Mod UNDERTALE the easy way.
Determination is a free mod manager for Mac and Windows. Drag in a mod, press PLAY. No command line, no Wine, and your saves stay safe.
Free · Mac (Apple Silicon) and Windows 10/11 · v0.1.0
* How it works
- 1Get the app
Download Determination and open it. It finds UNDERTALE in your Steam library by itself.
- 2Add mods
Drag a mod (.zip, .xdelta or .csx) onto the window, or grab one from this site.
- 3Press PLAY
Determination builds the modded game, keeps your original safe, and launches it.
* Why Determination
Every profile has its own saves. Your real UNDERTALE save is never touched unless you want it to be.
Like CurseForge instances: keep a "Cheats" profile, a "Randomizer" profile and plain Vanilla side by side.
Patches, scripts, music packs. On a Mac, even Windows-only patches work using Steam's own Windows files.
Every game you launch gets WASD controls and fullscreen on F11, Alt+Enter or Ctrl+F.
The original game is backed up before anything changes. RESTORE puts it back exactly, byte for byte.
Turn on several mods at once and set the load order. Determination tells you plainly if two don't fit together.
PLAY starts the game directly (and Steam in the background if needed). QUIT closes it.
* Mods
See all →* Guides
All guides →Rewrite dialogue, make anything talk, swap music and test any fight, all by clicking. A walkthrough of Determination Studio.
How to mod UNDERTALE on MacThe easy way to install UNDERTALE mods on a Mac: no Windows, no Wine, no command line. Takes about a minute.
Use Windows UNDERTALE mods on MacMost 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.
Make your own UNDERTALE modA beginner-friendly look at how UNDERTALE mods work and how to make a simple one that changes the game's text.
* FAQ
Is it free?
Determination (the mod manager) and every mod on this site are free. Determination Studio has a free 3-day trial for dialogue and rooms; Pro (paid) unlocks every tool.
Will it break my game or my save?
Determination backs up the original game before changing anything, and RESTORE puts it back exactly. Every profile gets its own save, so mods never touch your real save unless you tell them to.
Mac or Windows?
Both. Macs with Apple Silicon (M1 or newer) and Windows 10/11 PCs. You need UNDERTALE from Steam.
Do Windows UNDERTALE mods work?
Yes. On Windows they just work. On a Mac, Determination uses Steam's own Windows game files (free if you own UNDERTALE on Steam, one command to download) and runs them on your Mac. Script mods (.csx) work everywhere.
What else does it add?
Every game you launch from Determination gets WASD controls and easy fullscreen (F11, Alt+Enter or Ctrl+F) on top of the normal keys.
How do the Studio trial and Pro work?
The free trial lasts 3 days, and the clock only runs while Studio is open. It lets you change dialogue, make anything talk and edit rooms. Pro is a one-time purchase that unlocks everything: music, stats, battles, fights and every new tool. Your mods are never locked: after the trial you can still open, test, save and share them.
macOS says the app "can't be opened"?
The app isn't notarized by Apple yet. Right-click the app, choose Open, then Open again. You only need to do this once.
Windows says "Windows protected your PC"?
The app isn't signed yet. Click More info, then Run anyway. You only need to do this once.
Is this official?
No. Determination is a fan-made tool. UNDERTALE is made by Toby Fox. This site isn't affiliated with Toby Fox, Valve or Steam, and no game files are included: you need your own copy.
Ready? It takes about a minute.
DOWNLOAD: COMING SOON