at the correct deadline.
+
+Manual-only schedules (no automated wakeup source, e.g. TriggerImmediately-only) idle out like any other schedule when unpaused - `lastEventTime` is advanced by manual triggers via `recentActions`, matching V1's `RetentionTime` semantics. Sentinels are exempt from `held_open` and anchor their idle deadline on `CreateTime` directly - they exist to reserve a schedule ID and must close regardless of their otherwise-paused/empty state.
+
 ### Specification Processor
 
 The Specification Processor (`SpecProcessor` in code) is responsible for looping through the action times derived from the schedule's specification for a given range of time. It is used by both the `Generator` and the `Backfiller` to generate their `BufferedStart` structs. 
@@ -391,4 +404,4 @@ flowchart TD
     K --> L
 ```
 
-*Figure: `SpecProcessor.ProcessTimeRange`'s decision tree (trivial branches omitted).*
\ No newline at end of file
+*Figure: `SpecProcessor.ProcessTimeRange`'s decision tree (trivial branches omitted).*
diff --git a/service/frontend/service.go b/service/frontend/service.go
index dacde0b073d..f114bb59136 100644
--- a/service/frontend/service.go
+++ b/service/frontend/service.go
@@ -169,6 +169,9 @@ type Config struct {
 	EnableChasm dynamicconfig.BoolPropertyFnWithNamespaceFilter
 	// Enable creation of new schedules on CHASM (V2) engine
 	EnableCHASMSchedulerCreation dynamicconfig.BoolPropertyFnWithNamespaceFilter
+	// Per-namespace percentage [0-100] of new schedules routed to CHASM when
+	// EnableCHASMSchedulerCreation is true. Default 0.
+	CHASMSchedulerCreationRolloutPercent dynamicconfig.IntPropertyFnWithNamespaceFilter
 	// Enable CHASM-first routing for schedule RPCs other than CreateSchedule
 	EnableCHASMSchedulerRouting dynamicconfig.BoolPropertyFnWithNamespaceFilter
 	// Enables ID-space collision sentinels, and must be enabled and propagated in
@@ -351,11 +354,12 @@ func NewConfig(
 
 		MaxFairnessWeightOverrideConfigLimit: dynamicconfig.MatchingMaxFairnessKeyWeightOverrides.Get(dc),
 
-		EnableSchedules:               dynamicconfig.FrontendEnableSchedules.Get(dc),
-		EnableChasm:                   dynamicconfig.EnableChasm.Get(dc),
-		EnableCHASMSchedulerCreation:  dynamicconfig.EnableCHASMSchedulerCreation.Get(dc),
-		EnableCHASMSchedulerRouting:   dynamicconfig.EnableCHASMSchedulerRouting.Get(dc),
-		EnableCHASMSchedulerSentinels: dynamicconfig.EnableCHASMSchedulerSentinels.Get(dc),
+		EnableSchedules:                      dynamicconfig.FrontendEnableSchedules.Get(dc),
+		EnableChasm:                          dynamicconfig.EnableChasm.Get(dc),
+		EnableCHASMSchedulerCreation:         dynamicconfig.EnableCHASMSchedulerCreation.Get(dc),
+		CHASMSchedulerCreationRolloutPercent: dynamicconfig.CHASMSchedulerCreationRolloutPercent.Get(dc),
+		EnableCHASMSchedulerRouting:          dynamicconfig.EnableCHASMSchedulerRouting.Get(dc),
+		EnableCHASMSchedulerSentinels:        dynamicconfig.EnableCHASMSchedulerSentinels.Get(dc),
 
 		// [cleanup-wv-pre-release]
 		EnableDeployments:        dynamicconfig.EnableDeployments.Get(dc),
diff --git a/service/frontend/workflow_handler.go b/service/frontend/workflow_handler.go
index d94f5a947ca..e75fb04d5b1 100644
--- a/service/frontend/workflow_handler.go
+++ b/service/frontend/workflow_handler.go
@@ -3748,7 +3748,7 @@ func (wh *WorkflowHandler) CreateSchedule(
 
 	namespaceName := namespace.Name(request.Namespace)
 
-	useChasmScheduler := wh.chasmSchedulerCreationEnabled(ctx, namespaceName.String())
+	useChasmScheduler := wh.chasmSchedulerCreationEnabled(ctx, namespaceName.String(), request.ScheduleId)
 	wh.logger.Debug("Received CreateSchedule",
 		tag.ScheduleID(request.ScheduleId),
 		tag.WorkflowNamespace(namespaceName.String()),
@@ -3778,18 +3778,32 @@ func (wh *WorkflowHandler) CreateSchedule(
 }
 
 // chasmSchedulerCreationEnabled returns true when CreateSchedule should create on
-// the CHASM scheduler.
-func (wh *WorkflowHandler) chasmSchedulerCreationEnabled(ctx context.Context, namespaceName string) bool {
-	return (headers.IsExperimentRequested(ctx, ChasmSchedulerExperiment) &&
-		wh.config.IsExperimentAllowed(ChasmSchedulerExperiment, namespaceName)) ||
-		wh.config.EnableCHASMSchedulerCreation(namespaceName)
+// the CHASM scheduler. The binary EnableCHASMSchedulerCreation flag is gated by a
+// per-namespace percentage rollout keyed on scheduleID; the experiment header path
+// is unconditional.
+func (wh *WorkflowHandler) chasmSchedulerCreationEnabled(ctx context.Context, namespaceName, scheduleID string) bool {
+	if headers.IsExperimentRequested(ctx, ChasmSchedulerExperiment) &&
+		wh.config.IsExperimentAllowed(ChasmSchedulerExperiment, namespaceName) {
+		return true
+	}
+	if !wh.config.EnableCHASMSchedulerCreation(namespaceName) {
+		return false
+	}
+	key := fmt.Appendf(nil, "%s\x00%s", namespaceName, scheduleID)
+	return dynamicconfig.RolloutAccepts(key, wh.config.CHASMSchedulerCreationRolloutPercent(namespaceName))
 }
 
 // chasmSchedulerEnabled returns true when schedule RPCs should route to CHASM
 // first. Handlers must be capable of falling back to V1 codepaths for schedules
-// that haven't been migrated to CHASM.
+// that haven't been migrated to CHASM. Routing follows the binary creation flag
+// (without the percentage gate) because schedules already created on CHASM at any
+// rollout percent must be looked up there.
 func (wh *WorkflowHandler) chasmSchedulerEnabled(ctx context.Context, namespaceName string) bool {
-	return wh.chasmSchedulerCreationEnabled(ctx, namespaceName) ||
+	if headers.IsExperimentRequested(ctx, ChasmSchedulerExperiment) &&
+		wh.config.IsExperimentAllowed(ChasmSchedulerExperiment, namespaceName) {
+		return true
+	}
+	return wh.config.EnableCHASMSchedulerCreation(namespaceName) ||
 		wh.config.EnableCHASMSchedulerRouting(namespaceName)
 }
 
diff --git a/service/frontend/workflow_handler_second_test.go b/service/frontend/workflow_handler_second_test.go
index 4efdbbbd593..ae7bb336ca3 100644
--- a/service/frontend/workflow_handler_second_test.go
+++ b/service/frontend/workflow_handler_second_test.go
@@ -63,6 +63,7 @@ func TestCHASMSchedulerRoutingAndCreationGates(t *testing.T) {
 
 			config := NewConfig(dc.NewNoopCollection(), 1)
 			config.EnableCHASMSchedulerCreation = dc.GetBoolPropertyFnFilteredByNamespace(tc.enableCreation)
+			config.CHASMSchedulerCreationRolloutPercent = dc.GetIntPropertyFnFilteredByNamespace(100)
 			config.EnableCHASMSchedulerRouting = dc.GetBoolPropertyFnFilteredByNamespace(tc.enableRouting)
 			config.AllowedExperiments = dc.GetTypedPropertyFnFilteredByNamespace(tc.allowedExp)
 
@@ -73,7 +74,7 @@ func TestCHASMSchedulerRoutingAndCreationGates(t *testing.T) {
 				ctx = metadata.NewIncomingContext(ctx, metadata.Pairs(headers.ExperimentHeaderName, ChasmSchedulerExperiment))
 			}
 
-			require.Equal(t, tc.expectCreation, wh.chasmSchedulerCreationEnabled(ctx, "test-namespace"))
+			require.Equal(t, tc.expectCreation, wh.chasmSchedulerCreationEnabled(ctx, "test-namespace", "test-schedule"))
 			require.Equal(t, tc.expectRouting, wh.chasmSchedulerEnabled(ctx, "test-namespace"))
 		})
 	}
diff --git a/service/worker/scanner/scanner.go b/service/worker/scanner/scanner.go
index 8e4681641f7..2cd1b7b25bb 100644
--- a/service/worker/scanner/scanner.go
+++ b/service/worker/scanner/scanner.go
@@ -15,6 +15,7 @@ import (
 	"go.temporal.io/server/api/matchingservice/v1"
 	"go.temporal.io/server/common"
 	"go.temporal.io/server/common/backoff"
+	"go.temporal.io/server/common/clock"
 	"go.temporal.io/server/common/config"
 	"go.temporal.io/server/common/dynamicconfig"
 	"go.temporal.io/server/common/headers"
@@ -26,8 +27,10 @@ import (
 	"go.temporal.io/server/common/persistence"
 	"go.temporal.io/server/common/persistence/serialization"
 	"go.temporal.io/server/common/persistence/visibility/manager"
+	"go.temporal.io/server/common/persistence/visibility/store/elasticsearch"
 	"go.temporal.io/server/common/sdk"
 	"go.temporal.io/server/service/worker/scanner/build_ids"
+	"go.temporal.io/server/service/worker/scanner/scheduleinvariants"
 )
 
 type (
@@ -71,6 +74,16 @@ type (
 		RemovableBuildIdDurationSinceDefault dynamicconfig.DurationPropertyFn
 		// BuildIdScavengerVisibilityRPS is the rate limit for visibility calls from the build ID scavenger
 		BuildIdScavengerVisibilityRPS dynamicconfig.FloatPropertyFn
+
+		// Schedule-invariants scanners. Each runs as an independent cron workflow; all three
+		// require advanced (Elasticsearch) visibility to be configured (otherwise they're skipped).
+		ScheduleInvariantsScannerOverdueNextActionTimeEnabled      dynamicconfig.BoolPropertyFn
+		ScheduleInvariantsScannerStuckOpenEnabled                  dynamicconfig.BoolPropertyFn
+		ScheduleInvariantsScannerUnknownStateEnabled               dynamicconfig.BoolPropertyFn
+		ScheduleInvariantsScannerOverdueNextActionTimeTolerance    dynamicconfig.DurationPropertyFn
+		ScheduleInvariantsScannerVisibilityRPS                     dynamicconfig.FloatPropertyFn
+		ScheduleInvariantsScannerScanInterval                      dynamicconfig.DurationPropertyFn
+		ScheduleInvariantsScannerStuckOpenIdleTimeBufferMultiplier dynamicconfig.IntPropertyFn
 	}
 
 	// scannerContext is the context object that gets
@@ -209,6 +222,68 @@ func (s *Scanner) Start() error {
 		}
 	}
 
+	scheduleInvariantsAnyEnabled := s.context.cfg.ScheduleInvariantsScannerOverdueNextActionTimeEnabled() ||
+		s.context.cfg.ScheduleInvariantsScannerStuckOpenEnabled() ||
+		s.context.cfg.ScheduleInvariantsScannerUnknownStateEnabled()
+	if scheduleInvariantsAnyEnabled && !s.context.visibilityManager.HasStoreName(elasticsearch.PersistenceName) {
+		s.context.logger.Info("schedule-invariants scanners are enabled but advanced (Elasticsearch) visibility is not configured; skipping")
+	} else if scheduleInvariantsAnyEnabled {
+		scheduleActivities := scheduleinvariants.NewActivities(
+			s.context.logger,
+			s.context.metricsHandler,
+			s.context.metadataManager,
+			s.context.visibilityManager,
+			s.context.namespaceRegistry,
+			s.context.sdkClientFactory,
+			s.context.currentClusterName,
+			clock.NewRealTimeSource(),
+			s.context.cfg.ScheduleInvariantsScannerVisibilityRPS,
+			s.context.cfg.ScheduleInvariantsScannerOverdueNextActionTimeTolerance,
+			s.context.cfg.ScheduleInvariantsScannerScanInterval,
+			s.context.cfg.ScheduleInvariantsScannerStuckOpenIdleTimeBufferMultiplier,
+		)
+
+		if s.context.cfg.ScheduleInvariantsScannerOverdueNextActionTimeEnabled() {
+			s.wg.Add(1)
+			go s.startWorkflowWithRetry(ctx, scheduleinvariants.OverdueNextActionTimeWFStartOptions, scheduleinvariants.OverdueNextActionTimeWorkflowName)
+
+			work := s.context.sdkClientFactory.NewWorker(s.context.sdkClientFactory.GetSystemClient(), scheduleinvariants.OverdueNextActionTimeTaskQueue, workerOpts)
+			work.RegisterWorkflowWithOptions(scheduleinvariants.OverdueNextActionTimeWorkflow, workflow.RegisterOptions{Name: scheduleinvariants.OverdueNextActionTimeWorkflowName})
+			work.RegisterActivityWithOptions(scheduleActivities.ScanOverdueNextActionTime, activity.RegisterOptions{Name: scheduleinvariants.OverdueNextActionTimeActivityName})
+
+			// TODO: Nothing is gracefully stopping these workers or listening for fatal errors.
+			if err := work.Start(); err != nil {
+				return err
+			}
+		}
+
+		if s.context.cfg.ScheduleInvariantsScannerStuckOpenEnabled() {
+			s.wg.Add(1)
+			go s.startWorkflowWithRetry(ctx, scheduleinvariants.StuckOpenWFStartOptions, scheduleinvariants.StuckOpenWorkflowName)
+
+			work := s.context.sdkClientFactory.NewWorker(s.context.sdkClientFactory.GetSystemClient(), scheduleinvariants.StuckOpenTaskQueue, workerOpts)
+			work.RegisterWorkflowWithOptions(scheduleinvariants.StuckOpenWorkflow, workflow.RegisterOptions{Name: scheduleinvariants.StuckOpenWorkflowName})
+			work.RegisterActivityWithOptions(scheduleActivities.ScanStuckOpen, activity.RegisterOptions{Name: scheduleinvariants.StuckOpenActivityName})
+
+			if err := work.Start(); err != nil {
+				return err
+			}
+		}
+
+		if s.context.cfg.ScheduleInvariantsScannerUnknownStateEnabled() {
+			s.wg.Add(1)
+			go s.startWorkflowWithRetry(ctx, scheduleinvariants.UnknownStateWFStartOptions, scheduleinvariants.UnknownStateWorkflowName)
+
+			work := s.context.sdkClientFactory.NewWorker(s.context.sdkClientFactory.GetSystemClient(), scheduleinvariants.UnknownStateTaskQueue, workerOpts)
+			work.RegisterWorkflowWithOptions(scheduleinvariants.UnknownStateWorkflow, workflow.RegisterOptions{Name: scheduleinvariants.UnknownStateWorkflowName})
+			work.RegisterActivityWithOptions(scheduleActivities.ScanUnknownState, activity.RegisterOptions{Name: scheduleinvariants.UnknownStateActivityName})
+
+			if err := work.Start(); err != nil {
+				return err
+			}
+		}
+	}
+
 	// TODO: There's no reason to register all activities and workflows on every task queue.
 	for _, tl := range workerTaskQueueNames {
 		work := s.context.sdkClientFactory.NewWorker(s.context.sdkClientFactory.GetSystemClient(), tl, workerOpts)
diff --git a/service/worker/scanner/scanner_test.go b/service/worker/scanner/scanner_test.go
index cfd1f6821c7..ef631387877 100644
--- a/service/worker/scanner/scanner_test.go
+++ b/service/worker/scanner/scanner_test.go
@@ -183,14 +183,17 @@ func (s *scannerTestSuite) TestScannerEnabled() {
 			scanner := New(
 				log.NewNoopLogger(),
 				&Config{
-					MaxConcurrentActivityExecutionSize:     dynamicconfig.GetIntPropertyFn(1),
-					MaxConcurrentWorkflowTaskExecutionSize: dynamicconfig.GetIntPropertyFn(1),
-					MaxConcurrentActivityTaskPollers:       dynamicconfig.GetIntPropertyFn(1),
-					MaxConcurrentWorkflowTaskPollers:       dynamicconfig.GetIntPropertyFn(1),
-					HistoryScannerEnabled:                  dynamicconfig.GetBoolPropertyFn(c.HistoryScannerEnabled),
-					BuildIdScavengerEnabled:                dynamicconfig.GetBoolPropertyFn(c.BuildIdScavengerEnabled),
-					ExecutionsScannerEnabled:               dynamicconfig.GetBoolPropertyFn(c.ExecutionsScannerEnabled),
-					TaskQueueScannerEnabled:                dynamicconfig.GetBoolPropertyFn(c.TaskQueueScannerEnabled),
+					MaxConcurrentActivityExecutionSize:                    dynamicconfig.GetIntPropertyFn(1),
+					MaxConcurrentWorkflowTaskExecutionSize:                dynamicconfig.GetIntPropertyFn(1),
+					MaxConcurrentActivityTaskPollers:                      dynamicconfig.GetIntPropertyFn(1),
+					MaxConcurrentWorkflowTaskPollers:                      dynamicconfig.GetIntPropertyFn(1),
+					HistoryScannerEnabled:                                 dynamicconfig.GetBoolPropertyFn(c.HistoryScannerEnabled),
+					BuildIdScavengerEnabled:                               dynamicconfig.GetBoolPropertyFn(c.BuildIdScavengerEnabled),
+					ExecutionsScannerEnabled:                              dynamicconfig.GetBoolPropertyFn(c.ExecutionsScannerEnabled),
+					TaskQueueScannerEnabled:                               dynamicconfig.GetBoolPropertyFn(c.TaskQueueScannerEnabled),
+					ScheduleInvariantsScannerOverdueNextActionTimeEnabled: dynamicconfig.GetBoolPropertyFn(false),
+					ScheduleInvariantsScannerStuckOpenEnabled:             dynamicconfig.GetBoolPropertyFn(false),
+					ScheduleInvariantsScannerUnknownStateEnabled:          dynamicconfig.GetBoolPropertyFn(false),
 					Persistence: &config.Persistence{
 						DefaultStore: c.DefaultStore,
 						DataStores: map[string]config.DataStore{
@@ -263,14 +266,17 @@ func (s *scannerTestSuite) TestScannerShutdown() {
 	scanner := New(
 		logger,
 		&Config{
-			MaxConcurrentActivityExecutionSize:     dynamicconfig.GetIntPropertyFn(1),
-			MaxConcurrentWorkflowTaskExecutionSize: dynamicconfig.GetIntPropertyFn(1),
-			MaxConcurrentActivityTaskPollers:       dynamicconfig.GetIntPropertyFn(1),
-			MaxConcurrentWorkflowTaskPollers:       dynamicconfig.GetIntPropertyFn(1),
-			HistoryScannerEnabled:                  dynamicconfig.GetBoolPropertyFn(true),
-			ExecutionsScannerEnabled:               dynamicconfig.GetBoolPropertyFn(false),
-			TaskQueueScannerEnabled:                dynamicconfig.GetBoolPropertyFn(false),
-			BuildIdScavengerEnabled:                dynamicconfig.GetBoolPropertyFn(false),
+			MaxConcurrentActivityExecutionSize:                    dynamicconfig.GetIntPropertyFn(1),
+			MaxConcurrentWorkflowTaskExecutionSize:                dynamicconfig.GetIntPropertyFn(1),
+			MaxConcurrentActivityTaskPollers:                      dynamicconfig.GetIntPropertyFn(1),
+			MaxConcurrentWorkflowTaskPollers:                      dynamicconfig.GetIntPropertyFn(1),
+			HistoryScannerEnabled:                                 dynamicconfig.GetBoolPropertyFn(true),
+			ExecutionsScannerEnabled:                              dynamicconfig.GetBoolPropertyFn(false),
+			TaskQueueScannerEnabled:                               dynamicconfig.GetBoolPropertyFn(false),
+			BuildIdScavengerEnabled:                               dynamicconfig.GetBoolPropertyFn(false),
+			ScheduleInvariantsScannerOverdueNextActionTimeEnabled: dynamicconfig.GetBoolPropertyFn(false),
+			ScheduleInvariantsScannerStuckOpenEnabled:             dynamicconfig.GetBoolPropertyFn(false),
+			ScheduleInvariantsScannerUnknownStateEnabled:          dynamicconfig.GetBoolPropertyFn(false),
 			Persistence: &config.Persistence{
 				DefaultStore: config.StoreTypeNoSQL,
 				DataStores: map[string]config.DataStore{
diff --git a/service/worker/scanner/scheduleinvariants/invariants.go b/service/worker/scanner/scheduleinvariants/invariants.go
new file mode 100644
index 00000000000..f7a68745206
--- /dev/null
+++ b/service/worker/scanner/scheduleinvariants/invariants.go
@@ -0,0 +1,411 @@
+package scheduleinvariants
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	enumspb "go.temporal.io/api/enums/v1"
+	"go.temporal.io/api/workflowservice/v1"
+	"go.temporal.io/sdk/activity"
+	"go.temporal.io/server/api/visibilityservice/v1"
+	"go.temporal.io/server/chasm"
+	"go.temporal.io/server/chasm/lib/scheduler"
+	"go.temporal.io/server/common"
+	"go.temporal.io/server/common/clock"
+	"go.temporal.io/server/common/dynamicconfig"
+	"go.temporal.io/server/common/log"
+	"go.temporal.io/server/common/log/tag"
+	"go.temporal.io/server/common/metrics"
+	"go.temporal.io/server/common/namespace"
+	"go.temporal.io/server/common/persistence"
+	"go.temporal.io/server/common/persistence/visibility/manager"
+	"go.temporal.io/server/common/quotas"
+	"go.temporal.io/server/common/sdk"
+)
+
+const (
+	// heartbeatInterval is how often the background heartbeat goroutine pings the
+	// activity. It must be comfortably below the activity's HeartbeatTimeout.
+	heartbeatInterval = 10 * time.Second
+	// scheduleListPageSize controls pagination of ListChasmExecutions inside
+	// ScanOverdueNextActionTime. Each page is also one unit of rate-limited progress.
+	scheduleListPageSize  = 100
+	namespaceListPageSize = 100
+)
+
+// Activities holds shared dependencies for all three schedule-invariants scanner activities.
+type Activities struct {
+	logger             log.Logger
+	metricsHandler     metrics.Handler
+	metadataManager    persistence.MetadataManager
+	visibilityManager  manager.VisibilityManager
+	namespaceRegistry  namespace.Registry
+	sdkClientFactory   sdk.ClientFactory
+	currentClusterName string
+	timeSource         clock.TimeSource
+
+	overdueTolerance dynamicconfig.DurationPropertyFn
+	// scanInterval is how often each scanner activity kicks off a fresh scan pass.
+	scanInterval dynamicconfig.DurationPropertyFn
+	// stuckOpenIdleTimeBufferMultiplier multiplies the configured schedule IdleTime to set
+	// how far past its idle-close deadline a schedule must be before ScanStuckOpen flags it.
+	stuckOpenIdleTimeBufferMultiplier dynamicconfig.IntPropertyFn
+	rateLimiter                       quotas.RateLimiter
+}
+
+func NewActivities(
+	logger log.Logger,
+	metricsHandler metrics.Handler,
+	metadataManager persistence.MetadataManager,
+	visibilityManager manager.VisibilityManager,
+	namespaceRegistry namespace.Registry,
+	sdkClientFactory sdk.ClientFactory,
+	currentClusterName string,
+	timeSource clock.TimeSource,
+	visibilityRPS dynamicconfig.FloatPropertyFn,
+	overdueTolerance dynamicconfig.DurationPropertyFn,
+	scanInterval dynamicconfig.DurationPropertyFn,
+	stuckOpenIdleTimeBufferMultiplier dynamicconfig.IntPropertyFn,
+) *Activities {
+	return &Activities{
+		logger:                            logger,
+		metricsHandler:                    metricsHandler.WithTags(metrics.OperationTag(metrics.ScheduleInvariantsScannerScope)),
+		metadataManager:                   metadataManager,
+		visibilityManager:                 visibilityManager,
+		namespaceRegistry:                 namespaceRegistry,
+		sdkClientFactory:                  sdkClientFactory,
+		currentClusterName:                currentClusterName,
+		timeSource:                        timeSource,
+		overdueTolerance:                  overdueTolerance,
+		scanInterval:                      scanInterval,
+		stuckOpenIdleTimeBufferMultiplier: stuckOpenIdleTimeBufferMultiplier,
+		rateLimiter:                       quotas.NewDefaultOutgoingRateLimiter(quotas.RateFn(visibilityRPS)),
+	}
+}
+
+// ScanOverdueNextActionTime is a long-running activity that periodically lists
+// schedules whose TemporalScheduleNextActionTime is in the past beyond the configured
+// tolerance, calls DescribeSchedule on each candidate, and filters out schedules that
+// are paused or legitimately blocked from firing by a BUFFER_ONE / BUFFER_ALL overlap
+// policy with a prior workflow still running. Whatever remains is counted as an
+// anomaly per namespace and logged for triage.
+func (a *Activities) ScanOverdueNextActionTime(ctx context.Context) error {
+	return a.runForeverWithInterval(ctx, func(scanCtx context.Context) error {
+		threshold := a.timeSource.Now().UTC().Add(-a.overdueTolerance()).Format(time.RFC3339Nano)
+		// the query's intention is:
+		// show running schedules where ScheduleNextActionTime is in the past (plus a small buffer)
+		query := fmt.Sprintf(
+			`%s < "%s" AND TemporalSchedulePaused = false AND ExecutionStatus = "Running"`,
+			scheduler.ScheduleNextActionTimeName,
+			threshold,
+		)
+		return a.runOverdueScan(scanCtx, query)
+	})
+}
+
+// ScanStuckOpen is a long-running activity that periodically scans for schedules whose
+// ScheduleIdleCloseTime deadline has already passed (beyond the configured buffer).
+// ScheduleIdleCloseTime is only emitted while a schedule is open, so an overdue value
+// means the idle-close task never fired and the schedule is stuck open. Paused schedules
+// are excluded since they are intentionally held open and do not idle-close.
+func (a *Activities) ScanStuckOpen(ctx context.Context) error {
+	return a.runForeverWithInterval(ctx, func(scanCtx context.Context) error {
+		// two weeks ago (IdleTime, scaled by the configured buffer multiplier)
+		buffer := scheduler.DefaultTweakables.IdleTime * time.Duration(a.stuckOpenIdleTimeBufferMultiplier())
+		threshold := a.timeSource.Now().UTC().Add(-buffer).Format(time.RFC3339Nano)
+
+		// select schedules which went idle before two weeks ago
+		// and which are not paused and still appear to be running
+		query := fmt.Sprintf(
+			`%s < "%s" AND TemporalSchedulePaused = false AND ExecutionStatus = "Running"`,
+			scheduler.ScheduleIdleCloseTimeName,
+			threshold,
+		)
+		return a.runScan(scanCtx, "stuck_open", query, metrics.ScheduleInvariantsScannerStuckOpenCount.Name())
+	})
+}
+
+// ScanUnknownState is a long-running activity that periodically scans for running,
+// unpaused schedules with no TemporalScheduleNextActionTime set.
+//
+// The ScheduleIdleCloseTime IS NULL clause excludes schedules that have legitimately
+// run out of actions and are sitting in the idle/retention window before idle-close:
+// those are still Running and unpaused with no next action time, but carry a (future)
+// ScheduleIdleCloseTime. Without this clause every exhausted-but-retained schedule would
+// be a false positive for up to the retention window, and genuinely stuck-open schedules
+// (past ScheduleIdleCloseTime) would be double-counted with ScanStuckOpen.
+func (a *Activities) ScanUnknownState(ctx context.Context) error {
+	return a.runForeverWithInterval(ctx, func(scanCtx context.Context) error {
+		query := fmt.Sprintf(
+			`%s IS NULL AND %s IS NULL AND TemporalSchedulePaused = false AND ExecutionStatus = "Running"`,
+			scheduler.ScheduleNextActionTimeName,
+			scheduler.ScheduleIdleCloseTimeName,
+		)
+		return a.runScan(scanCtx, "unknown_state", query, metrics.ScheduleInvariantsScannerUnknownStateCount.Name())
+	})
+}
+
+// runForeverWithInterval runs scanFn immediately, then again on every tick of scanInterval, until
+// the activity context is canceled or scanFn returns an error. A background goroutine
+// emits heartbeats so the parent activity stays alive between scans. All timing flows
+// through the injected TimeSource so tests can drive the loop deterministically.
+func (a *Activities) runForeverWithInterval(ctx context.Context, scanFn func(context.Context) error) error {
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
+
+	go a.heartbeatLoop(ctx)
+
+	if err := scanFn(ctx); err != nil {
+		return err
+	}
+
+	ch, timer := a.timeSource.NewTimer(a.scanInterval())
+	defer timer.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		case <-ch:
+			if err := scanFn(ctx); err != nil {
+				return err
+			}
+			timer.Reset(a.scanInterval())
+		}
+	}
+}
+
+func (a *Activities) heartbeatLoop(ctx context.Context) {
+	ch, timer := a.timeSource.NewTimer(heartbeatInterval)
+	defer timer.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ch:
+			activity.RecordHeartbeat(ctx)
+			timer.Reset(heartbeatInterval)
+		}
+	}
+}
+
+// runScan ties the per-scanner pieces together: list namespaces, fan out a single
+// visibility query per namespace, and emit per-namespace metrics. Cluster-wide
+// totals are computed by the metrics backend by summing across the namespace tag.
+// Used by the count-only scanners (StuckOpen, UnknownState).
+func (a *Activities) runScan(ctx context.Context, subScanner, query, metricName string) error {
+	namespaces, err := a.ListAllNamespaces(ctx)
+	if err != nil {
+		return err
+	}
+
+	for _, nsName := range namespaces {
+		err := a.forEachNamespace(ctx, nsName, query, func(count int64) {
+			a.emitCount(metricName, nsName, count)
+		})
+		if err != nil {
+			a.recordScanError(nsName, subScanner, err)
+			continue
+		}
+	}
+	return nil
+}
+
+// runOverdueScan lists individual matching schedules per namespace, calls
+// DescribeSchedule on each, and filters out schedules that are paused or
+// expected to not be able to fire (BUFFER_ONE / BUFFER_ALL waiting on a prior
+// running workflow). What remains is counted as an anomaly.
+func (a *Activities) runOverdueScan(ctx context.Context, query string) error {
+	const subScanner = "overdue_next_action_time"
+	metricName := metrics.ScheduleInvariantsScannerOverdueNextActionTimeCount.Name()
+
+	namespaces, err := a.ListAllNamespaces(ctx)
+	if err != nil {
+		return err
+	}
+
+	for _, nsName := range namespaces {
+		var nsAnomalies int64
+		err := a.forEachScheduleInNamespace(ctx, nsName, query, func(scheduleID string) {
+			if a.scheduleIsExpectedNotToFire(ctx, nsName, scheduleID) {
+				return
+			}
+			nsAnomalies++
+			a.logger.Warn("schedule-invariants - the schedule was expected to fire but doesn't appear to have done so",
+				tag.WorkflowNamespace(nsName),
+				tag.ScheduleID(scheduleID))
+		})
+		if err != nil {
+			a.recordScanError(nsName, subScanner, err)
+			continue
+		}
+		a.emitCount(metricName, nsName, nsAnomalies)
+	}
+	return nil
+}
+
+// ListAllNamespaces returns the names of every namespace that exists in the cluster
+// metadata store and is active in the current cluster. Pagination is handled internally.
+func (a *Activities) ListAllNamespaces(ctx context.Context) ([]string, error) {
+	var names []string
+	var pageToken []byte
+	for {
+		nsResp, err := a.metadataManager.ListNamespaces(ctx, &persistence.ListNamespacesRequest{
+			PageSize:       namespaceListPageSize,
+			NextPageToken:  pageToken,
+			IncludeDeleted: false,
+		})
+		if err != nil {
+			return nil, err
+		}
+		for _, ns := range nsResp.Namespaces {
+			nsEntry, err := a.namespaceRegistry.GetNamespaceByID(namespace.ID(ns.Namespace.Info.Id))
+			if err != nil {
+				a.logger.Warn("namespace lookup failed, skipping",
+					tag.WorkflowNamespaceID(ns.Namespace.Info.Id), tag.Error(err))
+				continue
+			}
+			// Only the active cluster for this namespace should report; otherwise
+			// global namespaces would be reported once per cluster.
+			if !nsEntry.ActiveInCluster(a.currentClusterName) {
+				continue
+			}
+			names = append(names, nsEntry.Name().String())
+		}
+		pageToken = nsResp.NextPageToken
+		if len(pageToken) == 0 {
+			break
+		}
+	}
+	return names, nil
+}
+
+// forEachNamespace runs `query` against one namespace (scoped to the Scheduler archetype)
+// and passes the resulting count to resultsFilter. Rate-limited by the shared limiter.
+// Intended to be called once per namespace returned by ListAllNamespaces.
+//
+// Note: the parameter is named `nsName` rather than `namespace` to avoid shadowing the
+// imported `namespace` package used internally.
+func (a *Activities) forEachNamespace(
+	ctx context.Context,
+	nsName string,
+	query string,
+	resultsFilter func(count int64),
+) error {
+	if err := a.rateLimiter.Wait(ctx); err != nil {
+		if common.IsContextDeadlineExceededErr(err) {
+			return context.DeadlineExceeded
+		}
+		return err
+	}
+	nsID, err := a.namespaceRegistry.GetNamespaceID(namespace.Name(nsName))
+	if err != nil {
+		return err
+	}
+	resp, err := a.visibilityManager.CountChasmExecutions(ctx, &visibilityservice.CountChasmExecutionsRequest{
+		ArchetypeId: chasm.SchedulerArchetypeID,
+		NamespaceId: nsID.String(),
+		Namespace:   nsName,
+		Query:       query,
+	})
+	if err != nil {
+		return err
+	}
+	resultsFilter(resp.GetCount())
+	return nil
+}
+
+// forEachScheduleInNamespace paginates ListChasmExecutions for the given namespace
+// (scoped to the Scheduler archetype) and invokes visit for every schedule entity in
+// the result. The rate limiter is consumed per page.
+func (a *Activities) forEachScheduleInNamespace(
+	ctx context.Context,
+	nsName string,
+	query string,
+	visit func(scheduleID string),
+) error {
+	nsID, err := a.namespaceRegistry.GetNamespaceID(namespace.Name(nsName))
+	if err != nil {
+		return err
+	}
+
+	var pageToken []byte
+	for {
+		if err := a.rateLimiter.Wait(ctx); err != nil {
+			if common.IsContextDeadlineExceededErr(err) {
+				return context.DeadlineExceeded
+			}
+			return err
+		}
+		resp, err := a.visibilityManager.ListChasmExecutions(ctx, &visibilityservice.ListChasmExecutionsRequest{
+			ArchetypeId:   chasm.SchedulerArchetypeID,
+			NamespaceId:   nsID.String(),
+			Namespace:     nsName,
+			Query:         query,
+			PageSize:      scheduleListPageSize,
+			NextPageToken: pageToken,
+		})
+		if err != nil {
+			return err
+		}
+		for _, exec := range resp.GetExecutions() {
+			visit(exec.GetBusinessId())
+		}
+		pageToken = resp.GetNextPageToken()
+		if len(pageToken) == 0 {
+			return nil
+		}
+	}
+}
+
+// scheduleIsExpectedNotToFire returns true when a schedule flagged as "overdue" in
+// visibility is actually in a state that legitimately explains the stale NextActionTime:
+// either the schedule is paused, or it's holding under BUFFER_ONE / BUFFER_ALL with a
+// prior workflow still running.
+func (a *Activities) scheduleIsExpectedNotToFire(ctx context.Context, nsName, scheduleID string) bool {
+	if err := a.rateLimiter.Wait(ctx); err != nil {
+		a.logger.Warn("rate-limiter wait failed during DescribeSchedule; counting schedule as anomaly",
+			tag.WorkflowNamespace(nsName), tag.ScheduleID(scheduleID), tag.Error(err))
+		return false
+	}
+	desc, err := a.sdkClientFactory.GetSystemClient().WorkflowService().DescribeSchedule(ctx, &workflowservice.DescribeScheduleRequest{
+		Namespace:  nsName,
+		ScheduleId: scheduleID,
+	})
+	if err != nil {
+		a.logger.Warn("DescribeSchedule failed",
+			tag.WorkflowNamespace(nsName), tag.ScheduleID(scheduleID), tag.Error(err))
+		return false
+	}
+
+	if desc.GetSchedule().GetState().GetPaused() {
+		return true
+	}
+	overlap := desc.GetSchedule().GetPolicies().GetOverlapPolicy()
+	if (overlap == enumspb.SCHEDULE_OVERLAP_POLICY_BUFFER_ONE || overlap == enumspb.SCHEDULE_OVERLAP_POLICY_BUFFER_ALL) &&
+		len(desc.GetInfo().GetRunningWorkflows()) > 0 {
+		return true
+	}
+	return false
+}
+
+func (a *Activities) emitCount(metricName, namespaceTagValue string, count int64) {
+	if count <= 0 {
+		return
+	}
+	a.metricsHandler.
+		WithTags(metrics.NamespaceTag(namespaceTagValue)).
+		Counter(metricName).
+		Record(count)
+}
+
+func (a *Activities) recordScanError(nsName, subScanner string, err error) {
+	a.logger.Warn("schedule-invariants scan failed for namespace",
+		tag.WorkflowNamespace(nsName),
+		tag.NewStringTag("sub_scanner", subScanner),
+		tag.Error(err))
+	metrics.ScheduleInvariantsScannerErrorCount.With(a.metricsHandler.WithTags(
+		metrics.NamespaceTag(nsName),
+		metrics.StringTag("sub_scanner", subScanner),
+	)).Record(1)
+}
diff --git a/service/worker/scanner/scheduleinvariants/invariants_test.go b/service/worker/scanner/scheduleinvariants/invariants_test.go
new file mode 100644
index 00000000000..dcd72568033
--- /dev/null
+++ b/service/worker/scanner/scheduleinvariants/invariants_test.go
@@ -0,0 +1,432 @@
+package scheduleinvariants
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+	commonpb "go.temporal.io/api/common/v1"
+	enumspb "go.temporal.io/api/enums/v1"
+	schedulepb "go.temporal.io/api/schedule/v1"
+	"go.temporal.io/api/workflowservice/v1"
+	chasmspb "go.temporal.io/server/api/chasm/v1"
+	persistencespb "go.temporal.io/server/api/persistence/v1"
+	"go.temporal.io/server/api/visibilityservice/v1"
+	"go.temporal.io/server/chasm"
+	"go.temporal.io/server/common/clock"
+	"go.temporal.io/server/common/dynamicconfig"
+	"go.temporal.io/server/common/log"
+	"go.temporal.io/server/common/metrics"
+	"go.temporal.io/server/common/namespace"
+	"go.temporal.io/server/common/persistence"
+	"go.temporal.io/server/common/persistence/visibility/manager"
+	"go.temporal.io/server/common/quotas"
+	"go.temporal.io/server/common/sdk"
+	"go.temporal.io/server/common/testing/mockapi/workflowservicemock/v1"
+	"go.temporal.io/server/common/testing/mocksdk"
+	"go.uber.org/mock/gomock"
+)
+
+const testClusterName = "test-cluster"
+
+type testDeps struct {
+	ctrl              *gomock.Controller
+	metadataManager   *persistence.MockMetadataManager
+	visibilityManager *manager.MockVisibilityManager
+	namespaceRegistry *namespace.MockRegistry
+	sdkClientFactory  *sdk.MockClientFactory
+	sdkClient         *mocksdk.MockClient
+	frontendClient    *workflowservicemock.MockWorkflowServiceClient
+	timeSource        *clock.EventTimeSource
+}
+
+func newTestDeps(t *testing.T) *testDeps {
+	t.Helper()
+	ctrl := gomock.NewController(t)
+	d := &testDeps{
+		ctrl:              ctrl,
+		metadataManager:   persistence.NewMockMetadataManager(ctrl),
+		visibilityManager: manager.NewMockVisibilityManager(ctrl),
+		namespaceRegistry: namespace.NewMockRegistry(ctrl),
+		sdkClientFactory:  sdk.NewMockClientFactory(ctrl),
+		sdkClient:         mocksdk.NewMockClient(ctrl),
+		frontendClient:    workflowservicemock.NewMockWorkflowServiceClient(ctrl),
+		timeSource:        clock.NewEventTimeSource(),
+	}
+	// The DescribeSchedule path always goes via system client → frontend stub.
+	d.sdkClientFactory.EXPECT().GetSystemClient().Return(d.sdkClient).AnyTimes()
+	d.sdkClient.EXPECT().WorkflowService().Return(d.frontendClient).AnyTimes()
+	return d
+}
+
+func (d *testDeps) newActivities() *Activities {
+	// A very high RPS rate-limiter so Wait() never blocks under test.
+	rl := quotas.NewDefaultOutgoingRateLimiter(quotas.RateFn(dynamicconfig.GetFloatPropertyFn(10000.0)))
+	return &Activities{
+		logger:                            log.NewNoopLogger(),
+		metricsHandler:                    metrics.NoopMetricsHandler,
+		metadataManager:                   d.metadataManager,
+		visibilityManager:                 d.visibilityManager,
+		namespaceRegistry:                 d.namespaceRegistry,
+		sdkClientFactory:                  d.sdkClientFactory,
+		currentClusterName:                testClusterName,
+		timeSource:                        d.timeSource,
+		overdueTolerance:                  dynamicconfig.GetDurationPropertyFn(10 * time.Minute),
+		scanInterval:                      dynamicconfig.GetDurationPropertyFn(15 * time.Minute),
+		stuckOpenIdleTimeBufferMultiplier: dynamicconfig.GetIntPropertyFn(2),
+		rateLimiter:                       rl,
+	}
+}
+
+func nsDetail(id, name string) *persistence.GetNamespaceResponse {
+	return &persistence.GetNamespaceResponse{
+		Namespace: &persistencespb.NamespaceDetail{
+			Info: &persistencespb.NamespaceInfo{Id: id, Name: name},
+		},
+	}
+}
+
+func localNS(id, name, activeCluster string) *namespace.Namespace {
+	return namespace.NewLocalNamespaceForTest(
+		&persistencespb.NamespaceInfo{Id: id, Name: name},
+		nil,
+		activeCluster,
+	)
+}
+
+// globalNS builds a global (replicated) namespace whose active cluster is
+// activeCluster. Only global namespaces return false from ActiveInCluster when the
+// active cluster doesn't match; local namespaces are always "active" in every cluster.
+func globalNS(id, name, activeCluster string) *namespace.Namespace {
+	return namespace.NewGlobalNamespaceForTest(
+		&persistencespb.NamespaceInfo{Id: id, Name: name},
+		nil,
+		&persistencespb.NamespaceReplicationConfig{
+			ActiveClusterName: activeCluster,
+			Clusters:          []string{activeCluster, "other-cluster"},
+		},
+		0,
+	)
+}
+
+func TestListAllNamespaces_PaginatesAndFiltersInactive(t *testing.T) {
+	d := newTestDeps(t)
+
+	d.metadataManager.EXPECT().ListNamespaces(gomock.Any(), &persistence.ListNamespacesRequest{
+		PageSize:       namespaceListPageSize,
+		NextPageToken:  nil,
+		IncludeDeleted: false,
+	}).Return(&persistence.ListNamespacesResponse{
+		Namespaces:    []*persistence.GetNamespaceResponse{nsDetail("id-1", "ns-1"), nsDetail("id-2", "ns-2")},
+		NextPageToken: []byte("page2"),
+	}, nil)
+	d.metadataManager.EXPECT().ListNamespaces(gomock.Any(), &persistence.ListNamespacesRequest{
+		PageSize:       namespaceListPageSize,
+		NextPageToken:  []byte("page2"),
+		IncludeDeleted: false,
+	}).Return(&persistence.ListNamespacesResponse{
+		Namespaces:    []*persistence.GetNamespaceResponse{nsDetail("id-3", "ns-3")},
+		NextPageToken: nil,
+	}, nil)
+
+	d.namespaceRegistry.EXPECT().GetNamespaceByID(namespace.ID("id-1")).Return(globalNS("id-1", "ns-1", testClusterName), nil)
+	d.namespaceRegistry.EXPECT().GetNamespaceByID(namespace.ID("id-2")).Return(globalNS("id-2", "ns-2", "other-cluster"), nil)
+	d.namespaceRegistry.EXPECT().GetNamespaceByID(namespace.ID("id-3")).Return(globalNS("id-3", "ns-3", testClusterName), nil)
+
+	names, err := d.newActivities().ListAllNamespaces(context.Background())
+	require.NoError(t, err)
+	require.Equal(t, []string{"ns-1", "ns-3"}, names)
+}
+
+func TestListAllNamespaces_SkipsRegistryLookupFailures(t *testing.T) {
+	d := newTestDeps(t)
+
+	d.metadataManager.EXPECT().ListNamespaces(gomock.Any(), gomock.Any()).Return(&persistence.ListNamespacesResponse{
+		Namespaces: []*persistence.GetNamespaceResponse{
+			nsDetail("good", "good-ns"),
+			nsDetail("bad", "bad-ns"),
+		},
+		NextPageToken: nil,
+	}, nil)
+
+	d.namespaceRegistry.EXPECT().GetNamespaceByID(namespace.ID("good")).Return(localNS("good", "good-ns", testClusterName), nil)
+	d.namespaceRegistry.EXPECT().GetNamespaceByID(namespace.ID("bad")).Return(nil, errors.New("not found"))
+
+	names, err := d.newActivities().ListAllNamespaces(context.Background())
+	require.NoError(t, err)
+	require.Equal(t, []string{"good-ns"}, names)
+}
+
+func TestListAllNamespaces_ReturnsErrorOnMetadataFailure(t *testing.T) {
+	d := newTestDeps(t)
+	d.metadataManager.EXPECT().ListNamespaces(gomock.Any(), gomock.Any()).
+		Return(nil, errors.New("boom"))
+
+	_, err := d.newActivities().ListAllNamespaces(context.Background())
+	require.Error(t, err)
+}
+
+func TestForEachNamespace_InvokesCallbackWithCount(t *testing.T) {
+	d := newTestDeps(t)
+
+	d.namespaceRegistry.EXPECT().GetNamespaceID(namespace.Name("ns-1")).Return(namespace.ID("id-1"), nil)
+	d.visibilityManager.EXPECT().CountChasmExecutions(gomock.Any(), &visibilityservice.CountChasmExecutionsRequest{
+		ArchetypeId: chasm.SchedulerArchetypeID,
+		NamespaceId: "id-1",
+		Namespace:   "ns-1",
+		Query:       "some-query",
+	}).Return(&visibilityservice.CountChasmExecutionsResponse{Count: 7}, nil)
+
+	var got int64
+	err := d.newActivities().forEachNamespace(context.Background(), "ns-1", "some-query", func(count int64) {
+		got = count
+	})
+	require.NoError(t, err)
+	require.Equal(t, int64(7), got)
+}
+
+func TestForEachNamespace_PropagatesVisibilityError(t *testing.T) {
+	d := newTestDeps(t)
+
+	d.namespaceRegistry.EXPECT().GetNamespaceID(namespace.Name("ns-1")).Return(namespace.ID("id-1"), nil)
+	d.visibilityManager.EXPECT().CountChasmExecutions(gomock.Any(), gomock.Any()).
+		Return(nil, errors.New("count failed"))
+
+	called := false
+	err := d.newActivities().forEachNamespace(context.Background(), "ns-1", "q", func(count int64) {
+		called = true
+	})
+	require.Error(t, err)
+	require.False(t, called, "callback should not fire on error")
+}
+
+func chasmExec(id string) *chasmspb.VisibilityExecutionInfo {
+	return &chasmspb.VisibilityExecutionInfo{BusinessId: id}
+}
+
+func TestForEachScheduleInNamespace_PaginatesAndVisitsEachSchedule(t *testing.T) {
+	d := newTestDeps(t)
+
+	d.namespaceRegistry.EXPECT().GetNamespaceID(namespace.Name("ns-1")).Return(namespace.ID("id-1"), nil)
+
+	d.visibilityManager.EXPECT().ListChasmExecutions(gomock.Any(), &visibilityservice.ListChasmExecutionsRequest{
+		ArchetypeId:   chasm.SchedulerArchetypeID,
+		NamespaceId:   "id-1",
+		Namespace:     "ns-1",
+		Query:         "q",
+		PageSize:      scheduleListPageSize,
+		NextPageToken: nil,
+	}).Return(&visibilityservice.ListChasmExecutionsResponse{
+		Executions:    []*chasmspb.VisibilityExecutionInfo{chasmExec("sched-1"), chasmExec("sched-2")},
+		NextPageToken: []byte("p2"),
+	}, nil)
+	d.visibilityManager.EXPECT().ListChasmExecutions(gomock.Any(), &visibilityservice.ListChasmExecutionsRequest{
+		ArchetypeId:   chasm.SchedulerArchetypeID,
+		NamespaceId:   "id-1",
+		Namespace:     "ns-1",
+		Query:         "q",
+		PageSize:      scheduleListPageSize,
+		NextPageToken: []byte("p2"),
+	}).Return(&visibilityservice.ListChasmExecutionsResponse{
+		Executions:    []*chasmspb.VisibilityExecutionInfo{chasmExec("sched-3")},
+		NextPageToken: nil,
+	}, nil)
+
+	var visited []string
+	err := d.newActivities().forEachScheduleInNamespace(context.Background(), "ns-1", "q", func(scheduleID string) {
+		visited = append(visited, scheduleID)
+	})
+	require.NoError(t, err)
+	require.Equal(t, []string{"sched-1", "sched-2", "sched-3"}, visited)
+}
+
+func TestForEachScheduleInNamespace_StopsOnError(t *testing.T) {
+	d := newTestDeps(t)
+
+	d.namespaceRegistry.EXPECT().GetNamespaceID(namespace.Name("ns-1")).Return(namespace.ID("id-1"), nil)
+	d.visibilityManager.EXPECT().ListChasmExecutions(gomock.Any(), gomock.Any()).
+		Return(nil, errors.New("list failed"))
+
+	err := d.newActivities().forEachScheduleInNamespace(context.Background(), "ns-1", "q", func(string) {
+		t.Fatal("should not visit any schedule")
+	})
+	require.Error(t, err)
+}
+
+func describeResp(paused bool, overlap enumspb.ScheduleOverlapPolicy, runningCount int) *workflowservice.DescribeScheduleResponse {
+	resp := &workflowservice.DescribeScheduleResponse{
+		Schedule: &schedulepb.Schedule{
+			State:    &schedulepb.ScheduleState{Paused: paused},
+			Policies: &schedulepb.SchedulePolicies{OverlapPolicy: overlap},
+		},
+		Info: &schedulepb.ScheduleInfo{},
+	}
+	for range runningCount {
+		resp.Info.RunningWorkflows = append(resp.Info.RunningWorkflows, &commonpb.WorkflowExecution{WorkflowId: "running"})
+	}
+	return resp
+}
+
+func TestScheduleIsExpectedNotToFire(t *testing.T) {
+	cases := []struct {
+		name string
+		resp *workflowservice.DescribeScheduleResponse
+		err  error
+		want bool
+	}{
+		{
+			name: "paused",
+			resp: describeResp(true, enumspb.SCHEDULE_OVERLAP_POLICY_SKIP, 0),
+			want: true,
+		},
+		{
+			name: "buffer_one_with_running_workflow",
+			resp: describeResp(false, enumspb.SCHEDULE_OVERLAP_POLICY_BUFFER_ONE, 1),
+			want: true,
+		},
+		{
+			name: "buffer_all_with_running_workflow",
+			resp: describeResp(false, enumspb.SCHEDULE_OVERLAP_POLICY_BUFFER_ALL, 2),
+			want: true,
+		},
+		{
+			name: "buffer_one_no_running_workflow",
+			resp: describeResp(false, enumspb.SCHEDULE_OVERLAP_POLICY_BUFFER_ONE, 0),
+			want: false,
+		},
+		{
+			name: "skip_policy_with_running_workflow",
+			resp: describeResp(false, enumspb.SCHEDULE_OVERLAP_POLICY_SKIP, 1),
+			want: false,
+		},
+		{
+			name: "cancel_other_policy",
+			resp: describeResp(false, enumspb.SCHEDULE_OVERLAP_POLICY_CANCEL_OTHER, 1),
+			want: false,
+		},
+		{
+			name: "describe_error",
+			err:  errors.New("describe failed"),
+			want: false,
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			d := newTestDeps(t)
+			d.frontendClient.EXPECT().DescribeSchedule(gomock.Any(), &workflowservice.DescribeScheduleRequest{
+				Namespace:  "ns-1",
+				ScheduleId: "sched-1",
+			}).Return(tc.resp, tc.err)
+
+			got := d.newActivities().scheduleIsExpectedNotToFire(context.Background(), "ns-1", "sched-1")
+			require.Equal(t, tc.want, got)
+		})
+	}
+}
+
+func TestRunOverdueScan_FiltersExpectedNotToFireSchedulesAndCountsRest(t *testing.T) {
+	d := newTestDeps(t)
+
+	d.metadataManager.EXPECT().ListNamespaces(gomock.Any(), gomock.Any()).Return(&persistence.ListNamespacesResponse{
+		Namespaces:    []*persistence.GetNamespaceResponse{nsDetail("id-1", "ns-1")},
+		NextPageToken: nil,
+	}, nil)
+	d.namespaceRegistry.EXPECT().GetNamespaceByID(namespace.ID("id-1")).Return(localNS("id-1", "ns-1", testClusterName), nil)
+	d.namespaceRegistry.EXPECT().GetNamespaceID(namespace.Name("ns-1")).Return(namespace.ID("id-1"), nil)
+
+	d.visibilityManager.EXPECT().ListChasmExecutions(gomock.Any(), gomock.Any()).Return(&visibilityservice.ListChasmExecutionsResponse{
+		Executions: []*chasmspb.VisibilityExecutionInfo{
+			chasmExec("sched-paused"),
+			chasmExec("sched-buffer-waiting"),
+			chasmExec("sched-actually-overdue"),
+		},
+		NextPageToken: nil,
+	}, nil)
+
+	d.frontendClient.EXPECT().DescribeSchedule(gomock.Any(), &workflowservice.DescribeScheduleRequest{
+		Namespace: "ns-1", ScheduleId: "sched-paused",
+	}).Return(describeResp(true, enumspb.SCHEDULE_OVERLAP_POLICY_SKIP, 0), nil)
+	d.frontendClient.EXPECT().DescribeSchedule(gomock.Any(), &workflowservice.DescribeScheduleRequest{
+		Namespace: "ns-1", ScheduleId: "sched-buffer-waiting",
+	}).Return(describeResp(false, enumspb.SCHEDULE_OVERLAP_POLICY_BUFFER_ONE, 1), nil)
+	d.frontendClient.EXPECT().DescribeSchedule(gomock.Any(), &workflowservice.DescribeScheduleRequest{
+		Namespace: "ns-1", ScheduleId: "sched-actually-overdue",
+	}).Return(describeResp(false, enumspb.SCHEDULE_OVERLAP_POLICY_SKIP, 0), nil)
+
+	err := d.newActivities().runOverdueScan(context.Background(), "q")
+	require.NoError(t, err)
+}
+
+func TestRunOverdueScan_ContinuesPastPerNamespaceErrors(t *testing.T) {
+	d := newTestDeps(t)
+
+	d.metadataManager.EXPECT().ListNamespaces(gomock.Any(), gomock.Any()).Return(&persistence.ListNamespacesResponse{
+		Namespaces: []*persistence.GetNamespaceResponse{
+			nsDetail("id-broken", "ns-broken"),
+			nsDetail("id-ok", "ns-ok"),
+		},
+		NextPageToken: nil,
+	}, nil)
+	d.namespaceRegistry.EXPECT().GetNamespaceByID(namespace.ID("id-broken")).Return(localNS("id-broken", "ns-broken", testClusterName), nil)
+	d.namespaceRegistry.EXPECT().GetNamespaceByID(namespace.ID("id-ok")).Return(localNS("id-ok", "ns-ok", testClusterName), nil)
+
+	d.namespaceRegistry.EXPECT().GetNamespaceID(namespace.Name("ns-broken")).Return(namespace.ID("id-broken"), nil)
+	d.visibilityManager.EXPECT().ListChasmExecutions(gomock.Any(), gomock.AssignableToTypeOf(&visibilityservice.ListChasmExecutionsRequest{})).
+		DoAndReturn(func(_ context.Context, req *visibilityservice.ListChasmExecutionsRequest) (*visibilityservice.ListChasmExecutionsResponse, error) {
+			if req.Namespace == "ns-broken" {
+				return nil, errors.New("list failed")
+			}
+			return &visibilityservice.ListChasmExecutionsResponse{
+				Executions:    []*chasmspb.VisibilityExecutionInfo{chasmExec("sched-1")},
+				NextPageToken: nil,
+			}, nil
+		}).AnyTimes()
+
+	d.namespaceRegistry.EXPECT().GetNamespaceID(namespace.Name("ns-ok")).Return(namespace.ID("id-ok"), nil)
+	d.frontendClient.EXPECT().DescribeSchedule(gomock.Any(), gomock.Any()).
+		Return(describeResp(false, enumspb.SCHEDULE_OVERLAP_POLICY_SKIP, 0), nil)
+
+	err := d.newActivities().runOverdueScan(context.Background(), "q")
+	require.NoError(t, err)
+}
+
+func TestRunScan_EmitsPerNamespaceCounts(t *testing.T) {
+	d := newTestDeps(t)
+
+	d.metadataManager.EXPECT().ListNamespaces(gomock.Any(), gomock.Any()).Return(&persistence.ListNamespacesResponse{
+		Namespaces: []*persistence.GetNamespaceResponse{
+			nsDetail("id-1", "ns-1"),
+			nsDetail("id-2", "ns-2"),
+		},
+		NextPageToken: nil,
+	}, nil)
+	d.namespaceRegistry.EXPECT().GetNamespaceByID(namespace.ID("id-1")).Return(localNS("id-1", "ns-1", testClusterName), nil)
+	d.namespaceRegistry.EXPECT().GetNamespaceByID(namespace.ID("id-2")).Return(localNS("id-2", "ns-2", testClusterName), nil)
+
+	d.namespaceRegistry.EXPECT().GetNamespaceID(namespace.Name("ns-1")).Return(namespace.ID("id-1"), nil)
+	d.namespaceRegistry.EXPECT().GetNamespaceID(namespace.Name("ns-2")).Return(namespace.ID("id-2"), nil)
+	d.visibilityManager.EXPECT().CountChasmExecutions(gomock.Any(), gomock.Any()).
+		DoAndReturn(func(_ context.Context, req *visibilityservice.CountChasmExecutionsRequest) (*visibilityservice.CountChasmExecutionsResponse, error) {
+			switch req.Namespace {
+			case "ns-1":
+				return &visibilityservice.CountChasmExecutionsResponse{Count: 3}, nil
+			case "ns-2":
+				return &visibilityservice.CountChasmExecutionsResponse{Count: 0}, nil
+			}
+			return &visibilityservice.CountChasmExecutionsResponse{Count: 0}, nil
+		}).Times(2)
+
+	err := d.newActivities().runScan(context.Background(), "stuck_open", "q", metrics.ScheduleInvariantsScannerStuckOpenCount.Name())
+	require.NoError(t, err)
+}
+
+func TestEmitCount_IgnoresZeroAndNegative(t *testing.T) {
+	d := newTestDeps(t)
+	a := d.newActivities()
+	// emitCount is a no-op for count <= 0; mainly we verify it doesn't panic.
+	a.emitCount("metric", "ns", 0)
+	a.emitCount("metric", "ns", -1)
+	a.emitCount("metric", "ns", 5) // exercise positive path
+}
diff --git a/service/worker/scanner/scheduleinvariants/workflows.go b/service/worker/scanner/scheduleinvariants/workflows.go
new file mode 100644
index 00000000000..d029ffe5155
--- /dev/null
+++ b/service/worker/scanner/scheduleinvariants/workflows.go
@@ -0,0 +1,102 @@
+package scheduleinvariants
+
+import (
+	"time"
+
+	enumspb "go.temporal.io/api/enums/v1"
+	"go.temporal.io/sdk/client"
+	"go.temporal.io/sdk/temporal"
+	"go.temporal.io/sdk/workflow"
+)
+
+const (
+	OverdueNextActionTimeWorkflowName = "schedule-invariants-overdue-next-action-time-scanner"
+	OverdueNextActionTimeActivityName = "scan-schedule-invariants-overdue-next-action-time"
+	OverdueNextActionTimeWorkflowID   = "temporal-sys-schedule-invariants-overdue-next-action-time-scanner"
+	OverdueNextActionTimeTaskQueue    = "temporal-sys-schedule-invariants-overdue-next-action-time-scanner-taskqueue-0"
+
+	StuckOpenWorkflowName = "schedule-invariants-stuck-open-scanner"
+	StuckOpenActivityName = "scan-schedule-invariants-stuck-open"
+	StuckOpenWorkflowID   = "temporal-sys-schedule-invariants-stuck-open-scanner"
+	StuckOpenTaskQueue    = "temporal-sys-schedule-invariants-stuck-open-scanner-taskqueue-0"
+
+	UnknownStateWorkflowName = "schedule-invariants-unknown-state-scanner"
+	UnknownStateActivityName = "scan-schedule-invariants-unknown-state"
+	UnknownStateWorkflowID   = "temporal-sys-schedule-invariants-unknown-state-scanner"
+	UnknownStateTaskQueue    = "temporal-sys-schedule-invariants-unknown-state-scanner-taskqueue-0"
+
+	// activityStartToCloseTimeout is "effectively infinite" - the activity runs an
+	// internal scan ticker forever and is only ever expected to exit on transient
+	// failures (worker restart, network glitch, etc.).
+	activityStartToCloseTimeout = 365 * 24 * time.Hour
+	activityHeartbeatTimeout    = 30 * time.Second
+	// workflowRunTimeout bounds a single workflow run; the workflow continues-as-new
+	// after each activity exit, so this is just a safety net larger than the
+	// activity's own timeout plus retry budget.
+	workflowRunTimeout = 366 * 24 * time.Hour
+)
+
+var (
+	// retryPolicy applies to the long-running scan activity: 10 attempts spaced
+	// 5 minutes apart with no exponential backoff. Once exhausted the workflow
+	// loops via continue-as-new.
+	retryPolicy = &temporal.RetryPolicy{
+		InitialInterval:    5 * time.Minute,
+		BackoffCoefficient: 1.0,
+		MaximumAttempts:    10,
+	}
+
+	OverdueNextActionTimeWFStartOptions = client.StartWorkflowOptions{
+		ID:                    OverdueNextActionTimeWorkflowID,
+		TaskQueue:             OverdueNextActionTimeTaskQueue,
+		WorkflowIDReusePolicy: enumspb.WORKFLOW_ID_REUSE_POLICY_ALLOW_DUPLICATE,
+		WorkflowRunTimeout:    workflowRunTimeout,
+	}
+	StuckOpenWFStartOptions = client.StartWorkflowOptions{
+		ID:                    StuckOpenWorkflowID,
+		TaskQueue:             StuckOpenTaskQueue,
+		WorkflowIDReusePolicy: enumspb.WORKFLOW_ID_REUSE_POLICY_ALLOW_DUPLICATE,
+		WorkflowRunTimeout:    workflowRunTimeout,
+	}
+	UnknownStateWFStartOptions = client.StartWorkflowOptions{
+		ID:                    UnknownStateWorkflowID,
+		TaskQueue:             UnknownStateTaskQueue,
+		WorkflowIDReusePolicy: enumspb.WORKFLOW_ID_REUSE_POLICY_ALLOW_DUPLICATE,
+		WorkflowRunTimeout:    workflowRunTimeout,
+	}
+)
+
+// OverdueNextActionTimeWorkflow scans for schedules whose TemporalScheduleNextActionTime
+// is in the past beyond the configured tolerance. The activity runs an internal scan
+// ticker; this workflow continues-as-new each time the activity exits.
+func OverdueNextActionTimeWorkflow(ctx workflow.Context) error {
+	return runScanActivity(ctx, OverdueNextActionTimeActivityName, OverdueNextActionTimeWorkflow)
+}
+
+// StuckOpenWorkflow scans for schedules that appear stuck open long after their CloseTime.
+func StuckOpenWorkflow(ctx workflow.Context) error {
+	return runScanActivity(ctx, StuckOpenActivityName, StuckOpenWorkflow)
+}
+
+// UnknownStateWorkflow scans for running, unpaused schedules with no
+// TemporalScheduleNextActionTime.
+func UnknownStateWorkflow(ctx workflow.Context) error {
+	return runScanActivity(ctx, UnknownStateActivityName, UnknownStateWorkflow)
+}
+
+// runScanActivity executes the named long-running scan activity. The activity is
+// expected to outlive a single workflow run only on the steady-state path; when it
+// does eventually exit (retry budget exhausted, worker restart, etc.) the workflow
+// continues-as-new so the scan loop is reborn.
+func runScanActivity(ctx workflow.Context, activityName string, wfFn any) error {
+	activityCtx := workflow.WithActivityOptions(ctx, workflow.ActivityOptions{
+		StartToCloseTimeout: activityStartToCloseTimeout,
+		HeartbeatTimeout:    activityHeartbeatTimeout,
+		RetryPolicy:         retryPolicy,
+	})
+	// Ignore the activity result intentionally: the activity is expected to exit
+	// eventually (heartbeat timeout on worker restart, retry budget exhausted, etc.),
+	// and we want to continue-as-new and rebirth the scan loop in every case.
+	_ = workflow.ExecuteActivity(activityCtx, activityName).Get(ctx, nil)
+	return workflow.NewContinueAsNewError(ctx, wfFn)
+}
diff --git a/service/worker/scheduler/buffer.go b/service/worker/scheduler/buffer.go
index ddea9f2a83c..d2a20a2ecdb 100644
--- a/service/worker/scheduler/buffer.go
+++ b/service/worker/scheduler/buffer.go
@@ -22,7 +22,8 @@ type (
 		NeedCancel    bool
 		NeedTerminate bool
 		// Stats
-		OverlapSkipped int64
+		OverlapSkipped         int64
+		OverlapSkippedByPolicy map[enumspb.ScheduleOverlapPolicy]int64
 	}
 )
 
@@ -40,6 +41,7 @@ func ProcessBuffer[T Overlappable](
 	// affect them.
 
 	var action ProcessBufferResult[T]
+	action.OverlapSkippedByPolicy = make(map[enumspb.ScheduleOverlapPolicy]int64)
 	var zeroVal T
 
 	for _, start := range buffer {
@@ -64,12 +66,14 @@ func ProcessBuffer[T Overlappable](
 		case enumspb.SCHEDULE_OVERLAP_POLICY_SKIP:
 			// just skip
 			action.OverlapSkipped++
+			action.OverlapSkippedByPolicy[overlapPolicy]++
 		case enumspb.SCHEDULE_OVERLAP_POLICY_BUFFER_ONE:
 			// allow one (the first one) in the buffer
 			if len(action.NewBuffer) == 0 {
 				action.NewBuffer = append(action.NewBuffer, start)
 			} else {
 				action.OverlapSkipped++
+				action.OverlapSkippedByPolicy[overlapPolicy]++
 			}
 		case enumspb.SCHEDULE_OVERLAP_POLICY_BUFFER_ALL:
 			// always add to buffer
diff --git a/service/worker/scheduler/buffer_test.go b/service/worker/scheduler/buffer_test.go
index 85750e5cc9b..af48ac60624 100644
--- a/service/worker/scheduler/buffer_test.go
+++ b/service/worker/scheduler/buffer_test.go
@@ -51,6 +51,10 @@ func (s *processBufferSuite) TestProcessSkipRunning() {
 	s.Empty(action.NewBuffer)
 	s.False(action.NeedCancel)
 	s.False(action.NeedTerminate)
+	s.Equal(int64(3), action.OverlapSkipped)
+	s.Equal(map[enumspb.ScheduleOverlapPolicy]int64{
+		enumspb.SCHEDULE_OVERLAP_POLICY_SKIP: 3,
+	}, action.OverlapSkippedByPolicy)
 }
 
 func (s *processBufferSuite) TestProcessSkipNotRunning() {
@@ -71,6 +75,10 @@ func (s *processBufferSuite) TestProcessBufferOneRunning() {
 	s.Equal([]int{3}, jobIds(action.NewBuffer))
 	s.False(action.NeedCancel)
 	s.False(action.NeedTerminate)
+	s.Equal(int64(2), action.OverlapSkipped)
+	s.Equal(map[enumspb.ScheduleOverlapPolicy]int64{
+		enumspb.SCHEDULE_OVERLAP_POLICY_BUFFER_ONE: 2,
+	}, action.OverlapSkippedByPolicy)
 }
 
 func (s *processBufferSuite) TestProcessBufferOneNotRunning() {
@@ -168,4 +176,28 @@ func (s *processBufferSuite) TestProcessWithResolve() {
 	s.False(action.NeedTerminate)
 }
 
-// TODO: add test cases for mixed policies
+func (s *processBufferSuite) TestProcessMixedPoliciesTracksSkippedByResolvedPolicy() {
+	buffer := []*job{
+		{3, enumspb.SCHEDULE_OVERLAP_POLICY_SKIP},
+		{5, enumspb.SCHEDULE_OVERLAP_POLICY_BUFFER_ONE},
+		{7, enumspb.SCHEDULE_OVERLAP_POLICY_UNSPECIFIED},
+		{9, enumspb.SCHEDULE_OVERLAP_POLICY_UNSPECIFIED},
+	}
+	resolve := func(policy enumspb.ScheduleOverlapPolicy) enumspb.ScheduleOverlapPolicy {
+		if policy == enumspb.SCHEDULE_OVERLAP_POLICY_UNSPECIFIED {
+			return enumspb.SCHEDULE_OVERLAP_POLICY_SKIP
+		}
+		return policy
+	}
+
+	action := ProcessBuffer(buffer, true, resolve)
+	s.Empty(action.OverlappingStarts)
+	s.Nil(action.NonOverlappingStart)
+	s.Equal([]int{5}, jobIds(action.NewBuffer))
+	s.False(action.NeedCancel)
+	s.False(action.NeedTerminate)
+	s.Equal(int64(3), action.OverlapSkipped)
+	s.Equal(map[enumspb.ScheduleOverlapPolicy]int64{
+		enumspb.SCHEDULE_OVERLAP_POLICY_SKIP: 3,
+	}, action.OverlapSkippedByPolicy)
+}
diff --git a/service/worker/scheduler/fx.go b/service/worker/scheduler/fx.go
index 3389361a852..fd8d755342a 100644
--- a/service/worker/scheduler/fx.go
+++ b/service/worker/scheduler/fx.go
@@ -49,13 +49,15 @@ var VisibilityListQueryChasm = fmt.Sprintf(
 
 type (
 	workerComponent struct {
-		specBuilder              *SpecBuilder // workflow dep
-		activityDeps             activityDeps
-		enabledForNs             dynamicconfig.BoolPropertyFnWithNamespaceFilter
-		enableCHASMMigration     dynamicconfig.BoolPropertyFnWithNamespaceFilter
-		globalNSStartWorkflowRPS dynamicconfig.TypedSubscribableWithNamespaceFilter[float64]
-		maxBlobSize              dynamicconfig.IntPropertyFnWithNamespaceFilter
-		localActivitySleepLimit  dynamicconfig.DurationPropertyFnWithNamespaceFilter
+		specBuilder                  *SpecBuilder // workflow dep
+		activityDeps                 activityDeps
+		enabledForNs                 dynamicconfig.BoolPropertyFnWithNamespaceFilter
+		enableCHASMMigration         dynamicconfig.BoolPropertyFnWithNamespaceFilter
+		chasmMigrationRolloutPercent dynamicconfig.IntPropertyFnWithNamespaceFilter
+		migrateWithRunningWorkflows  dynamicconfig.BoolPropertyFnWithNamespaceFilter
+		globalNSStartWorkflowRPS     dynamicconfig.TypedSubscribableWithNamespaceFilter[float64]
+		maxBlobSize                  dynamicconfig.IntPropertyFnWithNamespaceFilter
+		localActivitySleepLimit      dynamicconfig.DurationPropertyFnWithNamespaceFilter
 	}
 
 	activityDeps struct {
@@ -85,13 +87,15 @@ func NewResult(
 ) fxResult {
 	return fxResult{
 		Component: &workerComponent{
-			specBuilder:              specBuilder,
-			activityDeps:             params,
-			enabledForNs:             dynamicconfig.WorkerEnableScheduler.Get(dc),
-			enableCHASMMigration:     dynamicconfig.EnableCHASMSchedulerMigration.Get(dc),
-			globalNSStartWorkflowRPS: dynamicconfig.SchedulerNamespaceStartWorkflowRPS.Subscribe(dc),
-			maxBlobSize:              dynamicconfig.BlobSizeLimitError.Get(dc),
-			localActivitySleepLimit:  dynamicconfig.SchedulerLocalActivitySleepLimit.Get(dc),
+			specBuilder:                  specBuilder,
+			activityDeps:                 params,
+			enabledForNs:                 dynamicconfig.WorkerEnableScheduler.Get(dc),
+			enableCHASMMigration:         dynamicconfig.EnableCHASMSchedulerMigration.Get(dc),
+			chasmMigrationRolloutPercent: dynamicconfig.CHASMSchedulerMigrationRolloutPercent.Get(dc),
+			migrateWithRunningWorkflows:  dynamicconfig.EnableCHASMSchedulerMigrationWithRunningWorkflows.Get(dc),
+			globalNSStartWorkflowRPS:     dynamicconfig.SchedulerNamespaceStartWorkflowRPS.Subscribe(dc),
+			maxBlobSize:                  dynamicconfig.BlobSizeLimitError.Get(dc),
+			localActivitySleepLimit:      dynamicconfig.SchedulerLocalActivitySleepLimit.Get(dc),
 		},
 	}
 }
@@ -103,9 +107,17 @@ func (s *workerComponent) DedicatedWorkerOptions(ns *namespace.Namespace) *worke
 }
 
 func (s *workerComponent) Register(registry sdkworker.Registry, ns *namespace.Namespace, details workercommon.RegistrationDetails) func() {
-	enableMigration := s.enableCHASMMigration(ns.Name().String())
+	nsName := ns.Name().String()
 	wfFunc := func(ctx workflow.Context, args *schedulespb.StartScheduleArgs) error {
-		return schedulerWorkflowWithSpecBuilder(ctx, args, s.specBuilder, enableMigration)
+		key := fmt.Appendf(nil, "%s\x00%s", nsName, args.State.ScheduleId)
+		enableMigration := func() bool {
+			return s.enableCHASMMigration(nsName) &&
+				dynamicconfig.RolloutAccepts(key, s.chasmMigrationRolloutPercent(nsName))
+		}
+		migrateWithRunningWorkflows := func() bool {
+			return s.migrateWithRunningWorkflows(nsName)
+		}
+		return schedulerWorkflowWithSpecBuilder(ctx, args, s.specBuilder, enableMigration, migrateWithRunningWorkflows)
 	}
 	registry.RegisterWorkflowWithOptions(wfFunc, workflow.RegisterOptions{Name: WorkflowType})
 
diff --git a/service/worker/scheduler/workflow.go b/service/worker/scheduler/workflow.go
index 6c235d587cc..b647c415d7b 100644
--- a/service/worker/scheduler/workflow.go
+++ b/service/worker/scheduler/workflow.go
@@ -112,9 +112,12 @@ type (
 		// SpecBuilder is technically a non-deterministic dependency, but it's safe as
 		// long as we only call methods on cspec inside of SideEffect (or in a query
 		// without modifying state).
-		specBuilder          *SpecBuilder
-		cspec                *CompiledSpec
-		enableCHASMMigration bool
+		specBuilder *SpecBuilder
+		cspec       *CompiledSpec
+		// enableCHASMMigration and migrateWithRunningWorkflows are re-evaluated
+		// every iteration inside the "tweakables" MutableSideEffect.
+		enableCHASMMigration        func() bool
+		migrateWithRunningWorkflows func() bool
 
 		tweakables TweakablePolicies
 
@@ -159,7 +162,8 @@ type (
 		Version                           SchedulerWorkflowVersion // Used to keep track of schedules version to release new features and for backward compatibility
 		// version 0 corresponds to the schedule version that comes before introducing the Version parameter
 
-		EnableCHASMMigration bool // Whether to automatically migrate this schedule to CHASM (V2)
+		EnableCHASMMigration        bool // Whether to automatically migrate this schedule to CHASM (V2)
+		MigrateWithRunningWorkflows bool // Whether to migrate this schedule to CHASM (V2) while it has running workflows
 
 		// When introducing a new field with new workflow logic, consider generating a new
 		// history for TestReplays using generate_history.sh.
@@ -224,10 +228,11 @@ var (
 )
 
 func SchedulerWorkflow(ctx workflow.Context, args *schedulespb.StartScheduleArgs) error {
-	return schedulerWorkflowWithSpecBuilder(ctx, args, NewSpecBuilder(), false)
+	disabled := func() bool { return false }
+	return schedulerWorkflowWithSpecBuilder(ctx, args, NewSpecBuilder(), disabled, disabled)
 }
 
-func schedulerWorkflowWithSpecBuilder(ctx workflow.Context, args *schedulespb.StartScheduleArgs, specBuilder *SpecBuilder, enableCHASMMigration bool) error {
+func schedulerWorkflowWithSpecBuilder(ctx workflow.Context, args *schedulespb.StartScheduleArgs, specBuilder *SpecBuilder, enableCHASMMigration func() bool, migrateWithRunningWorkflows func() bool) error {
 	scheduler := &scheduler{
 		StartScheduleArgs: args,
 		ctx:               ctx,
@@ -237,8 +242,9 @@ func schedulerWorkflowWithSpecBuilder(ctx workflow.Context, args *schedulespb.St
 			"namespace":                args.State.Namespace,
 			metrics.ScheduleBackendTag: metrics.ScheduleBackendLegacy,
 		}),
-		specBuilder:          specBuilder,
-		enableCHASMMigration: enableCHASMMigration,
+		specBuilder:                 specBuilder,
+		enableCHASMMigration:        enableCHASMMigration,
+		migrateWithRunningWorkflows: migrateWithRunningWorkflows,
 	}
 	return scheduler.run()
 }
@@ -322,7 +328,8 @@ func (s *scheduler) run() error {
 			)
 		}
 
-		if s.tweakables.EnableCHASMMigration {
+		if !s.State.PendingMigration && s.tweakables.EnableCHASMMigration &&
+			(s.tweakables.MigrateWithRunningWorkflows || len(s.Info.RunningWorkflows) == 0) {
 			s.State.PendingMigration = true
 		}
 		if s.State.PendingMigration {
@@ -678,6 +685,7 @@ func (s *scheduler) processTimeRange(
 	}
 
 	lastAction := start
+	recordedGenerateLatency := false
 	var next GetNextTimeResult
 	for next = s.getNextTime(start); !(next.Next.IsZero() || next.Next.After(end)); next = s.getNextTime(next.Next) {
 		if !s.hasMinVersion(BatchAndCacheTimeQueries) && !s.canTakeScheduledAction(manual, false) {
@@ -689,9 +697,21 @@ func (s *scheduler) processTimeRange(
 			// hasMinVersion because this condition couldn't happen in previous versions.
 			continue
 		}
+		// Record generate latency only for the first action in the batch to
+		// avoid inflating the metric when catching up over a large time range.
+		if !manual && !recordedGenerateLatency {
+			s.metrics.Timer(metrics.ScheduleGenerateLatency.Name()).Record(end.Sub(next.Next))
+			recordedGenerateLatency = true
+		}
 		if !manual && end.Sub(next.Next) > catchupWindow {
 			s.logger.Warn("Schedule missed catchup window", "now", end, "time", next.Next)
-			s.metrics.Counter(metrics.ScheduleMissedCatchupWindow.Name()).Inc(1)
+			// Action's nominal time was already past the catchup window when
+			// the scheduler woke up. It was never buffered for execution.
+			// action_running is not included: the action was never a candidate
+			// for execution, so whether something is running is irrelevant.
+			s.metrics.WithTags(map[string]string{
+				metrics.ScheduleMissedReasonTag: metrics.ScheduleMissedReasonNotBuffered,
+			}).Counter(metrics.ScheduleMissedCatchupWindow.Name()).Inc(1)
 			s.Info.MissedCatchupWindow++
 			continue
 		}
@@ -914,7 +934,11 @@ func (s *scheduler) processWatcherResult(id string, f workflow.Future, long bool
 		}
 	}
 
-	// Update desired time of next start if it's buffered. This is used for metrics only.
+	// Update desired time of next start if it's buffered. This is used to
+	// compute ScheduleActionDelay (time between completing one action and
+	// starting the next). The legacy path doesn't use deferred starts
+	// (Attempt == -1) like CHASM, so BufferedStarts[0] is always the next
+	// pending start.
 	if long && len(s.State.BufferedStarts) > 0 {
 		s.State.BufferedStarts[0].DesiredTime = res.CloseTime
 	}
@@ -1265,10 +1289,11 @@ func (s *scheduler) checkConflict(token int64) error {
 
 func (s *scheduler) updateTweakables() {
 	// Use MutableSideEffect so that we can change the defaults without breaking determinism.
-	enableCHASMMigration := s.enableCHASMMigration
 	get := func(ctx workflow.Context) any {
 		p := CurrentTweakablePolicies
-		p.EnableCHASMMigration = enableCHASMMigration
+		// Re-evaluates migration dynamic config each iteration.
+		p.EnableCHASMMigration = s.enableCHASMMigration()
+		p.MigrateWithRunningWorkflows = s.migrateWithRunningWorkflows()
 		return p
 	}
 	eq := func(a, b any) bool { return a.(TweakablePolicies) == b.(TweakablePolicies) }
@@ -1346,6 +1371,11 @@ func (s *scheduler) processBuffer() bool {
 
 	s.State.BufferedStarts = action.NewBuffer
 	s.Info.OverlapSkipped += action.OverlapSkipped
+	for overlapPolicy, count := range action.OverlapSkippedByPolicy {
+		s.metrics.WithTags(map[string]string{
+			metrics.ScheduleOverlapPolicyTag: overlapPolicy.String(),
+		}).Counter(metrics.ScheduleOverlapSkipped.Name()).Inc(count)
+	}
 
 	// Try starting whatever we're supposed to start now
 	allStarts := action.OverlappingStarts
@@ -1493,6 +1523,8 @@ func (s *scheduler) startWorkflow(
 			// record metric only for _scheduled_ actions, not trigger/backfill, otherwise it's not meaningful
 			desiredTime := cmp.Or(start.DesiredTime, start.ActualTime)
 			s.metrics.Timer(metrics.ScheduleActionDelay.Name()).Record(res.RealStartTime.AsTime().Sub(desiredTime.AsTime()))
+			// Record total delay from original schedule time, including any overlap policy wait.
+			s.metrics.Timer(metrics.ScheduleActionE2EDelay.Name()).Record(res.RealStartTime.AsTime().Sub(start.ActualTime.AsTime()))
 		}
 
 		actionResult := &schedulepb.ScheduleActionResult{
diff --git a/service/worker/scheduler/workflow_test.go b/service/worker/scheduler/workflow_test.go
index 2d6d43bbc2e..34e40992d88 100644
--- a/service/worker/scheduler/workflow_test.go
+++ b/service/worker/scheduler/workflow_test.go
@@ -2275,14 +2275,19 @@ func (s *workflowSuite) TestMigrateSuccess() {
 	// Mock MigrateSchedule activity to succeed.
 	s.env.OnActivity(new(activities).MigrateScheduleToChasm, mock.Anything, mock.Anything).Once().Return(nil)
 
-	// Send migrate signal after the first iteration.
+	// Enable migration and request it via signal after the first iteration.
+	enableMigration := false
 	s.env.RegisterDelayedCallback(func() {
+		enableMigration = true
 		s.env.SignalWorkflow(SignalNameMigrateToChasm, nil)
 	}, 1*time.Second)
 
 	CurrentTweakablePolicies.IterationsBeforeContinueAsNew = 100
 	s.env.SetStartTime(baseStartTime)
-	s.env.ExecuteWorkflow(SchedulerWorkflow, &schedulespb.StartScheduleArgs{
+	s.env.ExecuteWorkflow(func(ctx workflow.Context, args *schedulespb.StartScheduleArgs) error {
+		return schedulerWorkflowWithSpecBuilder(ctx, args, NewSpecBuilder(),
+			func() bool { return enableMigration }, func() bool { return true })
+	}, &schedulespb.StartScheduleArgs{
 		Schedule: &schedulepb.Schedule{
 			Spec: &schedulepb.ScheduleSpec{
 				Interval: []*schedulepb.IntervalSpec{{
@@ -2314,8 +2319,10 @@ func (s *workflowSuite) TestMigrateFailure() {
 			return errors.New("migration failed")
 		})
 
-	// Send migrate signal after the first iteration.
+	// Enable migration and request it via signal after the first iteration.
+	enableMigration := false
 	s.env.RegisterDelayedCallback(func() {
+		enableMigration = true
 		s.env.SignalWorkflow(SignalNameMigrateToChasm, nil)
 	}, 1*time.Second)
 
@@ -2328,7 +2335,10 @@ func (s *workflowSuite) TestMigrateFailure() {
 
 	CurrentTweakablePolicies.IterationsBeforeContinueAsNew = 100
 	s.env.SetStartTime(baseStartTime)
-	s.env.ExecuteWorkflow(SchedulerWorkflow, &schedulespb.StartScheduleArgs{
+	s.env.ExecuteWorkflow(func(ctx workflow.Context, args *schedulespb.StartScheduleArgs) error {
+		return schedulerWorkflowWithSpecBuilder(ctx, args, NewSpecBuilder(),
+			func() bool { return enableMigration }, func() bool { return true })
+	}, &schedulespb.StartScheduleArgs{
 		Schedule: &schedulepb.Schedule{
 			Spec: &schedulepb.ScheduleSpec{
 				Interval: []*schedulepb.IntervalSpec{{
@@ -2370,14 +2380,19 @@ func (s *workflowSuite) TestMigrateFailureThenRetrySuccess() {
 			return nil
 		})
 
-	// Send migrate signal after the first iteration.
+	// Enable migration and request it via signal after the first iteration.
+	enableMigration := false
 	s.env.RegisterDelayedCallback(func() {
+		enableMigration = true
 		s.env.SignalWorkflow(SignalNameMigrateToChasm, nil)
 	}, 1*time.Second)
 
 	CurrentTweakablePolicies.IterationsBeforeContinueAsNew = 100
 	s.env.SetStartTime(baseStartTime)
-	s.env.ExecuteWorkflow(SchedulerWorkflow, &schedulespb.StartScheduleArgs{
+	s.env.ExecuteWorkflow(func(ctx workflow.Context, args *schedulespb.StartScheduleArgs) error {
+		return schedulerWorkflowWithSpecBuilder(ctx, args, NewSpecBuilder(),
+			func() bool { return enableMigration }, func() bool { return true })
+	}, &schedulespb.StartScheduleArgs{
 		Schedule: &schedulepb.Schedule{
 			Spec: &schedulepb.ScheduleSpec{
 				Interval: []*schedulepb.IntervalSpec{{
@@ -2410,8 +2425,10 @@ func (s *workflowSuite) TestMigrateFailureThenSignal() {
 			return errors.New("migration failed")
 		})
 
-	// Send migrate signal after the first iteration.
+	// Enable migration and request it via signal after the first iteration.
+	enableMigration := false
 	s.env.RegisterDelayedCallback(func() {
+		enableMigration = true
 		s.env.SignalWorkflow(SignalNameMigrateToChasm, nil)
 	}, 1*time.Second)
 	// After migration failure, send a pause patch and verify it's processed,
@@ -2434,7 +2451,10 @@ func (s *workflowSuite) TestMigrateFailureThenSignal() {
 
 	CurrentTweakablePolicies.IterationsBeforeContinueAsNew = 100
 	s.env.SetStartTime(baseStartTime)
-	s.env.ExecuteWorkflow(SchedulerWorkflow, &schedulespb.StartScheduleArgs{
+	s.env.ExecuteWorkflow(func(ctx workflow.Context, args *schedulespb.StartScheduleArgs) error {
+		return schedulerWorkflowWithSpecBuilder(ctx, args, NewSpecBuilder(),
+			func() bool { return enableMigration }, func() bool { return true })
+	}, &schedulespb.StartScheduleArgs{
 		Schedule: &schedulepb.Schedule{
 			Spec: &schedulepb.ScheduleSpec{
 				Interval: []*schedulepb.IntervalSpec{{
@@ -2476,7 +2496,7 @@ func (s *workflowSuite) TestMigrateDynamicConfig() {
 
 	s.env.SetStartTime(baseStartTime)
 	s.env.ExecuteWorkflow(func(ctx workflow.Context, args *schedulespb.StartScheduleArgs) error {
-		return schedulerWorkflowWithSpecBuilder(ctx, args, NewSpecBuilder(), true)
+		return schedulerWorkflowWithSpecBuilder(ctx, args, NewSpecBuilder(), func() bool { return true }, func() bool { return true })
 	}, &schedulespb.StartScheduleArgs{
 		Schedule: &schedulepb.Schedule{
 			Spec: &schedulepb.ScheduleSpec{
@@ -2499,6 +2519,55 @@ func (s *workflowSuite) TestMigrateDynamicConfig() {
 	s.NoError(s.env.GetWorkflowError())
 }
 
+// TestMigrateDynamicConfigFlipsMidRun verifies that the enableCHASMMigration
+// closure is re-evaluated every iteration: the first iteration sees migration
+// disabled and does nothing, then the DC flips on, and a later iteration picks
+// it up and migrates without needing continue-as-new.
+func (s *workflowSuite) TestMigrateDynamicConfigFlipsMidRun() {
+	enabled := false
+	migrateCalls := 0
+	s.env.OnActivity(new(activities).MigrateScheduleToChasm, mock.Anything, mock.Anything).Return(
+		func(context.Context, *schedulerpb.CreateFromMigrationStateRequest) error {
+			migrateCalls++
+			return nil
+		})
+
+	prevTweakables := CurrentTweakablePolicies
+	// Enough iterations to span the flip; allow short intervals so the loop
+	// advances quickly under test time.
+	CurrentTweakablePolicies.IterationsBeforeContinueAsNew = 10
+	defer func() { CurrentTweakablePolicies = prevTweakables }()
+
+	// Flip the closure between iteration 1 and iteration 2 (1h interval).
+	s.env.RegisterDelayedCallback(func() {
+		enabled = true
+	}, 30*time.Minute)
+
+	s.env.SetStartTime(baseStartTime)
+	s.env.ExecuteWorkflow(func(ctx workflow.Context, args *schedulespb.StartScheduleArgs) error {
+		return schedulerWorkflowWithSpecBuilder(ctx, args, NewSpecBuilder(), func() bool { return enabled }, func() bool { return true })
+	}, &schedulespb.StartScheduleArgs{
+		Schedule: &schedulepb.Schedule{
+			Spec: &schedulepb.ScheduleSpec{
+				Interval: []*schedulepb.IntervalSpec{{
+					Interval: durationpb.New(1 * time.Hour),
+				}},
+			},
+			Action: s.defaultAction("myid"),
+		},
+		State: &schedulespb.InternalState{
+			Namespace:     "myns",
+			NamespaceId:   "mynsid",
+			ScheduleId:    "myschedule",
+			ConflictToken: InitialConflictToken,
+		},
+	})
+
+	s.True(s.env.IsWorkflowCompleted())
+	s.Require().NoError(s.env.GetWorkflowError(), "workflow should complete after the dynamic flip triggers migration")
+	s.Equal(1, migrateCalls, "migration should fire exactly once, after the DC flips")
+}
+
 func (s *workflowSuite) TestMigrateDynamicConfigFailure() {
 	// Enable migration by threading enableCHASMMigration=true through the closure (race-safe),
 	// but activity fails.
@@ -2515,7 +2584,7 @@ func (s *workflowSuite) TestMigrateDynamicConfigFailure() {
 
 	s.env.SetStartTime(baseStartTime)
 	s.env.ExecuteWorkflow(func(ctx workflow.Context, args *schedulespb.StartScheduleArgs) error {
-		return schedulerWorkflowWithSpecBuilder(ctx, args, NewSpecBuilder(), true)
+		return schedulerWorkflowWithSpecBuilder(ctx, args, NewSpecBuilder(), func() bool { return true }, func() bool { return true })
 	}, &schedulespb.StartScheduleArgs{
 		Schedule: &schedulepb.Schedule{
 			Spec: &schedulepb.ScheduleSpec{
diff --git a/service/worker/service.go b/service/worker/service.go
index 3db475993d2..78c299af0cf 100644
--- a/service/worker/service.go
+++ b/service/worker/service.go
@@ -212,6 +212,14 @@ func NewConfig(
 			ExecutionScannerHistoryEventIdValidator: dynamicconfig.ExecutionScannerHistoryEventIdValidator.Get(dc),
 			RemovableBuildIdDurationSinceDefault:    dynamicconfig.RemovableBuildIdDurationSinceDefault.Get(dc),
 			BuildIdScavengerVisibilityRPS:           dynamicconfig.BuildIdScavengerVisibilityRPS.Get(dc),
+
+			ScheduleInvariantsScannerOverdueNextActionTimeEnabled:      dynamicconfig.ScheduleInvariantsScannerOverdueNextActionTimeEnabled.Get(dc),
+			ScheduleInvariantsScannerStuckOpenEnabled:                  dynamicconfig.ScheduleInvariantsScannerStuckOpenEnabled.Get(dc),
+			ScheduleInvariantsScannerUnknownStateEnabled:               dynamicconfig.ScheduleInvariantsScannerUnknownStateEnabled.Get(dc),
+			ScheduleInvariantsScannerOverdueNextActionTimeTolerance:    dynamicconfig.ScheduleInvariantsScannerOverdueNextActionTimeTolerance.Get(dc),
+			ScheduleInvariantsScannerVisibilityRPS:                     dynamicconfig.ScheduleInvariantsScannerVisibilityRPS.Get(dc),
+			ScheduleInvariantsScannerScanInterval:                      dynamicconfig.ScheduleInvariantsScannerScanInterval.Get(dc),
+			ScheduleInvariantsScannerStuckOpenIdleTimeBufferMultiplier: dynamicconfig.ScheduleInvariantsScannerStuckOpenIdleTimeBufferMultiplier.Get(dc),
 		},
 		BatcherRPS:                           dynamicconfig.BatcherRPS.Get(dc),
 		BatcherConcurrency:                   dynamicconfig.BatcherConcurrency.Get(dc),
diff --git a/tests/schedule_migration_test.go b/tests/schedule_migration_test.go
index 3250c639118..b2abc212cac 100644
--- a/tests/schedule_migration_test.go
+++ b/tests/schedule_migration_test.go
@@ -170,6 +170,7 @@ func (s *ScheduleMigrationTestSuite) TestScheduleMigrationV2AlreadyExists() {
 	// and the V1 activity treats that as success (logs warning, returns nil).
 	// The V1 workflow terminates, but the pre-existing V2 schedule retains
 	// its original state -- the V1 state is not applied.
+	env.OverrideDynamicConfig(dynamicconfig.EnableCHASMSchedulerMigration, true)
 	_, err = env.AdminClient().MigrateSchedule(ctx, &adminservice.MigrateScheduleRequest{
 		Namespace:  nsName,
 		ScheduleId: sid,
@@ -398,6 +399,7 @@ func (s *ScheduleMigrationTestSuite) TestScheduleMigrationV1ToV2() {
 	}, 10*time.Second, 500*time.Millisecond)
 
 	// Issue migration from V1 to V2.
+	env.OverrideDynamicConfig(dynamicconfig.EnableCHASMSchedulerMigration, true)
 	_, err = env.AdminClient().MigrateSchedule(ctx, &adminservice.MigrateScheduleRequest{
 		Namespace:  nsName,
 		ScheduleId: sid,
@@ -1140,6 +1142,7 @@ func (s *ScheduleMigrationTestSuite) TestScheduleMigrationV1ToV2WithClosedV2() {
 	// Issue migration from V1 to V2. The previously deleted CHASM execution
 	// does not block creation of a new one -- StartExecution succeeds because
 	// closed executions allow reuse of the business ID.
+	env.OverrideDynamicConfig(dynamicconfig.EnableCHASMSchedulerMigration, true)
 	_, err = env.AdminClient().MigrateSchedule(ctx, &adminservice.MigrateScheduleRequest{
 		Namespace:  nsName,
 		ScheduleId: sid,
@@ -1259,8 +1262,10 @@ func TestScheduleMigrationV1ToV2NoDuplicateRecentActions(t *testing.T) {
 		return true
 	}, 15*time.Second, 500*time.Millisecond)
 
-	// Enable CHASM now so the migration activity can create the V2 schedule.
+	// Enable CHASM and migration, allowing it for schedules with running workflows.
 	env.OverrideDynamicConfig(dynamicconfig.EnableChasm, true)
+	env.OverrideDynamicConfig(dynamicconfig.EnableCHASMSchedulerMigration, true)
+	env.OverrideDynamicConfig(dynamicconfig.EnableCHASMSchedulerMigrationWithRunningWorkflows, true)
 
 	// Migrate from V1 to V2 while the workflow is still running.
 	_, err = env.AdminClient().MigrateSchedule(ctx, &adminservice.MigrateScheduleRequest{
@@ -1322,6 +1327,141 @@ func TestScheduleMigrationV1ToV2NoDuplicateRecentActions(t *testing.T) {
 	require.NoError(t, err)
 }
 
+// TestScheduleMigrationDeferredWithRunningWorkflow verifies that
+// EnableCHASMSchedulerMigrationWithRunningWorkflows gates automatic migration of a
+// V1 schedule that has a running workflow. When the gate is off (default),
+// auto-migration is deferred while a workflow is running (the scheduler keeps
+// running rather than migrating and completing); when the gate is turned on,
+// auto-migration proceeds even with a running workflow.
+func TestScheduleMigrationDeferredWithRunningWorkflow(t *testing.T) {
+	// Create the env without EnableChasm so that CreateSchedule produces a V1
+	// (workflow-backed) schedule rather than a CHASM sentinel.
+	env := testcore.NewEnv(
+		t,
+		testcore.WithWorkerService("V1 scheduler"),
+		testcore.WithDynamicConfig(dynamicconfig.EnableCHASMSchedulerMigrationWithRunningWorkflows, false),
+	)
+
+	ctx := testcore.NewContext()
+	sid := testcore.RandomizeStr("sched-migrate-defer-running")
+	wid := testcore.RandomizeStr("sched-migrate-defer-running-wf")
+	wt := testcore.RandomizeStr("sched-migrate-defer-running-wt")
+
+	nsName := env.Namespace().String()
+	nsID := env.NamespaceID().String()
+
+	// A workflow that blocks until signaled, so it stays running during migration.
+	resumeSignal := "resume"
+	workflowFn := func(ctx workflow.Context) error {
+		ch := workflow.GetSignalChannel(ctx, resumeSignal)
+		ch.Receive(ctx, nil)
+		return nil
+	}
+	env.SdkWorker().RegisterWorkflowWithOptions(workflowFn, workflow.RegisterOptions{Name: wt})
+
+	env.OverrideDynamicConfig(dynamicconfig.EnableChasm, false)
+
+	// Create a V1 schedule with a short interval, so the scheduler iterates often
+	// and re-evaluates the auto-migration decision, and an immediate trigger so it
+	// starts a workflow. SKIP keeps a single workflow running across intervals.
+	sched := &schedulepb.Schedule{
+		Spec: &schedulepb.ScheduleSpec{
+			Interval: []*schedulepb.IntervalSpec{{Interval: durationpb.New(2 * time.Second)}},
+		},
+		Policies: &schedulepb.SchedulePolicies{
+			OverlapPolicy: enumspb.SCHEDULE_OVERLAP_POLICY_SKIP,
+		},
+		Action: &schedulepb.ScheduleAction{
+			Action: &schedulepb.ScheduleAction_StartWorkflow{
+				StartWorkflow: &workflowpb.NewWorkflowExecutionInfo{
+					WorkflowId:   wid,
+					WorkflowType: &commonpb.WorkflowType{Name: wt},
+					TaskQueue:    &taskqueuepb.TaskQueue{Name: env.WorkerTaskQueue(), Kind: enumspb.TASK_QUEUE_KIND_NORMAL},
+				},
+			},
+		},
+	}
+	_, err := env.FrontendClient().CreateSchedule(ctx, &workflowservice.CreateScheduleRequest{
+		Namespace:  nsName,
+		ScheduleId: sid,
+		Schedule:   sched,
+		InitialPatch: &schedulepb.SchedulePatch{
+			TriggerImmediately: &schedulepb.TriggerImmediatelyRequest{},
+		},
+		Identity:  "test",
+		RequestId: uuid.NewString(),
+	})
+	require.NoError(t, err)
+
+	// Wait for the V1 scheduler to start the workflow and record it as running.
+	var runningWfID string
+	require.Eventually(t, func() bool {
+		descResp, err := env.FrontendClient().DescribeSchedule(ctx, &workflowservice.DescribeScheduleRequest{
+			Namespace:  nsName,
+			ScheduleId: sid,
+		})
+		if err != nil || len(descResp.GetInfo().GetRecentActions()) == 0 {
+			return false
+		}
+		a := descResp.Info.RecentActions[0]
+		if a.GetStartWorkflowStatus() != enumspb.WORKFLOW_EXECUTION_STATUS_RUNNING {
+			return false
+		}
+		runningWfID = a.GetStartWorkflowResult().GetWorkflowId()
+		return true
+	}, 15*time.Second, 500*time.Millisecond)
+
+	// Enable CHASM and migration; the running-workflow gate stays off, so only the
+	// running action workflow can defer migration here.
+	env.OverrideDynamicConfig(dynamicconfig.EnableChasm, true)
+	env.OverrideDynamicConfig(dynamicconfig.EnableCHASMSchedulerMigration, true)
+
+	// A successful migration completes the V1 scheduler workflow; while migration
+	// is deferred it stays running.
+	v1WorkflowID := scheduler.WorkflowIDPrefix + sid
+	v1Migrated := func() bool {
+		desc, err := env.GetTestCluster().HistoryClient().DescribeWorkflowExecution(
+			ctx,
+			&historyservice.DescribeWorkflowExecutionRequest{
+				NamespaceId: nsID,
+				Request: &workflowservice.DescribeWorkflowExecutionRequest{
+					Namespace: nsName,
+					Execution: &commonpb.WorkflowExecution{WorkflowId: v1WorkflowID},
+				},
+			},
+		)
+		return err == nil && desc.GetWorkflowExecutionInfo().GetStatus() == enumspb.WORKFLOW_EXECUTION_STATUS_COMPLETED
+	}
+
+	// With the gate off, auto-migration is deferred: the scheduler re-evaluates the
+	// decision every interval but keeps running rather than migrating.
+	require.Never(t, v1Migrated, 5*time.Second, 500*time.Millisecond,
+		"migration must be deferred while the schedule has a running workflow and the gate is off")
+
+	// Turn the gate on; auto-migration now proceeds even with the workflow running.
+	env.OverrideDynamicConfig(dynamicconfig.EnableCHASMSchedulerMigrationWithRunningWorkflows, true)
+	require.Eventually(t, v1Migrated, 15*time.Second, 500*time.Millisecond,
+		"migration should proceed once the gate allows running workflows")
+
+	// The V2 schedule should now exist.
+	_, err = env.GetTestCluster().SchedulerClient().DescribeSchedule(
+		ctx,
+		&schedulerpb.DescribeScheduleRequest{
+			NamespaceId:     nsID,
+			FrontendRequest: &workflowservice.DescribeScheduleRequest{Namespace: nsName, ScheduleId: sid},
+		},
+	)
+	require.NoError(t, err)
+
+	// Clean up: signal the running workflow to complete.
+	_, err = env.FrontendClient().SignalWorkflowExecution(ctx, &workflowservice.SignalWorkflowExecutionRequest{
+		Namespace:         nsName,
+		WorkflowExecution: &commonpb.WorkflowExecution{WorkflowId: runningWfID},
+		SignalName:        resumeSignal,
+	})
+	require.NoError(t, err)
+}
+
 // TestDeleteScheduleContextMetadata verifies that DeleteSchedule propagates the
 // correct context metadata (workflow-type, workflow-task-queue) for every
 // combination of CHASM and V1 state. This metadata is read by saas-temporal's
@@ -1840,6 +1980,132 @@ func TestScheduleMigration_StaleRunningDoesNotSkipPending(t *testing.T) {
 		"stale running entry must not cause the pending start to be dropped under SKIP overlap policy")
 }
 
+// TestScheduleMigrationRolloutPercent verifies that
+// CHASMSchedulerMigrationRolloutPercent gates per-schedule migration once
+// EnableCHASMSchedulerMigration is on: at 50%, two V1 schedules whose IDs
+// bucket on opposite sides of the rollout end up on different stacks — one
+// migrates and the other stays on V1.
+func (s *ScheduleMigrationTestSuite) TestScheduleMigrationRolloutPercent() {
+	t := s.T()
+	env := testcore.NewEnv(
+		t,
+		testcore.WithWorkerService("scheduler operations"),
+		testcore.WithDynamicConfig(dynamicconfig.EnableChasm, true),
+		testcore.WithDynamicConfig(dynamicconfig.EnableCHASMSchedulerMigration, true),
+		testcore.WithDynamicConfig(dynamicconfig.CHASMSchedulerMigrationRolloutPercent, 50),
+	)
+
+	ctx := testcore.NewContext()
+	nsName := env.Namespace().String()
+	nsID := env.NamespaceID().String()
+
+	migrateSID, stayV1SID := testcore.PickRolloutSplit(t, nsName, 50)
+
+	mkSchedule := func() *schedulepb.Schedule {
+		return &schedulepb.Schedule{
+			Spec: &schedulepb.ScheduleSpec{
+				Interval: []*schedulepb.IntervalSpec{{Interval: durationpb.New(1 * time.Hour)}},
+			},
+			Action: &schedulepb.ScheduleAction{
+				Action: &schedulepb.ScheduleAction_StartWorkflow{
+					StartWorkflow: &workflowpb.NewWorkflowExecutionInfo{
+						WorkflowId:   testcore.RandomizeStr("wid"),
+						WorkflowType: &commonpb.WorkflowType{Name: testcore.RandomizeStr("wt")},
+						TaskQueue:    &taskqueuepb.TaskQueue{Name: testcore.RandomizeStr("tq"), Kind: enumspb.TASK_QUEUE_KIND_NORMAL},
+					},
+				},
+			},
+		}
+	}
+
+	// Start each schedule directly as a V1 workflow, mirroring the pattern in
+	// TestScheduleMigrationDynamicConfig.
+	startV1 := func(sid string) {
+		startArgs := &schedulespb.StartScheduleArgs{
+			Schedule: mkSchedule(),
+			State: &schedulespb.InternalState{
+				Namespace:     nsName,
+				NamespaceId:   nsID,
+				ScheduleId:    sid,
+				ConflictToken: scheduler.InitialConflictToken,
+			},
+		}
+		inputPayloads, err := sdk.PreferProtoDataConverter.ToPayloads(startArgs)
+		require.NoError(t, err)
+		startReq := &workflowservice.StartWorkflowExecutionRequest{
+			Namespace:                nsName,
+			WorkflowId:               scheduler.WorkflowIDPrefix + sid,
+			WorkflowType:             &commonpb.WorkflowType{Name: scheduler.WorkflowType},
+			TaskQueue:                &taskqueuepb.TaskQueue{Name: primitives.PerNSWorkerTaskQueue},
+			Input:                    inputPayloads,
+			Identity:                 "test",
+			RequestId:                uuid.NewString(),
+			WorkflowIdReusePolicy:    enumspb.WORKFLOW_ID_REUSE_POLICY_ALLOW_DUPLICATE,
+			WorkflowIdConflictPolicy: enumspb.WORKFLOW_ID_CONFLICT_POLICY_FAIL,
+		}
+		_, err = env.GetTestCluster().HistoryClient().StartWorkflowExecution(
+			ctx,
+			common.CreateHistoryStartWorkflowRequest(nsID, startReq, nil, nil, time.Now().UTC()),
+		)
+		require.NoError(t, err)
+	}
+	startV1(migrateSID)
+	startV1(stayV1SID)
+
+	v1Status := func(sid string) enumspb.WorkflowExecutionStatus {
+		desc, err := env.GetTestCluster().HistoryClient().DescribeWorkflowExecution(
+			ctx,
+			&historyservice.DescribeWorkflowExecutionRequest{
+				NamespaceId: nsID,
+				Request: &workflowservice.DescribeWorkflowExecutionRequest{
+					Namespace: nsName,
+					Execution: &commonpb.WorkflowExecution{WorkflowId: scheduler.WorkflowIDPrefix + sid},
+				},
+			},
+		)
+		if err != nil {
+			return enumspb.WORKFLOW_EXECUTION_STATUS_UNSPECIFIED
+		}
+		return desc.GetWorkflowExecutionInfo().GetStatus()
+	}
+
+	// The accepted schedule's V1 workflow should complete (migrate to CHASM).
+	require.Eventually(t, func() bool {
+		return v1Status(migrateSID) == enumspb.WORKFLOW_EXECUTION_STATUS_COMPLETED
+	}, 30*time.Second, 500*time.Millisecond, "schedule %q should migrate to CHASM", migrateSID)
+
+	// And its CHASM-side description should be present.
+	_, err := env.GetTestCluster().SchedulerClient().DescribeSchedule(ctx, &schedulerpb.DescribeScheduleRequest{
+		NamespaceId:     nsID,
+		FrontendRequest: &workflowservice.DescribeScheduleRequest{Namespace: nsName, ScheduleId: migrateSID},
+	})
+	require.NoError(t, err)
+
+	// The rejected schedule's V1 workflow should remain running, and the CHASM
+	// handler should still report NotFound. Re-checked over a window to ensure
+	// the negative state is stable rather than racing a delayed migration.
+	require.Never(t, func() bool {
+		return v1Status(stayV1SID) == enumspb.WORKFLOW_EXECUTION_STATUS_COMPLETED
+	}, 10*time.Second, 1*time.Second, "schedule %q must not migrate while outside the rollout cohort", stayV1SID)
+
+	_, err = env.GetTestCluster().SchedulerClient().DescribeSchedule(ctx, &schedulerpb.DescribeScheduleRequest{
+		NamespaceId:     nsID,
+		FrontendRequest: &workflowservice.DescribeScheduleRequest{Namespace: nsName, ScheduleId: stayV1SID},
+	})
+	var notFoundErr *serviceerror.NotFound
+	require.ErrorAs(t, err, &notFoundErr,
+		"schedule %q rejected by the rollout must not appear on the CHASM handler", stayV1SID)
+
+	// And the schedule should still be reachable through the public frontend.
+	require.Eventually(t, func() bool {
+		_, err := env.FrontendClient().DescribeSchedule(ctx, &workflowservice.DescribeScheduleRequest{
+			Namespace:  nsName,
+			ScheduleId: stayV1SID,
+		})
+		return err == nil
+	}, 15*time.Second, 250*time.Millisecond, "frontend DescribeSchedule should succeed for V1 schedule %q", stayV1SID)
+}
+
 func TestScheduleMigration_NoRunningWorkflows_GeneratorStarts(t *testing.T) {
 	// Use a very short idle time so the schedule closes quickly once the generator
 	// determines there's nothing to do (empty spec → no next wakeup time → idle).
diff --git a/tests/schedule_test.go b/tests/schedule_test.go
index 116c42515cf..0d49109ac15 100644
--- a/tests/schedule_test.go
+++ b/tests/schedule_test.go
@@ -92,6 +92,8 @@ func TestScheduleCHASM(t *testing.T) {
 	t.Run("TestStateSizeBytesReported", func(t *testing.T) { testStateSizeBytesReported(t, newContext) })
 	t.Run("TestSingleDateScheduleCloses", func(t *testing.T) { testSingleDateScheduleCloses(t, newContext) })
 	t.Run("TestMultiDateScheduleCloses", func(t *testing.T) { testMultiDateScheduleCloses(t, newContext) })
+	t.Run("TestPausedDropsCatchup", func(t *testing.T) { testPausedDropsCatchup(t, newContext) })
+	t.Run("TestFutureActionTimesAdvanceWhilePaused", func(t *testing.T) { testFutureActionTimesAdvanceWhilePaused(t, newContext) })
 	t.Run("TestNextActionTimeVisibility", func(t *testing.T) { testScheduleNextActionTimeVisibility(t, newContext) })
 }
 
@@ -3381,6 +3383,84 @@ func testUpdateScheduleBlobSizeLimit(t *testing.T, newContext contextFactory) {
 	require.ErrorAs(t, err, &invalidArgBlob)
 }
 
+// TestScheduleCreationRolloutPercent verifies that
+// CHASMSchedulerCreationRolloutPercent acts as a per-schedule sampling gate
+// after EnableCHASMSchedulerCreation is on: at 50%, two schedules whose IDs
+// bucket on opposite sides of the rollout land on different stacks.
+func TestScheduleCreationRolloutPercent(t *testing.T) {
+	opts := append(scheduleCommonOpts(t),
+		// V1 worker is needed because at 50% rollout some schedules land on V1.
+		testcore.WithWorkerService("V1 scheduler"),
+		testcore.WithDynamicConfig(dynamicconfig.EnableCHASMSchedulerCreation, true),
+		testcore.WithDynamicConfig(dynamicconfig.CHASMSchedulerCreationRolloutPercent, 50),
+	)
+	s := testcore.NewEnv(t, opts...)
+	ctx := s.Context()
+	nsName := s.Namespace().String()
+	nsID := s.NamespaceID().String()
+
+	chasmSID, v1SID := testcore.PickRolloutSplit(t, nsName, 50)
+
+	mkSchedule := func() *schedulepb.Schedule {
+		return &schedulepb.Schedule{
+			Spec: &schedulepb.ScheduleSpec{
+				Interval: []*schedulepb.IntervalSpec{{Interval: durationpb.New(1 * time.Hour)}},
+			},
+			Action: &schedulepb.ScheduleAction{
+				Action: &schedulepb.ScheduleAction_StartWorkflow{
+					StartWorkflow: &workflowpb.NewWorkflowExecutionInfo{
+						WorkflowId:   testcore.RandomizeStr("wid"),
+						WorkflowType: &commonpb.WorkflowType{Name: testcore.RandomizeStr("wt")},
+						TaskQueue:    &taskqueuepb.TaskQueue{Name: s.WorkerTaskQueue(), Kind: enumspb.TASK_QUEUE_KIND_NORMAL},
+					},
+				},
+			},
+		}
+	}
+
+	for _, sid := range []string{chasmSID, v1SID} {
+		_, err := s.FrontendClient().CreateSchedule(ctx, &workflowservice.CreateScheduleRequest{
+			Namespace:  nsName,
+			ScheduleId: sid,
+			Schedule:   mkSchedule(),
+			Identity:   testcore.RandomizeStr("identity"),
+			RequestId:  uuid.NewString(),
+		})
+		require.NoError(t, err)
+	}
+
+	// A direct CHASM DescribeSchedule succeeds for CHASM-backed schedules and
+	// returns NotFound for V1-backed schedules (whose CHASM key is a sentinel).
+	describeOnCHASM := func(sid string) error {
+		_, err := s.GetTestCluster().SchedulerClient().DescribeSchedule(ctx, &schedulerpb.DescribeScheduleRequest{
+			NamespaceId:     nsID,
+			FrontendRequest: &workflowservice.DescribeScheduleRequest{Namespace: nsName, ScheduleId: sid},
+		})
+		return err
+	}
+
+	require.Eventually(t, func() bool { return describeOnCHASM(chasmSID) == nil }, 15*time.Second, 250*time.Millisecond,
+		"schedule %q bucketed into CHASM should be describable via the CHASM handler", chasmSID)
+
+	var notFoundErr *serviceerror.NotFound
+	require.ErrorAs(t, describeOnCHASM(v1SID), &notFoundErr,
+		"schedule %q bucketed into V1 should not be present on the CHASM handler", v1SID)
+
+	// Both schedules must remain describable through the public frontend
+	// regardless of which stack they live on — the V1 schedule in particular
+	// must round-trip through the frontend's fallback path. The V1 workflow
+	// takes a moment to be queryable after creation, so poll.
+	for _, sid := range []string{chasmSID, v1SID} {
+		require.Eventually(t, func() bool {
+			_, err := s.FrontendClient().DescribeSchedule(ctx, &workflowservice.DescribeScheduleRequest{
+				Namespace:  nsName,
+				ScheduleId: sid,
+			})
+			return err == nil
+		}, 15*time.Second, 250*time.Millisecond, "frontend DescribeSchedule should succeed for %q", sid)
+	}
+}
+
 // testSingleDateScheduleCloses verifies that a CHASM schedule configured with
 // a single calendar date closes after its one workflow completes.
 func testSingleDateScheduleCloses(t *testing.T, newContext contextFactory) {
@@ -3554,6 +3634,171 @@ func testMultiDateScheduleCloses(t *testing.T, newContext contextFactory) {
 	}, 15*time.Second, 200*time.Millisecond, "schedule should have closed")
 }
 
+// testFutureActionTimesAdvanceWhilePaused verifies that ListSchedules returns
+// up-to-date FutureActionTimes for a paused schedule. CHASM's always-on
+// Generator advances the high water mark and rebuilds FutureActionTimes
+// against the spec even when paused, so listed times stay in the future and
+// never roll into the past. The legacy V1 scheduler workflow does not advance
+// while paused, so its projected times would freeze at pause time and
+// eventually all sit in the past - hence this test is registered CHASM-only.
+func testFutureActionTimesAdvanceWhilePaused(t *testing.T, newContext contextFactory) {
+	s := testcore.NewEnv(t, scheduleCommonOpts(t)...)
+
+	sid := testcore.RandomizeStr("sched-future-actions-paused")
+	wid := testcore.RandomizeStr("sched-future-actions-paused-wf")
+	wt := testcore.RandomizeStr("sched-future-actions-paused-wt")
+
+	s.SdkWorker().RegisterWorkflowWithOptions(
+		func(ctx workflow.Context) error { return nil },
+		workflow.RegisterOptions{Name: wt},
+	)
+
+	ctx := newContext(s.Context())
+	_, err := s.FrontendClient().CreateSchedule(ctx, &workflowservice.CreateScheduleRequest{
+		Namespace:  s.Namespace().String(),
+		ScheduleId: sid,
+		Schedule: &schedulepb.Schedule{
+			Spec: &schedulepb.ScheduleSpec{
+				Interval: []*schedulepb.IntervalSpec{{Interval: durationpb.New(1 * time.Second)}},
+			},
+			State: &schedulepb.ScheduleState{Paused: true},
+			Action: &schedulepb.ScheduleAction{
+				Action: &schedulepb.ScheduleAction_StartWorkflow{
+					StartWorkflow: &workflowpb.NewWorkflowExecutionInfo{
+						WorkflowId:            wid,
+						WorkflowType:          &commonpb.WorkflowType{Name: wt},
+						TaskQueue:             &taskqueuepb.TaskQueue{Name: s.WorkerTaskQueue(), Kind: enumspb.TASK_QUEUE_KIND_NORMAL},
+						WorkflowIdReusePolicy: enumspb.WORKFLOW_ID_REUSE_POLICY_ALLOW_DUPLICATE,
+					},
+				},
+			},
+		},
+		Identity:  "test",
+		RequestId: uuid.NewString(),
+	})
+	s.NoError(err)
+
+	// Wait for visibility to surface an initial FutureActionTimes projection.
+	initial := getScheduleEntryFromVisibility(s, sid, newContext, func(ent *schedulepb.ScheduleListEntry) bool {
+		return len(ent.Info.FutureActionTimes) > 0
+	})
+	initialFirst := initial.Info.FutureActionTimes[0].AsTime()
+
+	// While still paused, the earliest projected time must advance past the
+	// initial value: the Generator keeps ticking and republishing the
+	// projection, even though no workflows fire.
+	getScheduleEntryFromVisibility(s, sid, newContext, func(ent *schedulepb.ScheduleListEntry) bool {
+		return len(ent.Info.FutureActionTimes) > 0 &&
+			ent.Info.FutureActionTimes[0].AsTime().After(initialFirst)
+	})
+}
+
+// testPausedDropsCatchup verifies that an action scheduled by the spec during
+// a paused window is NOT replayed when the schedule is unpaused. The test
+// uses a one-shot calendar entry near in the future so the assertion is
+// binary (runs == 0 vs runs == 1) and structural - no fixed-time sleeps and
+// no count-based bound checks.
+//
+// CHASM-only: with the always-on Generator fix, CHASM's HWM advances while
+// paused so unpause doesn't see the past spec time in the [HWM, now] window.
+// V1's scheduler workflow sleeps while paused (HWM is frozen), so V1 would
+// legitimately replay - that's a divergent backend behavior, not what this
+// test verifies.
+func testPausedDropsCatchup(t *testing.T, newContext contextFactory) {
+	shortIdleTime := 3 * time.Second
+	tweakables := chasmscheduler.DefaultTweakables
+	tweakables.IdleTime = shortIdleTime
+	s := testcore.NewEnv(t, append(scheduleCommonOpts(t),
+		testcore.WithDynamicConfig(chasmscheduler.CurrentTweakables, tweakables),
+	)...)
+
+	sid := testcore.RandomizeStr("sched-paused-drops-catchup")
+	wid := testcore.RandomizeStr("sched-paused-drops-catchup-wf")
+	wt := testcore.RandomizeStr("sched-paused-drops-catchup-wt")
+
+	var runs atomic.Int32
+	s.SdkWorker().RegisterWorkflowWithOptions(func(ctx workflow.Context) error {
+		runs.Add(1)
+		return nil
+	}, workflow.RegisterOptions{Name: wt})
+
+	// Single calendar entry a few seconds in the future. While paused, its
+	// time will pass. Offset must exceed worst-case CreateSchedule latency
+	// under parallel load so the entry is still genuinely in the future when
+	// the server materializes the schedule - otherwise the test passes for
+	// the wrong reason (HWM already past the entry at create time).
+	fireAt := time.Now().Add(10 * time.Second).UTC()
+	ctx := newContext(s.Context())
+	_, err := s.FrontendClient().CreateSchedule(ctx, &workflowservice.CreateScheduleRequest{
+		Namespace:  s.Namespace().String(),
+		ScheduleId: sid,
+		Schedule: &schedulepb.Schedule{
+			Spec: &schedulepb.ScheduleSpec{
+				Calendar: []*schedulepb.CalendarSpec{{
+					Year:       fmt.Sprintf("%d", fireAt.Year()),
+					Month:      fireAt.Month().String(),
+					DayOfMonth: fmt.Sprintf("%d", fireAt.Day()),
+					Hour:       fmt.Sprintf("%d", fireAt.Hour()),
+					Minute:     fmt.Sprintf("%d", fireAt.Minute()),
+					Second:     fmt.Sprintf("%d", fireAt.Second()),
+				}},
+			},
+			State: &schedulepb.ScheduleState{
+				Paused: true,
+			},
+			Action: &schedulepb.ScheduleAction{
+				Action: &schedulepb.ScheduleAction_StartWorkflow{
+					StartWorkflow: &workflowpb.NewWorkflowExecutionInfo{
+						WorkflowId:            wid,
+						WorkflowType:          &commonpb.WorkflowType{Name: wt},
+						TaskQueue:             &taskqueuepb.TaskQueue{Name: s.WorkerTaskQueue(), Kind: enumspb.TASK_QUEUE_KIND_NORMAL},
+						WorkflowIdReusePolicy: enumspb.WORKFLOW_ID_REUSE_POLICY_ALLOW_DUPLICATE,
+					},
+				},
+			},
+		},
+		Identity:  "test",
+		RequestId: uuid.NewString(),
+	})
+	s.NoError(err)
+
+	s.Eventually(func() bool {
+		desc, err := s.FrontendClient().DescribeSchedule(ctx, &workflowservice.DescribeScheduleRequest{
+			Namespace:  s.Namespace().String(),
+			ScheduleId: sid,
+		})
+		return err == nil && len(desc.Info.FutureActionTimes) == 0
+	}, 15*time.Second, 200*time.Millisecond,
+		"FutureActionTimes should empty out once the only calendar date passes (proves HWM advanced past it while paused)")
+
+	_, err = s.FrontendClient().PatchSchedule(ctx, &workflowservice.PatchScheduleRequest{
+		Namespace:  s.Namespace().String(),
+		ScheduleId: sid,
+		Patch:      &schedulepb.SchedulePatch{Unpause: "drops-catchup-test"},
+		Identity:   "test",
+		RequestId:  uuid.NewString(),
+	})
+	s.NoError(err)
+
+	// Wait for close - also ensures any post-unpause processing has completed
+	// before the runs assertion below.
+	s.Eventually(func() bool {
+		_, err := s.FrontendClient().DescribeSchedule(ctx, &workflowservice.DescribeScheduleRequest{
+			Namespace:  s.Namespace().String(),
+			ScheduleId: sid,
+		})
+		return err != nil
+	}, 30*time.Second, 200*time.Millisecond,
+		"schedule should close from idle after unpause (no future actions, no replay)")
+
+	// Quiet window: catches an in-flight SDK worker that started a catch-up
+	// workflow before close, whose runs.Add() may not have landed yet at the
+	// moment DescribeSchedule first errored.
+	s.Never(func() bool { return runs.Load() > 0 },
+		2*time.Second, 100*time.Millisecond,
+		"paused-window calendar entry must not be replayed on unpause")
+}
+
 // testScheduleNextActionTimeVisibility asserts that the
 // TemporalScheduleNextActionTime search attribute is published to visibility.
 func testScheduleNextActionTimeVisibility(t *testing.T, newContext contextFactory) {
diff --git a/tests/testcore/dynamic_config_overrides.go b/tests/testcore/dynamic_config_overrides.go
index 0b5b2868058..6fccde14de9 100644
--- a/tests/testcore/dynamic_config_overrides.go
+++ b/tests/testcore/dynamic_config_overrides.go
@@ -67,5 +67,12 @@ var (
 		dynamicconfig.RefreshNexusEndpointsMinWait.Key():                    1 * time.Millisecond,
 		nexusoperations.RecordCancelRequestCompletionEvents.Key():           true,
 		nexusoperations.UseSystemCallbackURL.Key():                          true,
+
+		// CHASM scheduler rollout percents default to 0 in production; in tests we
+		// dial them to 100 so existing tests that only flip the binary enable flag
+		// continue to route every schedule through CHASM. Tests that want to
+		// exercise the percent gate can override per-test.
+		dynamicconfig.CHASMSchedulerCreationRolloutPercent.Key():  100,
+		dynamicconfig.CHASMSchedulerMigrationRolloutPercent.Key(): 100,
 	}
 )
diff --git a/tests/testcore/utils.go b/tests/testcore/utils.go
index 608e05221d9..c462c583c79 100644
--- a/tests/testcore/utils.go
+++ b/tests/testcore/utils.go
@@ -3,6 +3,7 @@ package testcore
 import (
 	"fmt"
 	"regexp"
+	"testing"
 
 	"github.com/google/uuid"
 	"github.com/stretchr/testify/require"
@@ -13,11 +14,37 @@ import (
 	historyspb "go.temporal.io/server/api/history/v1"
 	"go.temporal.io/server/api/historyservice/v1"
 	replicationspb "go.temporal.io/server/api/replication/v1"
+	"go.temporal.io/server/common/dynamicconfig"
 	"go.temporal.io/server/common/payloads"
 	"go.temporal.io/server/common/persistence/versionhistory"
 	"google.golang.org/protobuf/proto"
 )
 
+// PickRolloutSplit returns two distinct business IDs in the given namespace
+// such that the first is accepted by RolloutAccepts at percent and the second
+// is rejected. Fails the test if no split is found.
+func PickRolloutSplit(t *testing.T, namespace string, percent int) (accepted, rejected string) {
+	t.Helper()
+	base := RandomizeStr("rollout")
+	for i := range 1000 {
+		if accepted != "" && rejected != "" {
+			break
+		}
+		id := fmt.Sprintf("%s-%d", base, i)
+		key := fmt.Appendf(nil, "%s\x00%s", namespace, id)
+		if dynamicconfig.RolloutAccepts(key, percent) {
+			if accepted == "" {
+				accepted = id
+			}
+		} else if rejected == "" {
+			rejected = id
+		}
+	}
+	require.NotEmpty(t, accepted, "could not find an ID accepted by rollout at %d%%", percent)
+	require.NotEmpty(t, rejected, "could not find an ID rejected by rollout at %d%%", percent)
+	return accepted, rejected
+}
+
 // TODO (alex): move this to functional_test_base.go as methods.
 
 func RandomizeStr(id string) string {