UndertaleModTool Switch

UndertaleModTool
General
Author	krzys-h
Type	Hack Utilities
Version	0.9.1.2
License	GPL-3.0
Last Updated	2026/07/13
Links
Bleeding Edge - Stable Download
Website
Source

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

UndertaleModTool has a few different versions to choose from. The differences are as follows:

• GUI (default) - the tool has a full graphical interface, making data file viewing and manipulation convenient.
• CLI - the tool is accessible only via a command-line interface, which is useful for automation and quick tasks, but is more difficult to use.
• Single file - the tool is only one executable, with all dependencies embedded within it. This makes your folders cleaner, however it can also cause stability issues in certain cases.
• Non-single file (default) - all dependencies are not embedded within the executable, and are now located right next to it. Choose this if you don't care about finding the right executable within ~300 DLL files, or if the stability issues from the Single file build affect you.

Documentation is available on UndertaleModTool wiki.

Features

• Can read every single byte from the data file for latest versions of Undertale, Deltarune, and most other GameMaker games, 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 GML VM code editing. This means you can add any custom code to a game, either using the built-in GML compiler or GML assembly. (YYC is not supported for this.)
• High-level GML decompiler and compiler. Supports a large range of GameMaker versions, and most important GML features (still missing a few).
• Support for running scripts that automatically modify your data file (or perform other nefarious tasks). This can be used for mod distribution, aside from other methods such as file patches/project systems.
• All core functionality extracted into a library for use in external tools.
• Can generate a .yydebug file for the GM:S debugger so that you can edit variables live! (see here)
• Automatic file associations for all GameMaker related data files. This is opt-in at the first boot of the tool, and can also be disabled by having a dna.txt file next to the executable.

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