Patching Undertale and Deltarune with XDelta

Which file do you patch?

Undertale and Deltarune are made with GameMaker, which keeps the game’s content in a single data file next to the executable. That file is what mods patch:

Patches are built for one specific file, so a mod normally ships a separate patch per platform, with names like mod_win.xdelta, mod_unx.xdelta and mod_mac.xdelta. Using the Windows patch on a Linux data file will fail with a checksum error, even though it’s the same game.

Finding the game folder

On Steam, right-click the game in your library, then choose Manage → Browse local files. That opens the install folder directly. The default locations are:

Outside Steam, an itch.io or standalone copy keeps the data file in whichever folder you extracted it to. On Windows you can also right-click a shortcut and choose Open file location.

Applying the patch safely

The install folder is usually protected, and patching in place is the main way people end up with a broken game. Work on a copy instead:

1. Back up the data file

   Copy data.win (or game.unx / game.ios) to a folder you control, such as Documents\Undertale mod. Keep a second, untouched copy somewhere safe: that is your way back.

2. Put the patch beside it

   Extract the .xdelta from its archive into the same working folder, and check the mod’s readme for the game version it expects.

3. Apply the patch

   In Delta Patcher, choose the copied data file as the Original file and the .xdelta as the XDelta patch. Tick Backup original file in the gear menu, then click Apply patch.

4. Rename the result

   With the backup option on, you get dataPATCHED.win. The game only loads a file called data.win, so rename it. (On Linux and macOS the equivalents are gamePATCHED.unx and gamePATCHED.ios.)

5. Put it back in the game folder

   Move your original data file out of the install folder (or rename it to data.win.original), then copy the patched file in. Confirm any prompt asking for administrator permission.

6. Launch the game

   Start it as usual. If the mod is a translation, you should see it immediately on the title screen or in the first dialogue.

Steam updates undo your mod

When the game updates, Steam replaces the data file with the official one, and the mod disappears. Worse, a new game version usually means the old patch no longer matches, so you can’t simply re-apply it.

• Keep the patched file. Store a copy outside the game folder so you can put it back after an update.
• Slow updates down. In Steam, open the game’s Properties → Updates and set it to only update when you launch it.
• Check the mod first. After a game update, see whether the mod author has released a patch for the new version before re-patching.
• Restore the original any time. In Properties → Installed Files, use Verify integrity of game files and Steam re-downloads a clean data file.

“The file you are trying to patch is not the right one”

This is the usual failure, and for these games it almost always means one of three things:

Mods don’t stack either: two patches built against the clean data file can’t both be applied, unless the author says otherwise. The full checksum mismatch guide covers the general case.

Other things that go wrong