Mixin apply failed (MixinTransformerError)
Mixin is the framework most modern Forge and Fabric mods use to patch Minecraft's internals at runtime. When a mixin 'apply failed', it means the mod tried to inject code into a method or class that has changed shape — either because another mod beat it to the target, or because the Minecraft version has a different layout than the mod expected.
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How to fix it
- 1
Find the failing mixin in the trace
The error names the mixin file and target, e.g. 'mod.mixins.json:FooMixin' or 'MixinChunkHolder'. The mixin file is owned by exactly one mod — the part before mixins.json is the mod ID.
- 2
Check for a newer version of the offending mod
Mixin failures are the #1 reason mod authors push patch releases. Open the mod's CurseForge or Modrinth page and grab the newest version that targets your MC version. 9 out of 10 mixin crashes are fixed by a one-mod update.
- 3
Look for a conflicting mod
If updating doesn't help, another mod might be patching the same target first. Common conflicts: Sodium + OptiFine, Embeddium + Iris on Forge, Performant + Lithium. Try disabling one mod at a time until the crash stops.
- 4
Disable the mod as a last resort
If no update or alternative is available, the broken mod has to go. Move its .jar out of mods/ and report the crash to the mod's issue tracker — the author needs the report to fix it.
Alternative causes
These can produce the same error message — worth ruling out if the steps above don't resolve it.
Mixin failure during Forge -> NeoForge migration
Forge and NeoForge share the same MC mappings but diverged enough that some mixins designed for Forge no longer apply on NeoForge. If you just migrated, find NeoForge-native versions of every mixin-using mod.
Mixin failure on Connector (Fabric-on-Forge)
Sinytra Connector lets Fabric mods run on Forge but doesn't translate every mixin pattern. If the failing mixin owner is a Fabric mod loaded via Connector, that's the real issue — see our Sinytra Connector guide.
Frequently asked
What does InvalidInjectionException mean?
It's a specific mixin failure: the @Inject annotation pointed at a method or instruction that doesn't exist in the target class. Usually the target was renamed or removed in your MC version vs what the mod expected.
Can I ignore a mixin warning?
Warnings during mixin loading are usually fine. 'Mixin apply failed' is a crash, not a warning — the mod won't function and the game can't continue.
Why does the same mod work fine on someone else's server?
Mod compatibility depends on which OTHER mods are in your folder. A mixin from mod A only fails when mod B has already patched the same target. The 'works for them, breaks for you' answer is almost always a different mod set.
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Last reviewed 2026-06-05. If a step is wrong or out of date, tell us — we'll fix the article and the auto-pattern at the same time.