UnsupportedClassVersionError: matching Java to Minecraft

Class file version 65.0 means Java 21. Here is the full mapping, which Minecraft version needs which Java, and what to do when a single mod is the one complaining.

This error is unusually honest. It tells you exactly what it found and exactly what it can handle, in numbers that mean nothing until somebody gives you the table.

#Reading the error

java.lang.UnsupportedClassVersionError: com/example/Thing has been compiled by a
more recent version of the Java Runtime (class file version 65.0), this version
of the Java Runtime only recognizes class file versions up to 61.0

Two numbers, both class file versions rather than Java versions. Here is the mapping:

So the example above says: this code needs Java 21, the server is running Java 17.

It never goes the other way round. Newer Java runs older code perfectly happily; older Java cannot run newer code, because the format did not exist when it was written.

#Which Java each Minecraft version needs

Mods can raise this. A mod compiled for Java 21 needs Java 21 regardless of what Minecraft version it targets, which is where the single-mod version of this error comes from.

Mods rarely lower it, but 1.12.2 packs are a genuine exception: much of that ecosystem expects Java 8 specifically, and running it on anything newer produces its own separate collection of problems.

#On Hostd

The Java version is selected to match the Minecraft version you are running, so the plain version of this error is uncommon. When it appears, it is usually one of:

• A modpack pinned to a Minecraft version whose mods have moved ahead of it. A pack on 1.20.1 that has pulled in a mod built for Java 21 will hit this.
• A hand-uploaded mod jar built for a newer Java. Check the mod's page for the Java version it targets, which is usually stated alongside the Minecraft version.
• A world or pack imported from another host that was running a different Java version than the game version implies.

If the whole server fails to start with this error rather than a single mod, open a ticket with the crash: the runtime and game version are out of step and that is ours to fix.

#What you cannot do about it

You cannot make a mod run on an older Java by changing a setting. The class file format is not negotiable and there is no compatibility flag.

The options are: get a build of the mod for your Java version, or move the server to a Minecraft version that runs on the Java the mod needs. For a single mod in a hand-built list, the first. For a pack, the second, which usually means picking a newer release of the pack.

#The reverse case

Occasionally 1.12.2 servers throw errors that look related but are not: a pack built for Java 8 running on Java 17 tends to fail with reflection and module access errors rather than an UnsupportedClassVersionError, because the code loads fine and then cannot reach the internals it expects.

If you are running a 1.12.2 pack such as RLCraft and seeing errors about illegal reflective access, that is the shape of it.

Written and maintained by the Hostd engineering team. Last updated 2026-08-05. Notice a mistake? Tell us.