How do I read a Sims 4 lastException file?
Sims 4 LastException reader: find the mod that broke your game
Paste your lastException file, and the reader names the mod nearest the error, your game version, and the part of the game that broke. The mod nearest the error is usually the one to update or remove. The file never leaves your browser.
The file stays in your browser. Nothing is uploaded.
Where to find the file
Open Documents, then Electronic Arts, then The Sims 4. The game writes lastException.txt (or lastException_ with numbers) when its Python code stops, and lastUIException.txt when one of its screens does. Sort the folder by date and take the newest one.
What the parts of the file mean
- buildsignature holds your game version, like 1.129.39.1030. A version older than the latest patch causes errors with mods that were updated for it.
- categoryid is where the game caught the error: a file of its code and a line.
- desyncdata holds the traceback: every function that ran, oldest first. Lines with a path under Scripts are the game's own code. Lines with a path under Mods, or a .ts4script file, are a mod.
- The last line names the error, like AttributeError or KeyError, and what was missing.
Fix it in four steps
- Update the game. A patch for the game often comes with updates for the mods.
- Update the mod the reader names, from its creator's page. If there is no update yet, move it out of the Mods folder.
- Delete the old lastException files, play until the problem comes back, and read the new file.
- No mod in the trace? Move half of the Mods folder out and test, then half of the half. Tuning mods never show up in a traceback.
What did the last patch change? Every hidden change in the October 27, 2026 update.
Questions
Where are the lastException files?
In the game's user folder: Documents, Electronic Arts, The Sims 4. They are named lastException.txt or lastException_ followed by numbers, and lastUIException for errors of the game's screens.
Can I delete lastException files?
Yes. The game writes a new one when the next error happens. Delete the old ones, play until the problem comes back, and read only the new file.
The trace names no mod. Are my mods fine?
Not always. Script mods show up in a trace, but tuning mods (.package files) change the data the game's code reads, so the error then shows only the game's own code. Test with half of the Mods folder moved out, then half of the half.
Why did my mods break after an update?
Each patch changes the game's code and tuning. The October 27, 2026 update changed 964 tuning records. A mod built for the old version can call code that moved, so update every script mod after a patch.