How to Fix UnsupportedClassVersionError on a Minecraft Server

Direct answer

UnsupportedClassVersionError means the JVM is trying to read class files built for a newer Java class-file version than that runtime supports. Run java -version in the same environment that starts the server, then install/select the Java version required by the server or plugin.

Current Paper mapping

For current Paper, 1.20–1.21.11 uses Java 21 and 26.1+ uses Java 25. If the error appears before Paper starts, the server runtime itself is too old. If it appears only while one plugin loads, that plugin may have been built for a newer Java baseline than your server environment.

The operating system may still launch old Java

Installing a new JDK does not guarantee the java command changed. Reopen the shell and verify PATH. On a game panel, change the Java/container image rather than the Java installed on your own desktop.

Do not edit class-file versions

Targeted diagnosis: Read the class-file version from the error and determine which Java runtime is actually launching the server. Check java -version in the same environment as the panel or systemd unit, not only in your interactive shell.

Changing a JAR name or editing plugin.yml cannot make bytecode compiled for a newer JVM compatible. Use a supported runtime or obtain a build of the software that targets your current Java/server line.

Fix and verify: Install the Java release required by the target server/plugin build, or use a compatible older artifact. Do not attempt to repair the issue by changing Minecraft port, deleting worlds or adding RAM.

What the Java error literally means

Oracle defines UnsupportedClassVersionError as the JVM rejecting a class because its major/minor class-file version is not supported. That makes it a runtime compatibility problem, not a world or networking problem.

Official references

These references were checked while preparing this fix. Version-specific behavior can change, so use the project documentation for your exact release when a detail differs.

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