Gradle: 'Unsupported class file major version 65' (or 69) / 'BUG! exception in phase semantic analysis'. Why does the build fail after installing a newer JDK, and how do I fix it?

Cause: Gradle itself runs on a JDK, and each Gradle version only supports running on Java versions up to a certain release. When the JDK that launches Gradle (JAVA_HOME, the IDE's Gradle JVM setting, or the CI image's default Java) is newer than your Gradle version supports, Gradle's own Groovy/Kotlin compilation fails with errors like 'Unsupported class file major version'. The number is the class-file version of the JDK: 65 is Java 21, 69 is Java 25. Per Gradle's compatibility matrix, running on Java 21 needs Gradle 8.5+, Java 23 needs 8.10+, Java 24 needs 8.14+, and Java 25 needs 9.1.0+. Fix (pick one): 1. Upgrade the Gradle wrapper to a version that supports your JDK: ./gradlew wrapper --gradle-version <version> (run it with a JDK the current Gradle still supports), then commit gradle/wrapper/gradle-wrapper.properties. 2. Or run Gradle on an older, supported JDK: point JAVA_HOME (or the IDE's 'Gradle JVM' setting, or the CI setup-java version) at that JDK. 3. Separate 'the JDK that runs Gradle' from 'the JDK that compiles your code' with Java toolchains: in the build, java { toolchain { languageVersion = JavaLanguageVersion.of(21) } }. Gradle then finds or provisions that JDK for compiling and testing, independent of which JDK launches Gradle (which still has to be supported by your Gradle version). 4. Clear stale state after switching: stop daemons with ./gradlew --stop, and in the IDE re-sync the project. Verify: ./gradlew --version shows the Gradle version and the JVM it runs on, and that pair is in the compatibility matrix; the build then runs. Sources: https://docs.gradle.org/current/userguide/compatibility.html https://docs.gradle.org/current/userguide/toolchains.html