Fix "Mod Crashed" on Minecraft Startup

A crash on startup usually means one mod is missing a dependency, is built for the wrong loader, or conflicts with another mod. The crash report names the culprit more often than people realize. This guide shows you where to look, how to read the stack trace, and how to isolate the failing mod when the log is unclear.

Step 1: Find the crash report

Open your Minecraft folder and look for crash-reports. The most recent .txt file is the one you want.

Windows: %appdata%\.minecraft\crash-reports
macOS: ~/Library/Application Support/minecraft/crash-reports
Linux: ~/.minecraft/crash-reports

If the game does not launch at all and no crash report appears, check logs/latest.log. Some crashes happen before the crash-report handler is loaded.

Step 2: Read the top section

The top of the crash report is titled ---- Minecraft Crash Report ----. It contains:

Description — a short phrase, e.g. "Ticking entity" or "Initializing game."
Time — when it happened.
Crash report description — often names the specific class that failed.
Stack trace — the at ... lines. This is the most important part.

Step 3: Find the first mod in the stack trace

Scroll through the at ... lines. Skip anything starting with:

net.minecraft — vanilla code.
java / sun / jdk — Java runtime.
net.fabricmc — Fabric loader.
net.minecraftforge / net.neoforged — Forge / NeoForge loader.
org.spongepowered — Mixin framework.

The first line that contains a mod's package name is usually the culprit. Example:

at snownee.jade.util.WailaExceptionHandler.handleErr(WailaExceptionHandler.java:42)
at snownee.jade.overlay.OverlayRenderer.render(OverlayRenderer.java:78)

The mod ID is snownee — that is Jade. The failing mod is likely Jade, or a mod that Jade is trying to interact with.

This is a heuristic, not proof. A mod that wraps another mod's code can appear responsible when the real issue is elsewhere. But it is the right place to start.

Step 4: Check the "Caused by" chain

Below the main stack trace, look for lines starting with Caused by:. Each one narrows the failure:

Caused by: java.lang.NoClassDefFoundError: net/mehvahdjukaar/moonlight/api/events/LowLevelBlockEvent
    at java.base/java.lang.ClassLoader.defineClass1(Native Method)

This means a class from a mod (Moonlight) is missing. The mod that triggered the load is the one before this line. The fix is to install Moonlight, or to check which mod requires it.

Step 5: Check for Mixin errors

If the crash report contains Caused by: org.spongepowered.asm.mixin anywhere, it is a Mixin crash. Search the report for .mixins.json or Mixin apply for mod. The mod ID appears there:

Mixin apply for mod memoryleakfix failed memoryleakfix.mixins.json:LoadingScreenMixin

The mod ID is memoryleakfix. The mod is Memory Leak Fix. Mixin crashes usually mean the mod is outdated relative to your loader or another mod that also patches the same code.

Step 6: Check for missing dependencies

If the error says NoClassDefFoundError or ClassNotFoundException, a required library is missing. The class name often hints at the mod:

net/mehvahdjukaar/moonlight/... — Moonlight library.
dev/architectury/... — Architectury API.
software/bernie/geckolib/... — GeckoLib.
me/shedaniel/clothconfig/... — Cloth Config API.

Check the mod's page for dependencies. Install each one and try again.

Step 7: Binary search (when the log is unclear)

If you cannot identify the culprit from the log, disable half your mods. Launch the game. If it still crashes, the problem is in the half you kept. If it launches, the problem is in the half you disabled. Repeat with the problematic half until you isolate the mod.

This is faster than removing mods one by one, even with 100+ mods. It takes about 7 launches to narrow 100 mods down to 1.

Common causes and fixes

Symptom	Likely cause	Fix
Crash on launch, log mentions a mod	Wrong loader or version	Check the mod page for exact loader and MC version
NoClassDefFoundError	Missing dependency	Install the library listed on the mod page
Mixin apply error	Mod conflict or outdated mod	Update the mod or remove it
Out of memory	Too many mods for allocated RAM	Increase -Xmx in the launcher profile
Game launches but mod does not appear	JAR in wrong folder	Check that it is in the mods folder, not a subfolder
Crash after 5 minutes of play	Memory leak or tick loop issue	Try removing mods added in the last week
NullPointerException in tick handler	Mod conflicts with vanilla entity	Check the mod's issue tracker for a known fix

What not to do

Do not download "crash fixer" tools from unknown sites. They are usually malware or adware.
Do not disable the crash report. It is the only diagnostic information you have.
Do not install mods from sites that require an installer or ask for your Microsoft credentials. A mod is a .jar file, nothing more.
Do not delete the crash report before you have identified the cause. You may need to share it for help.

Where to get help

If the crash log is unclear, the mod author's issue tracker on GitHub is usually the fastest source of an answer. Search the tracker for the exact error message before posting — most crashes have been reported already.

Frequently asked questions

I do not have a crash report. Where is it?

If Minecraft crashes before loading the crash-report handler, check logs/latest.log instead. Some crashes only produce a log, not a crash report.

Can a modpack crash even if all mods are correct?

Yes. Memory allocation, conflicting mod versions, and corrupt world saves can all cause crashes without a mod being "wrong." Check the RAM allocated to the profile and try a new world.

What does "Mixin apply failed" mean?

Mixin is a bytecode patching system that mods use to inject code into Minecraft. "Mixin apply failed" means a mod tried to patch a class and either the class does not exist (wrong version) or another mod patched it first (conflict).

How do I increase allocated RAM?

In the Minecraft Launcher, open Installations, select your profile, click More Options. In the "JVM Arguments" field, change -Xmx2G to -Xmx4G (or higher). Do not allocate more than half your total system RAM.

Will removing mods break my world?

It depends on the mod. Removing a mod that added blocks or items will leave missing block IDs in the world, which can cause crashes. Back up your world before removing mods that change world content.

Related guides

How to Install Mods — Fabric and Forge setup.

Iris vs OptiFine — shader loader comparison.