warning

ProtocolLib does not match this Minecraft version

Q: What does "[ProtocolLib] Version (MC 1.21.4) has not yet been tested! Proceed with caution." mean?

ProtocolLib does not match this Minecraft version. ProtocolLib, or a plugin built on it, predates the Minecraft version you run. Its internals reach into server code that moved. Install the latest ProtocolLib build for your exact Minecraft version (dev builds after an update). If errors continue, the plugin using it needs updating too.

Q: How do I fix "[ProtocolLib] Version (MC 1.21.4) has not yet been tested! Proceed with caution."?

1. Match the build to the game: Check the ProtocolLib page for a release that lists your Minecraft version. After a game update the working build is often a development snapshot from the project CI, linked from the same page, before a numbered release exists. 2. Replace the jar and restart: Delete the old jar from plugins, add the new one. Two ProtocolLib jars in the folder will fail in a different way. 3. Update the plugins built on it: If errors mention com.comphenix.protocol below a plugin name, that plugin is calling a ProtocolLib API that changed. It needs its own update. 4. If nothing exists yet, wait or roll back: A brand-new Minecraft version can be days ahead of ProtocolLib. Either run the previous game version until it catches up or run without the plugins that need it.

Applies to Paper, Vanilla.

What it looks like in your log

[ProtocolLib] Version (MC 1.21.4) has not yet been tested! Proceed with caution.

What causes it

ProtocolLib, or a plugin built on it, predates the Minecraft version you run. Its internals reach into server code that moved.

How to fix it

Install the latest ProtocolLib build for your exact Minecraft version (dev builds after an update). If errors continue, the plugin using it needs updating too.

Why it happens

ProtocolLib reaches inside the server to read and rewrite packets, which means it depends on the internal layout of the exact Minecraft version. Every game update moves that layout. A ProtocolLib build from before the update warns that it has not been tested, and then either works by luck or breaks every plugin that uses it: packet-based anti-cheat, cosmetics, holograms, custom GUIs, tab list plugins.

The warning is accurate. It means the author has not released a build for your version yet, or you have not installed it.

Fix it step by step

1Match the build to the gameCheck the ProtocolLib page for a release that lists your Minecraft version. After a game update the working build is often a development snapshot from the project CI, linked from the same page, before a numbered release exists.
2Replace the jar and restartDelete the old jar from plugins, add the new one. Two ProtocolLib jars in the folder will fail in a different way.
3Update the plugins built on itIf errors mention com.comphenix.protocol below a plugin name, that plugin is calling a ProtocolLib API that changed. It needs its own update.
4If nothing exists yet, wait or roll backA brand-new Minecraft version can be days ahead of ProtocolLib. Either run the previous game version until it catches up or run without the plugins that need it.

If that didn't work

FieldAccessException or "Unsupported minecraft version" under a ProtocolLib stack is the same problem after the warning was ignored. The fix is the same.

Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.

Check your own log