A GitHub posting reported in February 2019 was described as a decompilation of Undertale—not a release of Toby Fox’s original source code. The report said the files appeared to contain game assets, images, scripts and audio, but it did not establish who decompiled them or verify that they were authentic original source. A separate, currently public UndertaleDecomp project describes a decompilation of the Xbox One v1.13X release.
What was released on GitHub in 2019?
BleepingComputer reported on February 9, 2019, that a GitHub repository had been created by a user called TotallyTobyFox. According to the article, that user said someone else had supplied the files for posting. The report characterized the contents as what appeared to be Undertale assets, images, scripts and audio; it did not identify the person who performed the decompilation. Read the February 2019 report.
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The headline’s “source code” wording should not be read as proof that Toby Fox released the game’s original development files. The report said the poster was not claiming to be Fox, and described readers’ view that the project looked like a handcrafted decompilation rather than a leak of the original source. It also reported that an individual associated with the release denied doing the decompilation and suggested it might have originated on a forum that was no longer available. Those details leave the decompilation’s creator and provenance unverified.
Why the old compilation errors do not settle the question
The 2019 article said people trying to compile the posted files ran into errors. Users speculated that a mismatch between GameMaker Studio versions might explain them, but the report did not confirm that cause. These are observations about that historical posting, not evidence about whether today’s separate UndertaleDecomp project builds successfully.
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What is the current UndertaleDecomp project?
The public UndertaleDecomp repository describes itself as a decompilation of the Xbox One v1.13X release, ported to GameMaker Studio 2 Beta with optional enhancements. It is not identified by its maintainers as the same repository as the 2019 posting, so the two should not be conflated.
Branches
| Branch | What the repository says it contains |
|---|---|
main |
Enhancements and bug fixes. |
vanilla |
Decompilation-exclusive features removed, for a closer-to-vanilla modding experience. |
These descriptions come from the repository maintainers; they describe the branches’ intended differences, not an independent assessment of their behavior.
What the repository says users need
The maintainers’ stated setup is for developers who own a copy of Undertale. They say to clone the project, run its asset-import setup script, then open the resulting NXTALE.yyp in GameMaker Studio 2 Beta. The repository recommends the Xbox version and names the Switch version as the closest alternative. It says other versions may work, but platform-exclusive material may be replaced with placeholders.
The repository does not provide an easy guide to extracting Xbox data, and it does not explain Switch modification or dumping. Its instructions are not a guide to obtaining game files; use only a game copy you are entitled to use, and consult the repository for its current project-specific setup information.
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How the repository describes its distribution conditions
The maintainers say they initially took the project down after a misunderstanding following contact with the Fangamer Undertale contact. They say they later received permission to make a public project subject to conditions: start a new repository without the earlier upload’s history, remove Undertale-specific assets, and exclude those assets from both source distribution and compiled executables. The repository describes asset importing as a process for developers who own a game copy. These are the repository’s account of its arrangements; they do not independently establish the correspondence or determine copyright law.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How UndertaleModTool differs
UndertaleModTool is a separate project, not another name for UndertaleDecomp and not the 2019 GitHub posting. Its maintainers describe it as a tool for modding, decompiling and unpacking Undertale and other GameMaker games. Its documentation covers reading GameMaker data files and editing or decompiling GML code.
The repository describes GUI and command-line builds. Its Windows quick start is to download and open the GUI; for macOS and Linux it points users to CLI or other options rather than documenting an official GUI method for those systems. Version, requirements and platform support can change, so check the project’s current release documentation before installing or relying on a particular workflow.
Quick Recap
Which project should you look for?
- Trying to understand the February 2019 headline: use the BleepingComputer report as a historical account. It documents a reported posting, not a verified release of Toby Fox’s original source.
- Looking for a current Undertale decompilation project: consult the separate UndertaleDecomp repository and its branch and setup notes.
- Looking for a tool to inspect or modify GameMaker data: consult UndertaleModTool’s current documentation; it is a general-purpose tool project, not the decompilation repository.
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