The most complete tool for modding, decompiling and unpacking Undertale (and other GameMaker games!)
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.
Publisher’s description
What is UndertaleModTool?
UndertaleModTool by krzys-h is open-source Windows software in the text editors category. It is written in C#.
Installation and requirements
The publisher provides Portable installers for Any CPU Windows. It runs without installation (portable).
Updates and version history
The current version is 0.9.2.0, released on August 23, 2026. The version history goes back to December 27, 2023: 15 releases, all linked to the official files. That is about 5.6 releases a year.
Popularity and file types
The source code on GitHub (UnderminersTeam/UndertaleModTool) has 2,093 stars. On Windows it registers 5 file types, including .dat, .droid, .ios, .unx and .win. Similar programs include Notepad++, Atom and Neovim.
Project description on GitHub: “The most complete tool for modding, decompiling and unpacking Undertale (and other GameMaker games!)”
Pros / Cons
Pros
- Open source: the code can be inspected and built by anyone
- Portable: runs without installation
In short: UndertaleModTool is an open-source text editor tool for Any CPU PCs. It is actively maintained.
Is UndertaleModTool safe to download?
- The file is served by github.com, the publisher’s own domain
- The download link uses HTTPS
- A SHA256 hash is published, so you can verify the file:
7060496BC1B0047C4309EDE53D564F0E187D43C894253EA3733B48E08E77E3CF· Open the VirusTotal report for this hash - The link has not been checked yet
- The source code is public
- ExeAtlas does not host, wrap or modify installers
Related guide: How to check a file’s SHA256 hash on Windows
Recent versions
All 15 versions ›15 versions over 3 years · about 5.6 a year
| Version | Date | Size | Files |
|---|---|---|---|
| 0.9.2.0Latest | August 23, 2026 | — | Any CPU ZIP |
| 0.9.1.2 | July 13, 2026 | — | Any CPU ZIP |
| 0.9.1.1 | June 29, 2026 | — | Any CPU ZIP |
| 0.9.1.0 | June 19, 2026 | — | Any CPU ZIP |
| 0.9.0.0 | April 28, 2026 | — | Any CPU ZIP |
| 0.8.4.1 | January 8, 2026 | — | Any CPU ZIP |
| 0.8.4.0 | January 4, 2026 | — | Any CPU ZIP |
| 0.8.3.0 | July 28, 2025 | — | 64-bit ZIP |
Install from the command line
Silent install ›winget install --id krzys-h.UndertaleModTool -eProgram files (.exe): undertalemodtool.exe
Related guide: How to use winget: install and update apps
Information
- Publisher
- krzys-h
- Category
- Text editors
- Licence
- GPL-3.0
- Price
- Free
- Source code
- github.com/UnderminersTeam/UndertaleModTool
- Written in
- C#
- Requires
- Windows 11 / 10
- Architecture
- Any CPU
- Installer
- Portable
- Version
- 0.9.2.0
- Released
- August 23, 2026
- Added to the catalogue
- December 27, 2023
- Versions listed
- 15
- Updates / year
- 5.6
- GitHub stars
- 2,093
- GitHub forks
- 366
- Website
- github.com
- winget ID
- krzys-h.UndertaleModTool
File types it opens (5)
Ratings and reviews
No reviews yet. Be the first to rate it.
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Frequently asked questions
Is UndertaleModTool free?
Yes. UndertaleModTool is open-source software under the GPL-3.0 licence.
Is UndertaleModTool safe to download?
The download comes from github.com, which belongs to the publisher; ExeAtlas does not host or change the file. Compare its SHA256 hash with the one shown here, or open the VirusTotal report for that hash.
How do I install UndertaleModTool with winget?
Open PowerShell or Windows Terminal and run the winget command shown on this page. winget downloads the official installer and checks its SHA256 hash before running it.
Does UndertaleModTool run on ARM (Windows on Arm)?
The installer is architecture-neutral.
Where can I download an older version of UndertaleModTool?
The versions page lists all 15 versions with links to their official installer files, back to December 27, 2023.
Can I use UndertaleModTool without administrator rights?
Yes, it is portable: unpack and run it.
What are alternatives to UndertaleModTool?
Notepad++, Atom, Neovim, Geany and FontForge. The alternatives page compares them.
