The most complete tool for modding, decompiling and unpacking Undertale (and other Game Maker: Studio games!)
UndertaleModTool is a comprehensive software designed to mod, decompile, and unpack Undertale, Deltarune, and other GameMaker: Studio games. It provides powerful tools for editing game data at a low level while maintaining compatibility with the original file format.
Key Features:
Reads every byte of game files and recreates exact copies from decoded data.
Properly handles pointers to prevent corruption when modifying game content.
Includes an editor for nearly all game values, including unknown or custom data.
Supports GML VM code editing, allowing custom scripts to be added without YYC.
Features a high-level GML decompiler and compiler for easy script modification.
Automatic file associations for GameMaker data files (opt-in).
Audience & Benefit:
Ideal for game modders, researchers, and developers who want to deeply customize Undertale or other GameMaker games without breaking their functionality. It enables precise control over game mechanics, scripts, and assets while ensuring compatibility with the original game engine.
UndertaleModTool can be installed via winget.
README
UndertaleModTool
(seeing such an amazing tool fills you with DETERMINATION.)
Heya. I heard you like digging deep into GameMaker games like Undertale/Deltarune, so here's a tool just for you!
Downloads
Both the latest stable release and the most cutting edge version can be downloaded from the table below!
Note, that you can update to the bleeding edge releases at any time from within the settings menu of UndertaleModTool.
Releases
Status
Stable
Bleeding edge
It's worth noting that 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.
Main 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.
UndertaleModTool comes with a decently-sized collection of C# scripts that extend its functionality.
For more information on them, consult the SCRIPTS.md file.
Contributing
All contributions are welcome! If you find a bug, a data file that does not load etc., please report it on the issues page. Pull requests are welcome too! Here is a current list of stuff that needs to be worked on:
Upgrading profile system to a better, more source-control friendly project system.
Continuing to improve support for a wider variety of GameMaker versions (especially recent ones).
Further GML compiler and decompiler work, mainly over on Underanalyzer.
Making structural changes to clean up the library (an incremental effort).
Eventually, making the GUI cross-platform if possible, and improving it in general.
General usability improvements, bugfixes, and so on.
Compilation Instructions
In order to compile the repo yourself, the .NET Core 8 SDK or later is required.
The following projects can be compiled:
UndertaleModLib: The core library used by all other projects.
UndertaleModCli: A command line interface for interacting with GameMaker data files and applying scripts. Currently is very primitive in what it can do.
UndertaleModTool: The main graphical user interface for interacting with GameMaker data files. Windows is required in order to compile this.
Compiling Via IDE
Open the UndertaleModTool.sln in the IDE of your choice (Visual Studio, JetBrains Rider, Visual Studio Code etc.)
Select the project you want to compile
Compile
Compiling Via Command Line
Open a terminal and navigate to the directory of UndertaleModTool.sln
Execute dotnet publish where `` is one of the projects listed above.
You can also provide arguments for compiling, such as --no-self-contained or -c release. For a full list of arguments, consult the Microsoft Documentation.
GameMaker Data File Format
Interested in the file and instruction format research I've done while working on this? Check out the Wiki
for full details and documentation.
Special thanks
Special thanks to everybody who did previous research on unpacking and decompiling Undertale, it was a really huge help:
