diff --git a/src/event_model/__init__.py b/src/event_model/__init__.py
index 3e39b96e..0a50efd8 100644
--- a/src/event_model/__init__.py
+++ b/src/event_model/__init__.py
@@ -1529,6 +1529,11 @@ def __init__(
         self._resources: dict = {}
         self._stream_resources: dict = {}
 
+        # Map RunStart UID to the list of Resource UIDs and StreamResource UIDs.
+        # These are used to facilitate efficient cleanup of the caches above.
+        self._start_to_resources: defaultdict = defaultdict(list)
+        self._start_to_stream_resources: defaultdict = defaultdict(list)
+
         # Old-style Resources that do not have a RunStart UID
         self._unlabeled_resources: deque = deque(maxlen=10000)
 
@@ -1699,6 +1704,7 @@ def resource(self, doc: Resource) -> None:
         else:
             self._fillers[start_uid].resource(doc)
             self._resources[doc["uid"]] = doc["run_start"]
+            self._start_to_resources[start_uid].append(doc["uid"])
             for callback in self._factory_cbs_by_start[start_uid]:
                 callback("resource", doc)
             for callback in self._subfactory_cbs_by_start[start_uid]:
@@ -1708,6 +1714,7 @@ def stream_resource(self, doc: StreamResource) -> None:
         start_uid = doc["run_start"]  # No need for Try
         self._fillers[start_uid].stream_resource(doc)
         self._stream_resources[doc["uid"]] = doc["run_start"]
+        self._start_to_stream_resources[start_uid].append(doc["uid"])
         for callback in self._factory_cbs_by_start[start_uid]:
             callback("stream_resource", doc)
         for callback in self._subfactory_cbs_by_start[start_uid]:
@@ -1728,7 +1735,10 @@ def stop(self, doc: RunStop) -> None:
             self._descriptor_to_start.pop(descriptor_uid, None)
             self._factory_cbs_by_descriptor.pop(descriptor_uid, None)
             self._subfactory_cbs_by_descriptor.pop(descriptor_uid, None)
-        self._resources.pop(start_uid, None)
+        for resource_uid in self._start_to_resources.pop(start_uid, ()):
+            self._resources.pop(resource_uid, None)
+        for stream_resource_uid in self._start_to_stream_resources.pop(start_uid, ()):
+            self._stream_resources.pop(stream_resource_uid, None)
         self._start_to_start_doc.pop(start_uid, None)
 
 
diff --git a/src/event_model/documents/run_start.py b/src/event_model/documents/run_start.py
index b2971709..ce515a0b 100644
--- a/src/event_model/documents/run_start.py
+++ b/src/event_model/documents/run_start.py
@@ -137,6 +137,9 @@ class RunStart(TypedDict):
     Unix group to associate this data with
     """
     hints: NotRequired[Hints]
+    """
+    Start-level hints
+    """
     owner: NotRequired[str]
     """
     Unix owner to associate this data with
diff --git a/src/event_model/tests/test_run_router.py b/src/event_model/tests/test_run_router.py
index 8912ff4f..f1465f8d 100644
--- a/src/event_model/tests/test_run_router.py
+++ b/src/event_model/tests/test_run_router.py
@@ -359,3 +359,81 @@ def exception_callback_factory(start_doc_name, start_doc):
 
     event_document = {"descriptor": "ghijkl", "uid": "mnopqr"}
     rr.event(event_document)
+
+
+def test_run_router_resource_cleanup(tmp_path):
+    """Resources and StreamResources must be removed from internal caches after stop.
+
+    Regression test for a bug where _resources was popped using the wrong key
+    (start_uid instead of resource_uid), so all Resource documents from completed
+    runs accumulated in _resources indefinitely.  StreamResource documents were
+    never cleaned up at all.
+    """
+    rr = event_model.RunRouter([])
+
+    # --- Run 1: classic Resource / Datum documents ---
+    bundle1 = event_model.compose_run()
+    start1 = bundle1.start_doc
+    resource_bundle = bundle1.compose_resource(
+        spec="TIFF",
+        root=str(tmp_path),
+        resource_path="stack.tiff",
+        resource_kwargs={},
+    )
+    resource_doc = resource_bundle.resource_doc
+    stop1 = bundle1.compose_stop()
+
+    rr("start", start1)
+    rr("resource", resource_doc)
+    rr("stop", stop1)
+
+    # After stop, neither the resource nor the start should remain in any cache.
+    assert resource_doc["uid"] not in rr._resources, (
+        "_resources still contains the resource uid after stop"
+    )
+    assert start1["uid"] not in rr._start_to_resources, (
+        "_start_to_resources still contains the start uid after stop"
+    )
+    assert start1["uid"] not in rr._start_to_start_doc, (
+        "_start_to_start_doc still contains the start uid after stop"
+    )
+
+    # --- Run 2: StreamResource / StreamDatum documents ---
+    bundle2 = event_model.compose_run()
+    start2 = bundle2.start_doc
+    sres_doc, _ = bundle2.compose_stream_resource(
+        mimetype="image/tiff",
+        data_key="det",
+        uri="file://localhost" + str(tmp_path) + "/frames",
+        parameters={},
+    )
+    stop2 = bundle2.compose_stop()
+
+    rr("start", start2)
+    rr("stream_resource", sres_doc)
+    rr("stop", stop2)
+
+    assert sres_doc["uid"] not in rr._stream_resources, (
+        "_stream_resources still contains the stream_resource uid after stop"
+    )
+    assert start2["uid"] not in rr._start_to_stream_resources, (
+        "_start_to_stream_resources still contains the start uid after stop"
+    )
+
+    # --- Ensure caches stay empty across multiple runs ---
+    for _ in range(3):
+        bundle = event_model.compose_run()
+        sres, _ = bundle.compose_stream_resource(
+            mimetype="image/tiff",
+            data_key="det",
+            uri="file://localhost" + str(tmp_path) + "/frames",
+            parameters={},
+        )
+        rr("start", bundle.start_doc)
+        rr("stream_resource", sres)
+        rr("stop", bundle.compose_stop())
+
+    assert len(rr._resources) == 0
+    assert len(rr._stream_resources) == 0
+    assert len(rr._start_to_resources) == 0
+    assert len(rr._start_to_stream_resources) == 0