Wrong mod loader

Forge mod dropped into a Fabric instance (and the other way round)

What the log looks like

noclassdeffounderror: *net/minecraftforge/
noclassdeffounderror: *net/neoforged/
classnotfoundexception: *net\.minecraftforge\.
does not contain a fabric\.mod\.json

How to fix it

  1. Forge and Fabric jars are not interchangeable — the file names often look identical
  2. Re-download the build whose file name carries your loader
  3. Some mods have no build for your loader at all
LoadersFabric, Quilt
SeverityStops the game
CategoryWrong mod loader

Check your own log

Related errors

Wrong mod loaderAccessWidenerFormatException: Namespace (named) does not match current runtime namespace (intermediary)Check the crash log for the line 'Failed to read accessWidener file from mod <modname>' to identify the brokenWrong mod loaderFabric mod dropped into a Forge or NeoForge instanceA Fabric jar has fabric.mod.json; Forge/NeoForge want META-INF/mods.toml