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NeoForge Crash

A NeoForge server crashes because a mod tried to load a class improperly. NeoForge evolves quickly, and mods built for an older or newer build can break against the one you run.

What does this error mean?
NeoForge is the actively developed fork of Forge. When a mod attempts to load or transform a class in a way the current NeoForge build does not allow, loading aborts, usually a sign the mod and the loader are on incompatible versions. 
Mod X attempted to load class Y improperly
Most Common Causes
  • A mod built for a different NeoForge build than installed.
  • Mixing Forge-era mods with NeoForge.
  • A NeoForge update that changed internals a mod relied on.
  • Two mods conflicting over the same class.
  • A missing transitive dependency.
How To Diagnose
  1. Check the crash report for the mod and class involved.
  2. Compare the mod's required NeoForge version against yours.
  3. Confirm all mods are NeoForge builds (not legacy Forge).
  4. Bisect mods to isolate the one that fails to load.
Recommended Fixes
  • Match the NeoForge version
    Install the NeoForge build the mods were made for, or update the mods to match yours.
  • Use NeoForge builds
    Replace legacy Forge mods with their NeoForge versions.
  • Remove the conflicting mod
    Drop the mod the crash report blames if no compatible build exists.
  • Add missing dependencies
    Install any libraries the mod requires for your version.
Analyze My Log
Frequently Asked Questions

It is a separate, more actively maintained fork. Mods must target NeoForge specifically.

No. Use mods built for the loader you run, they are not cross-compatible.

Fast-moving internals change between builds, so mods must be rebuilt against the new version.