Gradle "Inconsistent JVM-target compatibility detected" in CI

When a project compiles both Java and Kotlin, Gradle checks that compileJava and compileKotlin target the same JVM bytecode version. A mismatch (for example Java targeting 17 while Kotlin targets 1.8) fails the build because the outputs would be inconsistent.

What this error means

The build fails with "‘compileJava’ task (current target is 17) and ‘compileKotlin’ task (current target is 1.8) jvm target compatibility should be set to the same Java version."

> Inconsistent JVM-target compatibility detected for tasks 'compileJava'
  (17) and 'compileKotlin' (1.8).
  This will become an error in future versions of the Kotlin Gradle plugin.

Common causes

Java and Kotlin target different versions

The Java toolchain or sourceCompatibility targets one version while kotlinOptions.jvmTarget (or the Kotlin compiler options) targets another.

A toolchain set for one language but not the other

Applying a Java toolchain without aligning the Kotlin JVM target leaves the two compile tasks disagreeing.

How to fix it

Set one toolchain for both

Configure a single Java toolchain so both compile tasks derive the same target.

kotlin {
  jvmToolchain(17)
}

Pin Java and Kotlin targets explicitly to match

If you set targets directly, make the Kotlin JVM target equal the Java target.

java { sourceCompatibility = JavaVersion.VERSION_17 }
tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile> {
  compilerOptions.jvmTarget.set(org.jetbrains.kotlin.gradle.dsl.JvmTarget.JVM_17)
}

How to prevent it

• Prefer jvmToolchain so both compilers share one version.
• Keep Java and Kotlin JVM targets in sync when set manually.
• Review target settings when upgrading the Kotlin plugin.

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