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UndertaleModTool

github.com/UnderminersTeam/UndertaleModTool Game Guides, Wikis & Tools Open-source projectOpen source

UndertaleModTool supports modding, decompiling, and unpacking Undertale and other GameMaker games.

Visit UndertaleModTool 24 alternatives
github.com/UnderminersTeam/UndertaleModTool
UndertaleModTool homepage screenshot

About UndertaleModTool

UndertaleModTool is an open-source tool for modding, decompiling and unpacking Undertale, Deltarune and most other GameMaker games. It opens game data files such as data.win, game.ios and game.unx, and can edit values, rooms and GML VM code, decompile and compile GML, and run scripts that modify data files.

The graphical version is provided for Windows, with stable and nightly downloads in GUI and command-line versions. There is no official method for running the GUI on macOS or Linux as of writing; options listed are the CLI version, a work-in-progress cross-platform port, or Wine. YYC is not supported for GML VM code editing.

Key features

  • Reads and rewrites Undertale, Deltarune and GameMaker data files
  • Edits values while preserving file pointers when content changes
  • Edits GML VM code with built-in compiler or assembly
  • Decompiles and compiles GML across many GameMaker versions
  • Includes simple room and level editor
  • Runs scripts to automatically modify data files
Best for: People who mod Undertale, Deltarune, or other GameMaker games.
Good for
  • Open-source repository with stable and nightly releases
  • Recreates byte-for-byte exact copy from decoded data
  • Core functionality available as a library for external tools
Watch out for
  • No official GUI method on macOS or Linux as of writing
  • YYC is not supported for GML VM code editing
  • Single-file build can cause stability issues in certain cases

UndertaleModTool at a glance

Type
Open-source project
Pricing
Not confirmed. Check the site
Platforms
iOS, Linux, macOS, Windows
Open source
Yes · Source code
Language
English
Last checked
Oct 5, 2026 · Online when checked
Listed
Oct 5, 2026
open sourcegamelinuxgame guides

UndertaleModTool alternatives

Tools and sites you could use instead, ranked by how closely they match.

All 24 alternatives

XXMI

github.com/SpectrumQT/XXMI-Launcher

XXMI provides a modding platform for Genshin Impact, Honkai: Star Rail, Wuthering Waves, and Zenless Zone Zero.

open sourcegamelinux
Open source
24 alternatives

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UndertaleModTool FAQ

How do I start using it on Windows?
Download the stable or nightly GUI or CLI ZIP, extract it to a folder without running from inside the archive, run UndertaleModTool.exe, then open the game data file via File → Open.
Can I use the GUI on macOS or Linux?
As of writing there is no official method. Listed options are using the CLI version, using a work-in-progress cross-platform port, or running the tool via Wine.
What is the difference between the GUI, CLI and single-file versions?
GUI provides a graphical interface for viewing and manipulation, CLI is command-line only for automation and quick tasks, single-file embeds dependencies in one executable but can cause stability issues, while non-single-file keeps dependencies beside the executable.
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Listing maintained by Chris McClure and the WebIndexly editors · Updated Oct 9, 2026 · How we rank