How to Fix Forge Crash on Startup in Minecraft

The Error

net.minecraftforge.fml.common.LoaderExceptionModCrash: Caught exception from MyMod
java.lang.NoClassDefFoundError: net/minecraftforge/common/ForgeHooks
---- Minecraft Crash Report ----
Description: Mod loading error has occurred

Forge crashes on startup when the mod loading process encounters a fatal error. This can be a mod incompatibility, a missing dependency, a wrong Java version, or a corrupt Forge installation. The crash report and logs always contain the specific reason.

Quick Fix

Remove all mods from the mods/ folder and test if Forge itself starts. If it does, add mods back in groups of 5 until the crash recurs. The last group added contains the problem mod.

Step-by-Step Diagnosis

1. Read the crash report. Open the most recent file in .minecraft/crash-reports/. Scroll to the "Mod List" section. If a mod is marked as "ERRORED", that is your culprit.
2. Check the log. Open .minecraft/logs/latest.log and search for "ERROR" or "FATAL". The log provides more detail than the crash report, including which mod threw the exception and why.
3. Verify Java version. Forge for Minecraft 1.21+ requires Java 21. Run java -version to check. Download Java 21 from Adoptium if needed.
4. Test vanilla Forge. Move all mods out of the mods/ folder. Start the game. If Forge loads, the issue is a mod. If Forge itself crashes, reinstall Forge.
5. Reinstall Forge. Download the Forge installer for your exact Minecraft version from files.minecraftforge.net. Run the installer and select "Install Client". Delete .minecraft/libraries/net/minecraftforge/ first to ensure a clean install.
6. Check for mod version mismatches. A mod built for Forge 1.20.4 will not work on Forge 1.21. Download the correct version of each mod for your specific Minecraft and Forge version.
7. Check for missing dependencies. The log will say "Missing or unsupported mandatory dependencies" followed by the library name and the mod that needs it. Download and install the missing library.

Common Forge Error Messages

• Unsupported class file major version 65 - The mod was compiled with Java 21 but you are running Java 17 (or vice versa). Match your Java version to what Forge requires.
• Coremods are present but unloadable - A coremod (low-level mod that modifies Forge itself) is incompatible. Remove coremods first when troubleshooting.
• Missing or unsupported mandatory dependencies - A mod requires a library that is not installed. The message names the missing library.
• Duplicate mods found - Two versions of the same mod are in the mods folder. Remove the duplicate.
• Mixin apply failed - A mixin mod conflicts with another mod modifying the same class. One of them must be removed or updated.

Common Causes

• Mod compiled for wrong version. The single most common cause. Each mod must match the Minecraft version and often the specific Forge build number.
• Missing library mods. Mods like GeckoLib, Architectury API, or Cloth Config are required by many popular mods.
• Corrupt Forge installation. A failed download or interrupted installation leaves partial files. Clean reinstall fixes this.
• Mixin conflicts. Two mods that modify the same vanilla class through Mixins are incompatible. One must be removed.
• Wrong Java version. Forge versions for 1.21+ need Java 21. Older Forge (1.16-) needed Java 8.

FAQ

How do I find which mod is causing the crash?

Binary search: remove half the mods, test, and repeat. If 20 mods are installed, remove 10. If it works, the problem is in the removed 10. Split those 10 in half and repeat until you find the single offender.

Can I use Forge and OptiFine together?

OptiFine includes Forge compatibility, but conflicts are common with other rendering mods. If you crash with OptiFine installed, remove it and test. Consider Embeddium (Forge Sodium port) as a more compatible alternative.

Does Forge support Minecraft 1.21+?

Yes. NeoForge is the active continuation of the Forge project for 1.21+. Download it from neoforged.net. The original Forge project has slower updates for newer versions.

Need a server that handles all this? Astroworld Hosting, NVMe SSDs, Pterodactyl panel, DDoS protection on every plan.