UndertaleModTool Switch

UndertaleModTool
UndertaleModTool
General
Author	krzys-h
Type	Hack Utilities
Version	0.9.1.0
License	GPL-3.0
Last Updated	2026/06/19
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.