DETERMINATION

Use Windows UNDERTALE mods on Mac

Most UNDERTALE mods are patches made for the Windows version. Here is how to run them on a Mac, legally and for free, with your Steam copy.

Most UNDERTALE mods you'll find (on GameBanana, Discord and so on) come as an .xdelta patch. A patch is a list of changes to one exact file, and almost every patch is made for the Windows game file, data.win. The Mac version uses a slightly different file (game.ios), so the patch doesn't fit.

The good news: if you own UNDERTALE on Steam, you're allowed to download the Windows files too, and the Mac version of UNDERTALE runs them fine. Determination handles the rest automatically.

Step 1: Download the Windows game files (one time)

  1. Open Steam and make sure you're logged in.
  2. Open Steam's console: in your web browser's address bar, type steam://open/console and press Enter. (Allow it to open Steam.)
  3. Click the box at the bottom of the console, paste this, and press Enter:
    download_depot 391540 391541
  4. Wait about a minute (it's around 120 MB). The console says "Depot download complete" when it's done.

That's it. This doesn't change your Mac game at all: Steam puts the Windows files in a separate folder. 391540 is UNDERTALE and 391541 is its Windows version.

Step 2: Install the mod like any other

Drag the mod onto Determination and press PLAY. When Determination sees a Windows patch, it says "That patch is for the Windows version. Rebuilding on the Windows game data..." and builds the mod from the Windows files instead. Under the game's name you'll see Windows mods: ready.

Can I mix Windows patches with other mods?

Yes. Put the patch mod first in the load order (Determination does this for you), then add script mods like the Cheat Menu on top. What doesn't work is two different .xdelta patches together, because each one expects the plain game file. That's true on Windows too.

Is this allowed?

Yes. Steam's console download only works for games you own, and you're running UNDERTALE through your own Steam account. Determination doesn't download or share any game files itself.