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Outdated client! Please use 1.21.x (protocol mismatch)

Minecraft's network protocol changes with every version. When the client's protocol doesn't match the server's, the server rejects the join with 'Outdated client' (client older) or 'Outdated server' (client newer). Nothing is broken — the two sides just speak different protocol versions.

Fastest path: paste your crash report into the Crash Doctor — it identifies this pattern and 40+ others automatically. No signup.

How to fix it

  1. 1

    Check the server's version

    The error names the version the server wants. Panel status pages and the boot log also show it.

  2. 2

    Match the client

    In the vanilla launcher, install and launch that exact version. Modded clients must match both MC version and loader — a 1.20.1 Fabric client won't join a 1.21 vanilla server.

  3. 3

    Or bridge versions with ViaVersion

    Server admins can install ViaVersion (allow newer clients on an older server) and ViaBackwards (older clients on a newer server). One plugin, big convenience.

  4. 4

    Bedrock players: Geyser + Floodgate

    Bedrock clients speak a different protocol entirely; Geyser translates it. See the crossplay guide below for the setup.

Alternative causes

These can produce the same error message — worth ruling out if the steps above don't resolve it.

Snapshot vs release mismatch

Snapshot protocol versions differ from their release counterparts. A snapshot client can't join a release server even from the same week.

Modded client on a vanilla server

Loader handshakes extend the protocol. A modded client usually can't join vanilla servers, and vice versa, regardless of matching MC versions.

Frequently asked

Can one server accept multiple Minecraft versions?

Yes, with ViaVersion/ViaBackwards plugins covering the protocol gaps. Not every modded setup tolerates them, but vanilla and many plugin servers do.

Why do modpacks lock everyone to one version?

Mods are compiled per MC version with exact API expectations. The pack's version is the whole ecosystem's version.

Is this the same as 'bad login'?

No. 'Bad login' is session/authentication; 'Outdated client' is purely protocol version. Different causes, different fixes.

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Last reviewed 2026-09-06. If a step is wrong or out of date, tell us — we'll fix the article and the auto-pattern at the same time.