Gradle Kotlin "kapt"/"ksp" Failed - Fix Annotation Processing in CI

A Kotlin annotation-processing task (kapt or ksp) failed. Common causes are the processor not declared on the right kapt/ksp configuration, a KSP plugin version mismatched to the Kotlin version, an actual processing error in your annotated code, or a kapt stub-generation OOM.

What this error means

The build fails at :kaptGenerateStubsKotlin/:kspKotlin with an annotation-processor error, error: [ksp] ..., a version-incompatibility message, or an OutOfMemoryError during kapt stub generation.

gradle output

> Task :app:kaptGenerateStubsKotlin FAILED
e: [ksp] com.example.MyProcessor: failed to process: ...
# or a version mismatch:
ksp-1.9.0-1.0.13 is too old for kotlin-1.9.24. Please upgrade ksp ...

Common causes

Processor not on the kapt/ksp configuration

Declaring the processor as a normal implementation instead of kapt(...)/ksp(...) means it never runs, and generated code is missing.

KSP/Kotlin version mismatch or kapt OOM

The KSP plugin version is tied to a specific Kotlin version; a mismatch fails the task. kapt stub generation is also memory-heavy and can OOM on small runners.

How to fix it

Declare the processor on the right configuration

Use the ksp(...)/kapt(...) configuration and a KSP version matched to your Kotlin version.

build.gradle.kts

plugins {
    kotlin("jvm") version "1.9.24"
    id("com.google.devtools.ksp") version "1.9.24-1.0.20"   // match Kotlin
}
dependencies {
    ksp("com.example:my-processor:1.4.0")
}

Give kapt more memory or prefer KSP

Raise the Kotlin/daemon heap, or migrate to KSP which is faster and lighter than kapt.

gradle.properties

# gradle.properties
org.gradle.jvmargs=-Xmx3g -XX:MaxMetaspaceSize=1g
kapt.use.worker.api=true

How to prevent it

Keep the KSP plugin version locked to the Kotlin version (<kotlin>-<ksp>).
Declare processors on ksp/kapt, never plain implementation.
Prefer KSP over kapt for speed and lower memory; size the daemon heap accordingly.