KSP2 `KspAATask` lost no-source skip optimization from KSP1 `KspTaskJvm`

[changusmc]

Summary

After upgrading from KSP 1.0.31 (KspTaskJvm) to KSP 2.0.4 (KspAATask), 38 modules that were previously skipped as NO-SOURCE now execute KSP unnecessarily. The root cause is that KspAATask declares its source inputs using unfiltered FileTree objects, while KspTaskJvm inherited Kotlin source file pattern filtering (*.kt, *.kts) from KotlinCompile.

Environment

• Kotlin: 2.1.20 → 2.2.21
• KSP: 2.1.20-1.0.31 → 2.2.21-2.0.4
• Gradle: 9.2.1 → 9.3.1

Evidence

We compared two warm-cache builds on the same codebase, differing only in Kotlin/KSP version:

Metric	KSP1 (KspTaskJvm)	KSP2 (KspAATask)
Total kspDebugKotlin tasks	567	556
no-source (skipped)	48	10
avoided_up_to_date	178	270
executed_cacheable	341	276

38 modules lost their NO-SOURCE skip. These modules have KSP processors on the classpath (via Hilt/Dagger convention plugins) but contain no processable annotations — KSP initializes, finds nothing, and exits. Each one still costs 300–500ms of pure overhead.

Data collected from Develocity (Gradle Enterprise) build scans across ~900 builds from February–May 2026.

Root cause

KSP1: KspTaskJvm inherits source filtering from KotlinCompile

KspTaskJvm extends KotlinCompile → AbstractKotlinCompile → AbstractKotlinCompileTool → implements KotlinCompileTool.

The KotlinCompileTool interface declares:

// KotlinCompileTool (JetBrains/kotlin)
@get:InputFiles
@get:SkipWhenEmpty
@get:IgnoreEmptyDirectories
@get:PathSensitive(PathSensitivity.RELATIVE)
val sources: FileCollection  // filtered through sourceFileFilter: *.kt, *.kts

And KspTaskJvm adds:

// KspTaskJvm (google/ksp, KotlinFactories.kt ~line 229)
override fun skipCondition(): Boolean = sources.isEmpty && javaSources.isEmpty

@get:InputFiles
@get:SkipWhenEmpty
override val javaSources: FileCollection = super.javaSources.filter {
    !destination.get().isParentOf(it)
}

sources is filtered through sourceFileFilter which only matches *.kt and *.kts. A source directory containing no Kotlin files produces an empty sources collection. Combined with the @SkipWhenEmpty annotation, Gradle marks the task as NO-SOURCE.

KSP2: KspAATask uses unfiltered file trees

KspAATask extends DefaultTask (not KotlinCompile) and declares sources on a nested config object:

// KspAATask (google/ksp, KspAATask.kt ~line 472)
abstract class KspGradleConfig {
    @get:InputFiles
    @get:SkipWhenEmpty
    @get:IgnoreEmptyDirectories
    @get:PathSensitive(PathSensitivity.RELATIVE)
    abstract val sourceRoots: ConfigurableFileCollection  // NO file extension filter

    @get:InputFiles
    @get:SkipWhenEmpty
    @get:IgnoreEmptyDirectories
    @get:PathSensitive(PathSensitivity.RELATIVE)
    abstract val javaSourceRoots: ConfigurableFileCollection  // NO file extension filter
}

Source wiring (~line 210):

cfg.sourceRoots.from(
    sourceSet.kotlin.srcDirs.filter { ... }.map {
        project.objects.fileTree().from(it)  // ALL files in directory
    }
)
cfg.javaSourceRoots.from(filtered)  // same unfiltered trees

fileTree().from(dir) includes all files in the directory — not just *.kt/*.java. Any file (resources, .gitkeep, build artifacts) makes the collection non-empty, preventing Gradle's @SkipWhenEmpty from triggering NO-SOURCE.

Comparison table

Aspect	KSP1 (KspTaskJvm)	KSP2 (KspAATask)
Base class	KotlinCompile	DefaultTask
Source file filter	*.kt, *.kts pattern	None (all files in dir)
@SkipWhenEmpty on	sources + javaSources	sourceRoots + commonSourceRoots + javaSourceRoots
skipCondition() override	sources.isEmpty && javaSources.isEmpty	None

Suggested fix

Add file extension filters to the source file trees in KspAATask:

// KspAATask.kt, registerKspAATask(), ~line 215
// Instead of:
project.objects.fileTree().from(it)

// Should be:
project.objects.fileTree().from(it).matching {
    it.include("**/*.kt", "**/*.kts", "**/*.java")
}

This restores the pattern filtering that KotlinCompile provides to KSP1, allowing Gradle to correctly mark modules with no Kotlin/Java sources as NO-SOURCE.

Impact

• 38 modules unnecessarily run KSP per full rebuild
• ~300–500ms overhead per module (KSP initialization + teardown with no processing)
• ~15–20s total serial time wasted per full rebuild
• Additionally contributes to cache key churn since these modules produce empty/trivial KSP outputs that change with compiler version bumps