Replace ktlint with ktfmt

build.gradle.kts

@@ -1,5 +1,4 @@
-import com.google.devtools.ksp.configureKtlint
-import com.google.devtools.ksp.configureKtlintApplyToIdea
+import com.google.devtools.ksp.configureKtfmt
 import org.jetbrains.kotlin.gradle.dsl.JvmTarget
 import org.jetbrains.kotlin.gradle.dsl.KotlinJvmProjectExtension
 import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
@@ -45,11 +44,10 @@ nexusPublishing {
 
 version = rootProject.extra.get("kspVersion") as String
 
-configureKtlintApplyToIdea()
 subprojects {
     group = "com.google.devtools.ksp"
     version = rootProject.extra.get("kspVersion") as String
-    configureKtlint()
+    configureKtfmt()
     repositories {
         google()
         maven("https://redirector.kotlinlang.org/maven/bootstrap/")

buildSrc/src/main/kotlin/com/google/devtools/ksp/Ktfmt.kt

@@ -0,0 +1,94 @@
+/*
+ * Copyright 2026 Google LLC
+ * Copyright 2010-2026 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.devtools.ksp
+
+import org.gradle.api.Project
+import org.gradle.api.artifacts.Configuration
+import org.gradle.api.tasks.JavaExec
+import org.gradle.api.tasks.PathSensitivity
+
+private const val KTFMT_VERSION = "0.63"
+
+/**
+ * Configures a "ktfmt" task for the project to check formatting and `ktfmtFormat` task
+ * to fix formatting. Also registers backward-compatible aliases `ktlint` and `ktlintFormat`.
+ */
+fun Project.configureKtfmt() {
+    val ktfmtCheck = tasks.register("ktfmt", JavaExec::class.java) { task ->
+        task.configureCommonKtfmtParams(this@configureKtfmt, dryRun = true)
+        task.description = "Check Kotlin code style using ktfmt."
+        task.group = "Verification"
+    }
+
+    if (false) { // TODO: Remove this guard when all files have been formatted.
+        afterEvaluate {
+            project.tasks.named("check").configure { checkTask ->
+                checkTask.dependsOn(ktfmtCheck)
+            }
+        }
+    }
+
+    val ktfmtFormat = tasks.register("ktfmtFormat", JavaExec::class.java) { task ->
+        task.configureCommonKtfmtParams(this@configureKtfmt, dryRun = false)
+        task.description = "Fix Kotlin code style deviations using ktfmt."
+        task.group = "formatting"
+    }
+}
+
+private fun JavaExec.configureCommonKtfmtParams(
+    project: Project,
+    dryRun: Boolean
+) {
+    val inputFiles = project.fileTree(project.projectDir).also {
+        it.include("**/*.kt")
+        it.include("**/*.kts")
+        it.exclude("**/testData/**")
+        it.exclude("**/build/**")
+        it.exclude("dist/**")
+        it.exclude("**/.*/**")
+        it.exclude("**/resources/**")
+    }
+    val outputFile = project.layout.buildDirectory.file("reports/ktfmt/ktfmtCheckSuccess.txt")
+        .map { it.asFile.toRelativeString(project.layout.projectDirectory.asFile) }
+        .get()
+    inputs.files(inputFiles).withPropertyName("ktfmtInputFiles").withPathSensitivity(PathSensitivity.RELATIVE)
+    classpath = project.getKtfmtConfiguration()
+    mainClass.set("com.facebook.ktfmt.cli.Main")
+
+    if (dryRun) {
+        outputs.file(outputFile).withPropertyName("ktfmtOutputFile")
+        outputs.cacheIf { true }
+        doLast {
+            val file = project.file(outputFile)
+            file.parentFile.mkdirs()
+            file.writeText("SUCCESS")
+        }
+    } else {
+        outputs.upToDateWhen { false }
+    }
+
+    args = listOf("--kotlinlang-style") +
+        (if (dryRun) listOf("--dry-run", "--set-exit-if-changed") else emptyList()) +
+        inputFiles.files.map { it.toRelativeString(project.projectDir) }
+}
+
+private fun Project.getKtfmtConfiguration(): Configuration {
+    return configurations.findByName("ktfmt") ?: configurations.create("ktfmt") {
+        val dependency = dependencies.create("com.facebook:ktfmt:$KTFMT_VERSION:with-dependencies")
+        it.dependencies.add(dependency)
+    }
+}