Fix "Mod Crashed" on 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.
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 [citation:11]
Step 2: Read the first section
The top of the crash report is titled ---- Minecraft Crash Report ----. It contains a description of the error and a stack trace. This is the most important part. Ignore -- Last Reload -- unless the crash happened during a resource reload (F3+T) [citation:11].
Step 3: Find the first mod in the stack trace
Scroll through the at ... lines. Skip anything starting with net.minecraft, java, sun, or the loader itself (net.fabricmc, net.minecraftforge). The first line that contains a mod's package name is usually the culprit.
Example from a real crash report: the stack trace contained at snownee.snow.block.ShapeCaches.get. The mod ID is snownee — that is the mod to investigate first [citation:11].
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 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. For example, memoryleakfix.mixins.json points to Memory Leak Fix [citation:8].
Step 5: Check for missing dependencies
If the error says NoClassDefFoundError or ClassNotFoundException, a required library is missing. The class name often hints at the mod. Check the mod's page for dependencies — Cloth Config, Architectury, and GeckoLib are common [citation:4].
Step 6: 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 [citation:8].
This is faster than removing mods one by one, even with 100+ mods.
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 [citation:6] |
| Game launches but mod does not appear | JAR in wrong folder | Check that it is in the mods folder, not a subfolder |
What not to do
Do not download "crash fixer" tools from unknown sites. Do not disable the crash report. Do not install mods from sites that require an installer or ask for your Microsoft credentials. A mod is a .jar file, nothing more [citation:6].