UndertaleModTool Switch

UndertaleModTool
General
Author	krzys-h
Type	Hack Utilities
Version	0.9.0.0
License	GPL-3.0
Last Updated	2026/04/29
Links
Bleeding Edge - Stable Download
Website
Source

The most complete tool for modding, decompiling and unpacking Undertale (and other Game Maker: Studio games).

Features

• Can read every single byte from the data file for lastest version of Undertale, Deltarune, and most other GameMaker: Studio games (GM:S 1.4 and GMS2 bytecode versions 13 to 17 are currently supported) for every platform and then recreate a byte-for-byte exact copy from the decoded data.
• Properly handles all of the pointers in the file so that if you add/remove stuff, make things longer/shorter, move them around etc. the file format won't break.
• An editor which lets you change (almost) every single value, including unknown ones.
• Includes a simple room/level editor.
• Allows for code disassembly and editing. This means you can add any custom code to the game, either using the built-in GML compiler or GML assembly.
• Experimental high-level decompiler. The output is accurate (except for the latest GameMaker versions), but it could use some more cleaning up of the high-level structures.
• Support for running scripts that automatically modify your data file (or perform other nefarious tasks) - this is the way to distribute mods, but creating them is a manual job for now. It also serves as a replacement for sharing hex editor offsets - if you make it into a file-format-aware script instead, there is much smaller change of it breaking after an update.
• All core IO functionality extracted into a library for use in external tools.
• Can generate an .yydebug file for the GM:S debugger so that you can edit variables live (see here).
• Automatic file associations for all GameMaker related files. This can be disabled by having a dna.txt file next to the executable.

User guide

UndertaleModTool has different builds per release. The differences are as follows:

• .NET bundled - Bundles the required .NET runtime version needed to run the tool alongside it. All stable releases are .NET bundled, so you don't have to worry about installing the necessary runtime.
• Single file - The tool is only one executable, with all dependencies embedded within it. This does make your folders cleaner, however it also causes some unexpected stability issues.
• Non-single File - All dependencies are not embedded within the executable, but are now located right next to it. Choose this if you don't care about finding the right executable in-between of ~300 dll's, or if the stability issues from the Single file build affect you.

Some test scripts are included:

• Universal:
  • Search - Basic text search within decompiled code entries.
  • Scripts to batch import and export various types of asset files.
• Undertale only:
  • EnableDebug - Activates Undertale's debug mode by enabling the global variable 'debug' at game start.
  • DebugToggler - Allows toggling the debug mode on and off with the F1 key.
  • BorderEnabler - Facilitates the importation of PlayStation exclusive borders into the PC version, while ensuring proper display by patching version checks.
  • Testing - Displays random text on the main menu, this was the first script ever created.
  • TTFFonts - Makes the game load fonts in TTF format from current directory instead of using the spritesheet fonts. You'll need to provide the font files yourself.
  • RoomOfDetermination - Adds a new room to Undertale 1.08. Although incomplete, it serves as a comprehensive example of adding content.
• Deltarune only:
  • DeltaHATE - HATE-inspired script for corrupting Deltarune
  • DeltaMILK - Replaces every non-background sprite with the K.Round healing milk. Don't ask why.
  • TheWholeWorldRevolving - Creates a spinning effect.
• Undertale and Deltarune only:
  • GoToRoom - Replaces the F3 button's debug mode functionality with a dialog allowing you to jump to any room by its ID.
  • ShowRoomName - Displays the current room name and ID on the screen during debug mode.

Additionally, there are some community-made scripts. For more information, consult the SCRIPTS.md file.

Documentation is available on UndertaleModTool wiki.

Screenshots

Undertale: Bits and Pieces

RIBBIT - The Deltarune Mod

Changelog

Release notes.

External links

• GitHub - https://github.com/krzys-h/UndertaleModTool
• GameBanana - https://gamebanana.com/tools/6647