Getting warning about Kotlin being incompatible with JDK 25 with Gradle 9.1.0, JDK 25, and Kotlin DSL

[tbroyer]

Current Behavior

Getting warnings:

> Task :build-logic:compileKotlin
Kotlin does not yet support 25 JDK target, falling back to Kotlin JVM_24 JVM target
w: ⚠️ Inconsistent JVM Target Compatibility Between Java and Kotlin Tasks
Inconsistent JVM-target compatibility detected for tasks 'compileJava' (25) and 'compileKotlin' (24).
This will become an error in Gradle 8.0.
Solution:
Consider using JVM Toolchain: https://kotl.in/gradle/jvm/toolchain
Learn more about JVM-target validation: https://kotl.in/gradle/jvm/target-validation

> Configure project :
Kotlin does not yet support 25 JDK target, falling back to Kotlin JVM_24 JVM target

Expected Behavior

No warning at all, as Gradle 9.1.0 is supposedly compatible with JDK 25.

Context (optional)

No response

Self-contained Reproducer Project

Using JDK 25 and Gradle 9.1.0.

1. Create a project using gradle init

   $ gradle init
   
   Select type of build to generate:
     1: Application
     2: Library
     3: Gradle plugin
     4: Basic (build structure only)
   Enter selection (default: Application) [1..4] 1
   
   Select implementation language:
     1: Java
     2: Kotlin
     3: Groovy
     4: Scala
     5: C++
     6: Swift
   Enter selection (default: Java) [1..6] 1
   
   Enter target Java version (min: 7, default: 21): 25
   
   Project name (default: test): 
   
   Select application structure:
     1: Single application project
     2: Application and library project
   Enter selection (default: Single application project) [1..2] 2
   
   Select build script DSL:
     1: Kotlin
     2: Groovy
   Enter selection (default: Kotlin) [1..2] 1
   
   Generate build using new APIs and behavior (some features may change in the next minor release)? (default: no) [yes, no] yes
   
   
   > Task :init
   Learn more about Gradle by exploring our Samples at https://docs.gradle.org/9.1.0/samples/sample_building_java_applications_multi_project.html
   
   BUILD SUCCESSFUL in 28s
   1 actionable task: 1 executed
   

2. run gradle tasks (or ./gradle tasks)

   $ gradle tasks
   Calculating task graph as no cached configuration is available for tasks: tasks
   
   > Configure project :app
   Kotlin does not yet support 25 JDK target, falling back to Kotlin JVM_24 JVM target
   
   > Task :build-logic:compileKotlin
   Kotlin does not yet support 25 JDK target, falling back to Kotlin JVM_24 JVM target
   w: ⚠️ Inconsistent JVM Target Compatibility Between Java and Kotlin Tasks
   Inconsistent JVM-target compatibility detected for tasks 'compileJava' (25) and 'compileKotlin' (24).
   This will become an error in Gradle 8.0.
   Solution:
   Consider using JVM Toolchain: https://kotl.in/gradle/jvm/toolchain
   Learn more about JVM-target validation: https://kotl.in/gradle/jvm/target-validation
   
   
   > Task :tasks
   […]
   

Gradle version

9.1.0

Build scan URL (optional)

https://scans.gradle.com/s/ml4t6riodsmh6

Your Environment (optional)

$ gradle --version

------------------------------------------------------------
Gradle 9.1.0
------------------------------------------------------------

Build time:    2025-09-18 21:55:32 UTC
Revision:      <unknown>

Kotlin:        2.2.0
Groovy:        4.0.28
Ant:           Apache Ant(TM) version 1.10.15 compiled on August 25 2024
Launcher JVM:  25.0.1 (Arch Linux 25.0.1)
Daemon JVM:    /usr/lib/jvm/java-25-openjdk (no JDK specified, using current Java home)
OS:            Linux 6.16.8-arch3-1 amd64

$ java --version
openjdk 25.0.1 2025-09-16
OpenJDK Runtime Environment (build 25.0.1)
OpenJDK 64-Bit Server VM (build 25.0.1, mixed mode, sharing)

[TWiStErRob]

It might not possible to fix this yet as no stable Kotlin is released to support JVM_25.
See: https://youtrack.jetbrains.com/issue/KT-81077/Add-JVM-target-bytecode-version-25

Although I wonder if kotlin-dsl should target the lowest possible version for Gradle (i.e. 17)? Might not be feasible, just a thought.

[hegyibalint]

This issue needs a decision from the team responsible for that area. They have been informed. Response time may vary.

[jbartok]

The warning is coming from KGP, not Gradle per se:
https://github.com/JetBrains/kotlin/blob/3d14798762df92c7d754d89cf16affac44b6c991/libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/plugin/diagnostics/KotlinToolingDiagnostics.kt#L792

So we'll probably need to:

• open an issue on their tracker and ask to provide a less confusing message
• check if anything could be improved on the actual Gradle side

[dejan2609]

@jbartok
JetBrains already knows about this: https://youtrack.jetbrains.com/issue/KT-80096/Strange-Inconsistent-JVM-Target-Compatibility-warning