sbt "Unsupported class file major version" in CI

By Daniel Zoghalchali·Latchkey

A bytecode-processing tool (ASM inside a plugin, a coverage or shading step) encountered class files from a JDK newer than it understands and threw "Unsupported class file major version N". The tool or the JDK must be aligned.

What this error means

sbt fails with "IllegalArgumentException: Unsupported class file major version 65" (or 64/63) from an assembly, coverage, or bytecode-manipulating plugin.

[error] java.lang.IllegalArgumentException: Unsupported class file major version 65
[error]   at org.objectweb.asm.ClassReader.<init>(ClassReader.java:...)

Common causes

A plugin bundles an old ASM version

The bytecode library inside a plugin predates the JDK running the build (major 65 is Java 21), so it cannot parse the newer class files.

The CI JDK is newer than the tooling supports

setup-java installed a JDK ahead of what the plugin ecosystem handles, so a build step that reads bytecode fails.

How to fix it

Upgrade the plugin that bundles ASM

1. Identify the plugin doing bytecode work (assembly, coverage, shading).
2. Upgrade it to a version that supports the JDK in use.
3. Re-run the step.

// project/plugins.sbt
addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "2.2.0")

Pin the CI JDK to a supported version

If a plugin cannot yet handle the newest JDK, set setup-java to a version it supports.

- uses: actions/setup-java@v4
  with:
    distribution: temurin
    java-version: '17'

How to prevent it

• Keep bytecode-manipulating plugins current with your JDK.
• Pin the CI JDK to a version your plugin set supports.
• Test a JDK bump against the full plugin toolchain before adopting it.

Frequently asked questions

Related guides

References

• Apache Maven guides
• Gradle user manual
• GitHub Actions documentation