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Minecraft JEI ClassNotFoundException Fix

If your Minecraft crash log mentions ClassNotFoundException, mezz.jei, or IJeiRuntime, the problem is usually connected to Just Enough Items, also called JEI, or a mod that expects JEI to be installed.

Quick answer

Install or update the correct JEI version for your exact Minecraft version and loader. Make sure you are not mixing Forge JEI with a Fabric modpack, or Fabric JEI with a Forge modpack. Then relaunch Minecraft and use a fresh crash log if it still fails.

What this crash usually means

A JEI ClassNotFoundException means Minecraft tried to load a JEI class, but that class was not available. That usually happens when JEI is missing, installed for the wrong loader, installed for the wrong Minecraft version, or when another mod expects a newer or older JEI API.

Common signs in the crash log

java.lang.ClassNotFoundException
mezz.jei.api.runtime.IJeiRuntime
mezz.jei
Just Enough Items
A mod failed to load correctly
Missing or incompatible dependency

How to fix it

1
Check your Minecraft version, such as 1.20.1, 1.19.2, or 1.18.2.
2
Check your loader: Forge, Fabric, Quilt, or NeoForge.
3
Download the JEI version that matches both your Minecraft version and loader.
4
Remove any old or duplicate JEI files from your mods folder.
5
Put the correct JEI file into the same mods folder as the rest of your modpack.
6
Relaunch Minecraft and let it create a fresh log.

Do not do this yet

Do not delete random mods before checking whether JEI is simply missing.
Do not install the newest JEI blindly if your modpack uses an older Minecraft version.
Do not mix Forge and Fabric versions of JEI.
Do not keep multiple JEI files in the mods folder unless the modpack specifically requires it.

FixMyGame tip

If FixMyGame says JEI is missing but you already installed it, relaunch Minecraft first and load the newest log. The log may be from before JEI was added.

Related Guides

Missing Dependencies

Learn what it means when a mod needs another mod installed before it can load.

Minecraft Fabric API Missing Dependency Fix

Fix crashes where a Fabric mod needs Fabric API before it can load.

Minecraft Java 17 UnsupportedClassVersionError Fix

Fix Minecraft crashes caused by running the wrong Java version.

Fresh Log vs Old Log

Learn why the newest crash log matters when diagnosing modded game crashes.