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
The most complete tool for modding, decompiling and unpacking Undertale (and other GameMaker games!)
> * (Seeing such a specialized tool, the culmination of work from many amazing contributors...)
> * (It fills you with determination.)
Quick Start
Windows
Find the latest stable (or nightly) release from the Downloads section below
Download the GUI version (e.g. UndertaleModTool_v0.8.4.1-Windows.zip), or the CLI version if you know what you're doing
Extract the ZIP file to a folder (do not run from inside the archive!)
Run UndertaleModTool.exe to start the tool
Open your game's data file (e.g. data.win, game.ios, game.unx, etc.) via File → Open
macOS/Linux
As of writing, there is no official method of running UndertaleModTool's GUI on macOS or Linux. However, there are some options available:
Use the CLI (command-line interface) version of the tool. This means there's no graphical interface, but it can be useful for automation and quick tasks.
Use a work-in-progress port of the GUI to a cross-platform framework, such as the one currently available here. As of writing, this port is incomplete, but it has support for many features that are commonly used.
Run the tool via Wine. This can be configured manually, or you can use an unofficial, community-maintained installer script such as this one. The Underminers team is not associated with these projects, so take care to ensure any scripts are safe before running them!
Downloads
Both the latest stable and nightly releases can be downloaded from the table below!
The nightly releases are more likely to have issues, but contain the most recent features and fixes.
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.
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 10 SDK or later is required.
When cloning using Git, make sure to recursively clone submodules (e.g. with --recurse-submodules), as a submodule is used for the Underanalyzer dependency.
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:
