How to find which mod is crashing your Minecraft server

The short answer

Forge annotates every stack frame with the jar it came from. Almost nobody outside mod development knows this, and it turns a 400-line trace into a one-line answer.

at com.example.wildlife.procedures.StalkerProcedure.execute
   (StalkerProcedure.java:250) ~[examplemod-1.20.1-2.1.11.jar%23205!/:?]

That ~[examplemod-1.20.1-2.1.11.jar...] is the mod. Not the package name — the actual file sitting in your mods/ folder.

The procedure

Open the newest file in crash-reports/. Read the stack trace from the top down, and find the first frame whose jar is not one of these:

• server-*.jar, client-*.jar, anything minecraft
• forge-*, neoforge-*, fmlcore, fmlloader, javafmllanguage, modlauncher, securejarhandler, bootstraplauncher
• mixin, asm, netty-*, guava, gson, log4j
• any frame with no ~[...] annotation at all — that is the JDK

The first jar left over is your answer.

Why the skip list is the whole trick

This is the part that goes wrong. A naive version of this blames netty-common on roughly half of all crashes, because Netty sits near the top of any network-related trace. Filter Netty out and it starts blaming fmlloader. Filter that and it blames modlauncher.

Every one of those is a confident, authoritative-looking wrong answer. If something tells you a Forge internal killed your server, it is wrong. Forge is in every trace; being present is not being responsible.

Always read Caused by: first

Forge wraps failures in its own exceptions. A report opening with java.lang.Exception: Mod Loading has failed tells you nothing at all. Scroll to the last Caused by: and start there. The same goes for Failed to initialize server and Exception in server tick loop — all three are envelopes, not causes.

When there is no stack trace

If the process vanished with no crash report, nothing crashed in Java — it was killed from outside. That is almost always the OS out-of-memory killer or your host's watchdog. Check heap size first.

Doing this automatically

Forge Server Doctor performs the whole procedure above — reads the report, finds the real cause under the wrappers, skips the vanilla frames, and names the mod jar. 24 failure modes, each with its fix. The free edition covers the six most common startup failures.