Finding Which Mod Caused a Crash

The crash report tells you more than the screen does

When the game stops, it writes a file into the crash-reports folder in your Minecraft game directory, named with the date and time. The on-screen message is a summary; the file is the evidence. Open the newest one in any text editor.

If the game closed before the world loaded, look in logs/latest.log instead — startup failures land there.

What to look for

Two parts of the report do most of the work:

• The description and the top of the stack trace. The first few lines after the exception name usually contain a package name such as com.somemod.something. That package name is the mod.
• The mod list further down. The report lists every loaded mod with its file name. Matching the package name from the trace against that list usually identifies the jar in one step.

Forge also marks a crash it believes a mod is responsible for, which saves you reading the trace at all in the common cases.

When the trace does not name a mod

Sometimes the top of the trace is all vanilla or loader code, which means two mods are conflicting rather than one failing. Then you bisect: move half the mods out of the folder, start the game, and see whether it still crashes. Repeat on whichever half breaks. It is slower, but it finds conflicts the log cannot.

Keep the mods you remove in a separate folder rather than deleting them, so you can put the set back exactly as it was.

Before you report it

Check the three version rules first — Minecraft version, loader, and any required library — because a mismatch produces crashes that look like mod bugs. Once you are sure the versions are right, the crash report file is what the mod's author will ask for, so upload the file itself rather than a photograph of the screen.

What Forge will not do

Forge loads mods; it does not sandbox them. A badly behaved mod can still crash the game, and Forge being up to date does not prevent that. The loader's job ends at getting the mods running.