Error Codes › java.lang.NoClassDefFoundError

java.lang.NoClassDefFoundError

COAL-009 · Class not found (NoClassDefFoundError)

A class the server needed at runtime was not there - usually a missing library mod, a version mismatch, or a client-only mod installed on a server.

What it looks like in the log

java.lang.NoClassDefFoundError: net/minecraft/client/Minecraft
	at com.example.somemod.SomeClass.<init>(SomeClass.java:42)

What it means

Code tried to use a class that was not on the classpath when it ran. The class name after the colon is the missing piece and the most useful thing on the line - read it before anything else. A path starting with net/minecraft/client/ means a mod is reaching for client-only code that a dedicated server does not ship at all.

How to fix it

If the class is under org/lwjgl, you have a client-only mod on the server - COAL-017 covers that case in full.
If it is under net/minecraft/client, same story: the mod named in the stack trace below the error is client-side only. Remove it from the server's mods folder.
Otherwise it is a missing library. The package name usually gives away which mod owns it - install that library mod at a version matching your Minecraft version.
Check for a version mismatch. A class that exists but moved between versions produces this too, so make sure every mod targets your exact Minecraft version.