The JavaPreCompileTask computes KSP and annotation processors that are consumed in downstream JavaCompile tasks. For annotation processors, the compilation task in AGP computes if any processors are non-incremental, and (correctly) reports a warning for any non-incremental processors found.

However, there's a bug in JavaPreCompileTask's logic as it merges the KSP and annotation processing classpath when computing, meaning that dependencies on the KSP classpath are incorrectly treated as if they are going to be on the JavaCompile annotationProcessorClasspath.

Essentially, in this bit of logic

    public override fun doTaskAction() {
        val annotationProcessorArtifacts =
            (annotationProcessorArtifacts?.artifacts?.map { SerializableArtifact(it) } ?: emptyList()) +
                    (kspProcessorArtifacts?.artifacts?.map { SerializableArtifact(it) }
                        ?: emptyList())

        workerExecutor.noIsolation().submit(JavaPreCompileWorkAction::class.java) {
            it.initializeFromAndroidVariantTask(this)
            it.annotationProcessorArtifacts.setDisallowChanges(annotationProcessorArtifacts)
            it.annotationProcessorClassNames.setDisallowChanges(annotationProcessorClassNames)
            it.annotationProcessorListFile.setDisallowChanges(annotationProcessorListFile)
        }
    }

It shouldn't be merging together the ksp and annotation processor artifacts, as KSP dependencies do not participate in the JavaCompile classpath.

Studio Build: N/A
Version of Gradle Plugin: 8.5.0-alpha02
Version of Gradle: 8.7
Version of Java: 21
OS: macOS Sonoma 14.4.1

In the meantime, I can work around this with the following snippet in Kotlin gradle DSL.

tasks.withType<JavaPreCompileTask>()
  .configureEach {
    doFirst {
      // JavaPreCompileTask incorrectly reads annotation processors from the ksp classpath
      // and then warns about them ending up in the JavaCompile tasks even though they're
      // not on the classpath. This works around that by clearing out that field before it
      // tries to merge them in with annotationProcessorArtifacts.
      JavaPreCompileTask::class.java.getDeclaredField("kspProcessorArtifacts")
        .apply { isAccessible = true }
        .set(this, null)
    }
  }