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Coremod Crash (Forge)

A Forge server crashes early with a ClassCastException tied to a coremod. Coremods rewrite Minecraft's code at load time, so when one is outdated or conflicts with another, the result is a hard crash before the game finishes loading.

What does this error mean?
A coremod uses bytecode transformation (ASM/Mixin) to patch classes as they load. A ClassCastException here usually means a transformer modified a class in a way another mod or the current Forge version no longer expects, producing an incompatible object at runtime. 
java.lang.ClassCastException
Most Common Causes
  • An outdated coremod built for an older Minecraft/Forge version.
  • Two coremods patching the same class in conflicting ways.
  • A Forge update that changed internals the coremod relied on.
  • Load-order issues between transforming mods.
  • A bug in the coremod's transformer.
How To Diagnose
  1. Open the crash report and find which coremod/transformer is named.
  2. Note the class involved in the ClassCastException.
  3. Check whether the coremod is up to date for your exact Forge build.
  4. Remove suspected coremods one at a time to isolate the conflict.
Recommended Fixes
  • Update the coremod
    Install the build that matches your Minecraft and Forge version.
  • Remove the conflicting mod
    If two coremods clash, drop the less essential one.
  • Match the Forge version
    Use the Forge version the mods were built against.
  • Adjust load order
    Where supported, set mod load order so transformers apply cleanly.
Analyze My Log
Frequently Asked Questions

A mod that rewrites Minecraft's own bytecode at load time to change deep behavior, more invasive than a normal mod.

They patch internal code. When Minecraft or Forge changes that code, the patch can no longer apply correctly.

The crash report names the transformer/coremod. Confirm by removing it and retesting.