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Fabric Loader Crash

A Fabric server crashes during loading with a mixin error. A Fabric mod tried to inject into a class it no longer matches, almost always a version-compatibility problem between mods, Fabric Loader, and Minecraft.

What does this error mean?
Fabric mods use Mixin to weave changes into Minecraft classes at load time. 'Error while applying mixin' means a mod's injection target changed or is missing, so the patch cannot be applied, typically because the mod is built for a different Minecraft version. 
Error while applying mixin
Most Common Causes
  • A mod built for a different Minecraft version than you run.
  • An outdated Fabric Loader or Fabric API.
  • Two mods mixing into the same method in incompatible ways.
  • A mod depending on internals changed by another mod.
  • A corrupted or partially downloaded mod jar.
How To Diagnose
  1. Read the log for the failing mixin and the mod that owns it.
  2. Check that the named mod targets your exact Minecraft version.
  3. Confirm Fabric Loader and Fabric API are current.
  4. Remove the named mod and retest to confirm the cause.
Recommended Fixes
  • Update the mod
    Install the build that matches your Minecraft version, this fixes most mixin failures.
  • Update Fabric Loader/API
    Run the latest Loader and Fabric API for your version.
  • Remove the conflicting mod
    If two mods clash on the same target, drop the less essential one.
  • Re-download the jar
    Replace a possibly corrupted mod file with a fresh download.
Analyze My Log
Frequently Asked Questions

A Fabric mechanism that injects code into existing Minecraft methods/classes at load time.

Mixins target specific code. A mod for another version aims at code that has since moved or changed.

Most Fabric mods require it. Install the version matching your Minecraft release.