Why does my modded Minecraft keep crashing? How to find the mod causing it

You add a few mods, hit Play, and the game closes. Then it does it again. When modded Minecraft keeps crashing on startup, in the CurseForge app, the Modrinth App or Prism, the launcher may say "the game crashed" or just show an exit code. Either way, one of your mods (or two of them together) is the problem. Here's how to find which one without losing your evening.

Step 1: Find the crash report

Every launcher keeps your pack in its own folder (the "instance" folder). Most have a button for it, usually labelled "Open folder" (just "Folder" in Prism Launcher). Inside, look for:

• crash-reports/: a file named with the date and time of the crash. This is the best place to start.
• logs/latest.log: everything the game printed in the last run, including crashes that happen too early to write a crash report.
• logs/debug.log: on NeoForge, a more detailed version. Useful when latest.log stops short.

If there's no crash report at all, the game probably crashed while loading mods, so go straight to latest.log.

Step 2: Read the right part

Crash reports are long, but you only need a few lines.

Look for "Caused by"

Scroll down the stack trace (the long list of at ... lines) and find the last line that starts with Caused by:. The deepest cause is usually the real one; the lines above it are the game reacting to it. The mod's name or its package (something like com.somemod.) often appears in that line or just below it.

Look for a mod loading screen

On NeoForge, a broken setup often doesn't crash outright. You get a screen listing problems such as missing or unsupported dependencies. That screen tells you exactly what's wrong: mod X needs mod Y, or needs a newer version of it. Install what it asks for, at the version it asks for.

Look for "Mixin"

Lines mentioning Mixin apply failed or mixin with a mod name mean two mods tried to change the same part of the game. The mod named in the mixin config (a file like somemod.mixins.json) is one of the two involved.

Step 3: Check the usual suspects

Before going hunting, check the four things that cause most launch crashes:

1. Wrong version. A mod built for 1.20.1 or 1.21.4 instead of 1.21.1, or for Forge or Fabric instead of NeoForge. Mod pages list the exact versions; check each one you added recently.
2. Missing dependency. A library mod the other one needs. The loading screen usually says which.
3. Wrong Java. Minecraft 1.21.1 needs Java 21. Launchers normally handle this, but a custom Java setting in your launcher can override it.
4. Two mods that do the same job. Two performance mods that change rendering, or two minimap mods, often clash.

Step 4: Halve the pack

If the log doesn't name a culprit, use the halving method, which is the one experienced pack makers use:

1. Back up your mods folder.
2. Remove half of your mods, keeping anything the remaining mods depend on.
3. Launch. If it still crashes, the problem is in the half you kept. If it works, it's in the half you removed.
4. Repeat with the guilty half.

Each round halves what's left, so even a big pack doesn't take long: 200 mods narrow to one in about eight launches. The catch is dependencies. If you remove a library, every mod that needs it fails too, which looks like a crash but isn't the one you're hunting. Keep libraries with the mods that need them.

Step 5: Fix it

Once you know the mod:

• Check for a newer (or older) version that fixes the crash. Its issue tracker often has the exact error.
• If it clashes with another mod, decide which one your pack needs more.
• If it's a client-only mod crashing your server, remove it from the server only.

Or never see a crash log again

Packspire does all of this before you download your pack. It starts the pack on a real server and on a real game client, reads the crash and blames the right mod; when the game crashes without naming one, it finds the mod by halving the pack automatically (respecting dependencies). The mod at fault is removed, or, if it can't be, you're told which one to take out and why.

Build a pack that starts first time.

Frequently asked questions

Why does my modded Minecraft keep crashing?

Almost always because of one mod: built for the wrong Minecraft version or loader, missing a library it needs, or clashing with another mod. The last Caused by: line in the newest crash report usually names it. The wrong Java version and too little memory are the next most common causes.

Where are Minecraft crash logs saved?

In your pack's instance folder: crash reports are in crash-reports/, and the full game log is logs/latest.log. NeoForge also writes a more detailed logs/debug.log. Most launchers have an "Open folder" button for the instance.

How do I know which mod is crashing Minecraft?

Read the deepest Caused by: line in the crash report and look for a mod's name or package nearby. Mixin errors name the mod's mixin config. If nothing is named, halve the mod list until the crash disappears.

How long does it take to find a crashing mod by halving?

Each launch halves the suspects, so about log2 of your mod count: around 7 launches for 100 mods and 8 for 200, as long as libraries stay with the mods that need them.

What Java version does Minecraft 1.21.1 need?

Java 21. Minecraft has required Java 21 since version 1.20.5.

Sources

• Minecraft: Java Edition 1.20.5 release notes, which made Java 21 a requirement