Mixin failures

java.lang.VerifyError: Bad type on operand stack

What the log looks like

java\.lang\.VerifyError:\s*Bad type on operand stack

How to fix it

  1. If you are using OptiFine and OptiFabric on Fabric, remove both from your mods folder—OptiFabric frequently collides with other mods' Mixins and triggers VerifyError crashes (use Sodium and Iris as replacements).
  2. Open logs/latest.log and look at the class name listed under 'Exception Details -> Location:' (such as net/minecraft/class_5944), then search latest.log for that class name to identify which mod's Mixin modified it.
  3. If you manually customized library versions in Prism Launcher or MultiMC (such as overriding sponge-mixin), reset your instance's Fabric/Quilt/Forge version page to default stable libraries.
  4. Update your mod loader and major rendering or optimization mods (like Sodium, Iris, or Fabric API) so all bytecode patches match your exact Minecraft version.
LoadersFabric, Quilt, Forge, Neoforge
SeverityStops the game
CategoryMixin failures

Check your own log

Related errors

Mixin failuresCritical injection failure — a mixin could not find its targetTwo mods are usually fighting over the same game methodMixin failuresInvalidMixinException: Mod Mixin Failed to ApplyOpen your latest.log or crash report and look for 'Mixin apply for mod' or the 'Config :' line inside the 'InvMixin failuresInvalidMixinException: Unexpecteded ClassMetadataNotFoundExceptionFind the line saying 'Mixin apply for mod <modname> failed' or 'from mod <modname>' right above the error in yMixin failuresMixin Transformation of net.minecraft.client.main.Main FailedOpen latest.log or the launcher crash output and look at the Caused by lines below Mixin transformation of netMixin failuresMixin apply failed — one mod could not patch the gameThe line names the mixin config, and its prefix is the mod id that brokeMixin failuresMixinExtras is missing or too oldUpdate the mod that needs it — modern mods bundle MixinExtras themselves