What does "Incompatible client! Please use 1.21.1" mean?

Players are being turned away for running a different game version. The server rejected joins because the client is on a different Minecraft version, so the two never agree on a protocol. The joining player picks the version the server runs in their launcher installation, or the server owner changes the server version to match the players.

How do I fix "Incompatible client! Please use 1.21.1"?

1. Match the client to the server: In the Minecraft launcher, create an installation on the exact version the message names and join from that. Everything else in the launcher stays as it was. 2. Or move the server: If most players have updated and the server is behind, update the server. Back up the world first: a world opened by a newer version cannot be reopened by the older one. 3. On a modpack, update the pack, not the jar: The pack pins the game version. Installing the pack's newer release on both sides moves both together. 4. Let old clients through, if you must: ViaVersion on the server lets newer clients join an older server, and ViaBackwards lets older clients join a newer one. It is a compatibility layer, not a substitute for matching versions, and some features will not work across it.

Roastd / Minecraft

warning

Players are being turned away for running a different game version

What it looks like in your log

Incompatible client! Please use 1.21.1

What causes it

The server rejected joins because the client is on a different Minecraft version, so the two never agree on a protocol.

How to fix it

The joining player picks the version the server runs in their launcher installation, or the server owner changes the server version to match the players.

Why it happens

Client and server negotiate a protocol version in the first packet. If they differ, the server refuses the join and tells the client which version it runs. "Incompatible client! Please use 1.21.1" means the server is on 1.21.1 and the client is not. Neither side is broken; they are speaking different dialects and the server declines to guess.

Version numbers with the same first two parts are not always compatible. 1.20.1 and 1.20.4 have different protocols. The number in the message is exact.

Fix it step by step

1Match the client to the serverIn the Minecraft launcher, create an installation on the exact version the message names and join from that. Everything else in the launcher stays as it was.
2Or move the serverIf most players have updated and the server is behind, update the server. Back up the world first: a world opened by a newer version cannot be reopened by the older one.
3On a modpack, update the pack, not the jarThe pack pins the game version. Installing the pack's newer release on both sides moves both together.
4Let old clients through, if you mustViaVersion on the server lets newer clients join an older server, and ViaBackwards lets older clients join a newer one. It is a compatibility layer, not a substitute for matching versions, and some features will not work across it.

If that didn't work

If the message names a mod loader ("Please use NeoForge 21.1.72"), the versions match and the loader does not. That is the loader mismatch page.

"Outdated server! I'm still on 1.20.1" is the same problem seen from a client that is newer than the server.

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