How to mod UNDERTALE on Mac

The easy way to install UNDERTALE mods on a Mac: no Windows, no Wine, no command line. Takes about a minute.

Modding UNDERTALE has always been a Windows thing. The main modding tool, UndertaleModTool, only has a Windows app, and most guides assume you have a PC. On a Mac you'd normally need Wine or a virtual machine. Determination fixes that: it's a Mac app that does the modding for you.

What you need

• A Mac with Apple Silicon (M1, M2, M3, M4 or newer)
• UNDERTALE from Steam, installed
• Determination (free)

Step 1: Get Determination

First, download Determination (free). Unzip it and drag Determination into your Applications folder. The first time you open it, macOS may say it "can't be opened" because the app isn't notarized by Apple yet: right-click the app, choose Open, then Open again. You only do this once.

Step 2: Let it find your game

Open Determination. It scans your Steam library and shows UNDERTALE on the left. Nothing changes in your game yet: Determination only backs up the original game file the first time you install a mod.

Step 3: Add a mod

Download a mod. Our Cheat Menu is a fun first one. Drag the .zip onto the Determination window. It shows up in your Library on the right.

Determination understands all the usual UNDERTALE mod formats:

• .xdelta patches (the most common kind, usually made for Windows: see using Windows mods on Mac)
• .csx scripts made for UndertaleModTool
• Whole replacement game files (data.win or game.ios)
• Music replacements (.ogg files)
• .zip files with any of the above inside

Step 4: Make a profile and press PLAY

Click the ♡ next to the mod. Determination asks you to name a new profile (like "Cheats"). Profiles work like CurseForge instances: each one has its own mods and its own saves, so your real UNDERTALE save stays safe.

Press PLAY. Determination builds the modded game (a few seconds), starts Steam in the background if it's closed, and launches UNDERTALE. While the game is open, the button turns into QUIT.

Going back to normal

Click the Vanilla profile and press PLAY, or press RESTORE. The original game file is put back exactly as it was, byte for byte, along with your normal saves.

If something goes wrong

• "This patch was made for a different version": it's a Windows patch. Follow this guide once and it'll just work.
• The game closes right after starting: UNDERTALE needs Steam open and logged in. Log in to Steam and press PLAY again.
• Two patch mods won't combine: most .xdelta patches expect the plain game, so only one can go first. Script mods (.csx) can be stacked freely.