UndertaleModTool

About

Undertale is a 2015 role-playing game created by indie developer Toby Fox, in which the player explores the Underground and can choose to fight or spare the monsters they meet, with dialogue, characters, and the story changing based on those choices. UndertaleModTool is a modding, decompiling, and unpacking tool for Undertale, Deltarune, and other GameMaker-engine games. It can read and rebuild a game's data file byte-for-byte, includes a room/level editor and a value editor covering nearly every field in the data, and features a high-level GML decompiler/compiler for directly editing a game's scripts and logic, plus support for running automation scripts against a data file.

Getting started

Windows users can download a ready-made GUI or CLI release and open a supported game's data file directly through the program; macOS and Linux users currently need the CLI build, an in-progress cross-platform GUI port, or Wine, since there is no official native GUI for those platforms yet.

History

The tool builds on years of community reverse-engineering work on Undertale's GameMaker data format, crediting earlier research efforts such as PoroCYon's UNDERTALE decompilation research and Donkeybonks's GameMaker bytecode research.

Known issues & limitations

• Nightly releases are more likely to have issues than stable releases.
• The single-file version can have stability issues in certain cases.
• There is no official way to run the GUI on macOS or Linux; the cross-platform GUI port is incomplete.
• YYC code is not supported for GML VM code editing.
• The GML decompiler and compiler are still missing some features.

What you need to run it

Features

• It includes a simple room and level editor.
• You can edit GML VM code, adding custom code with its built-in compiler or GML assembly; YYC is not supported.
• Scripts can automatically modify a data file and can be used to distribute mods.
• The tool can create a .yydebug file for the GM:S debugger, which lets you edit variables live.
• A CLI version is available for automation and quick tasks, but is more difficult to use.

How to install

1. On Windows, download a stable or nightly release, choose the GUI or CLI ZIP, extract it, then run UndertaleModTool.exe.

Media

Additional notes

Policy

The project's contributing guidelines state it does not accept AI-generated code contributions.

FAQ

What is UndertaleModTool?

UndertaleModTool is an open-source tool of Undertale. Tooling used for decompilation and recompilation work.