What does NoClassDefFoundError mean in modded Minecraft?

A mod used code that is not in the game: a class from Minecraft, the loader or a library mod that is missing or has a different version. Usually a required library mod is not installed, or a mod is made for another Minecraft version.

What does NoSuchMethodError mean in Minecraft?

The class exists but the method the mod calls does not, because the mod was built against another version of Minecraft or of a library mod. Install the versions the mod was made for.

How do I fix NoClassDefFoundError in Minecraft?

Read the class name in the error: its package tells you which mod or library it belongs to. Install that library (for example Fabric API, Architectury or Cloth Config) in the version your mods need, or install versions of your mods made for your exact Minecraft version and loader.

Crash help

Minecraft crash: NoClassDefFoundError and NoSuchMethodError

A mod used code this game does not have. Usually a library mod is missing, or the mod is made for another version of Minecraft, of the loader or of a library.

Updated October 2, 2026. Fabric, NeoForge and Forge.

What you see

The crash report or log has one of these, followed by a class name:

java.lang.NoClassDefFoundError: dev/architectury/event/events/common/TickEvent
java.lang.ClassNotFoundException: me.shedaniel.clothconfig2.api.ConfigBuilder
java.lang.NoSuchMethodError: 'void net.minecraft.world.level.Level.playSound(...)'
java.lang.NoSuchFieldError: TOOLTIP
java.lang.AbstractMethodError, IncompatibleClassChangeError, IllegalAccessError

They are the same problem in different places: the mod was compiled against code that is missing or different in your game.

Why it happens

A library mod is missing. Many mods need another mod to run: Fabric API, Architectury, Cloth Config, GeckoLib, Balm and others. Without it, the first class the mod asks for is not found.
A version does not match. NoSuchMethodError and NoSuchFieldError mean the class is there but changed: the mod was built for another version of Minecraft or of the library.
A mod for another loader. A Fabric mod in a NeoForge game (or the other way) asks for loader classes that are not there.

Find the mod

Open the newest file in crash-reports, or logs/latest.log.
Read the class name after the error. Its first parts name who owns it: dev.architectury is Architectury, me.shedaniel.clothconfig2 is Cloth Config, net.fabricmc.fabric is Fabric API, net.minecraft is Minecraft itself.
Then look at the lines below it (the stack trace) for the first one that is not Minecraft or Java: that is the mod that asked for the missing code.

Fix it

If the class belongs to a library, install it, in the version the mod's page lists for your Minecraft.
If the class is from Minecraft (net.minecraft), the mod is for another Minecraft version: get the version made for yours.
If the library is installed, update or downgrade it to the version the mod needs; mod pages list required versions under dependencies.

Let the game tell you

The Modkeel Companion reads the crash on the next start, matches the missing class to the mod that owns it and the mod that asked for it, and offers to turn the culprit off or go back to the mods that last worked. Free and open source, for Fabric, NeoForge and Forge.