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Fix “UnsupportedClassVersionError” when starting Minecraft Java

Applies to: Java launcher or plugin class loading on JVM-based Minecraft services; runtime and class requirements must match the exact server/plugin releases.
Documentation checked .

UnsupportedClassVersionError means Java encountered a class format that the running JVM does not support. It is different from a plugin's Minecraft API requirement and from running out of memory. The class name and version numbers in the complete error determine which part of the installation needs attention.

Preserve the useful evidence

Copy the first exception and its surrounding startup lines from the panel console. If Java stops before Minecraft starts, logs/latest.log may never receive the error. Record the runtime version printed by this particular server's startup, the Minecraft/Paper build and the last changed JAR. Redact credentials or tokens from the startup information before sharing it.

A message may compare the class file's required version with the highest version the current runtime recognises. Those are class-format numbers, not Minecraft release numbers. Do not assume a number such as 65 describes Minecraft, the amount of memory or a plugin version. Oracle's exception documentation defines this failure in terms of unsupported class-format versions.

Decide whether the server or a plugin failed

If the named class belongs to the server launcher or server implementation, compare that server release's documented Java requirement with the selected runtime. If Paper starts and the failure names a plugin class during plugin loading, check that plugin release's Java requirement as well. A server may be compatible with a runtime while one newly installed plugin is not.

Find the runtime selection in the game panel if your plan exposes it. The Java installed on your own computer, another game service or a host shell is not proof of the Java inside this service's container. Use the startup evidence from the affected service. If no runtime selector is available, send support the complete class-version message and required release rather than changing unrelated startup flags.

Choose a compatible recovery

Preserve a backup and the previous working JAR/configuration. During a planned stop, select a runtime supported by both the exact Minecraft/Paper release and your required plugins. Do not select the newest Java merely because its number is larger; compatibility must cover the whole stack. Requirements differ between historical 1.21.x builds and current 26.x releases.

Alternatively, if only a new plugin requires a different runtime, obtain an author-supported compatible plugin build or return to the previous compatible plugin state. Keep its data and review whether the attempted update already migrated configuration or a database. An older JAR alone may not reverse that migration.

Verify the actual restart

Start once and confirm the runtime banner reflects the intended selection. Check that the original class-version exception is gone, that the server finishes starting and that the relevant plugin feature works with an ordinary account. A panel setting showing the desired runtime is weaker evidence than the new process's output.

If the change fails, restore the previous supported runtime and compatible application state. Do not force a newer world into an older Minecraft release to avoid the Java requirement, edit class files or disable version safeguards. Provide the new first error to support; it may reveal a separate compatibility problem after this one is resolved.

Official references

Paper example of an outdated Java runtime. Oracle definition of UnsupportedClassVersionError. Paper release-specific Java requirements.

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