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