Wrong mod loader

Fabric Crash: Failed to Read classTweaker File (Namespace Mismatch)

What it looks like in the log

java.lang.RuntimeException: Failed to read classTweaker file from mod rich

How to fix it

  1. Look at the line 'Failed to read classTweaker file from mod <modid>' in your crash log to identify the offending mod, then remove that mod's JAR from your mods folder.
  2. If you downloaded the mod from a release page, check its filename and replace any '-dev.jar' or '-sources.jar' file with the regular release JAR.
  3. If you compiled this mod yourself, run './gradlew build' (which triggers 'remapJar') rather than the plain 'jar' task, and use the output file in 'build/libs/' that does not end in '-dev.jar'.
  4. If you are the mod developer and 'remapJar' still produces this crash, add 'accessWidenerPath = file("src/main/resources/<yourmod>.accesswidener")' inside the 'loom' block of your build.gradle and rebuild.
LoadersFabric
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 loaderClassTweakerFormatException: Namespace (named) Does Not Match Runtime (intermediary)Check the crash log for the line 'Failed to read classTweaker file from mod <mod_id>' to identify the broken mWrong mod loaderFabric mod dropped into a Forge or NeoForge instanceA Fabric jar has fabric.mod.json; Forge/NeoForge want META-INF/mods.tomlWrong mod loaderFailed to read accessWidener file: Namespace (named) does not match (intermediary)Open your instance's 'mods' folder and find the mod named in the error message (for example, 'iris').Wrong mod loaderFailed to read classTweaker file from mod (Namespace Mismatch)Find the broken mod's ID right after 'Failed to read classTweaker file from mod' in the error log.Wrong mod loaderForge mod dropped into a Fabric instance (and the other way round)Forge and Fabric jars are not interchangeable — the file names often look identical