(seeing such an amazing tool...
... fills you with DETERMINATION)

heya. I heard you like digging deep into Undertale data so I made a tool just for you! Downloads are here: https://github.com/krzys-h/UndertaleModTool/releases

• Can read every single byte from the data file for lastest version of Undertale, and probably other GameMaker: Studio games (GM:S 1.4 bytecode version 15 or 16 only, version 14 may be supported at some point but not yet, partial GM:S2 support is done) for every platform 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. A lot better than a hex editor, huh?
• Includes a (very) simple level editor
• Allows for code disassembly and EDITING. This means you can add any custom code to the game, as long as you are determined enough to write it in GML assembly.
• Experimental high-level decompiler. The output is accurate and I've not seen it totally break in a long time, but it could use some more cleaning up of the high-level structures.
• Support for running scripts that automatically modify your data file - this is the way to distribute mods, but creating them is manual job for now. It also serves as a replacement for sharing hex editor offsets - if you make it into a file-format-aware script instead, there is much smaller change of it breaking after an update.
• All core IO functionality extracted into a library for use in external tools
• Can generate an .yydebug file for the GM:S debugger so that you can edit variables live [EXPERIMENTAL]
• Partial GM:S 2 support. Loading and saving should work fine, although not all editors are updated yet. Should work with the latest Nintendo Switch release of Undertale.
• Supports DELTARUNE too!

I also included some of my test scripts. They are:

• Universal:
  • EnableDebug: does just that, makes the global variable 'debug' be enabled at game start. If you don't know about Undertale's debug mode, check out https://tcrf.net/Undertale/Debug_Mode
  • DebugToggler: similar to the above, but instead toggles the debug mode on and off with F12
  • GoToRoom: Replaces the debug mode functionality of the F3 button with a dialog that lets you jump to any room by ID
  • ShowRoomName: Displays the current room name and ID on screen in debug mode
• Undertale only:
  • BorderEnabler: lets you import the PlayStation exclusive borders into the PC version and patches all version checks so that they display properly
  • testing: nothing important, just displays random text on the main menu - the first script I ever made
  • TTFFonts: Makes the game load fonts in TTF format from current directory instead of using the spritesheet fonts. You will need to track down all the font files yourself, I can't host them here for licensing reasons :(
  • RoomOfDetermination: Adds a new room to Undertale 1.08. I wanted to add something more to it but never got around to it, so I guess I'm releasing it as is. Just start the game and you'll see. Probably the most complete sample of adding stuff you'll find.
• Deltarune only:
  • DeltaHATE: HATE-inspired script for corrupting Deltarune
  • DeltaMILK: Replaces every non-background sprite with the K.Round healing milk. Don't ask why.

All contributions are welcome! If you find a bug, a data file that does not load etc. please report it on the issues page. Pull requests and help with decoding the format is welcome too! Here is my current list of stuff that needs to be worked on:

• Discover meaning of the new stuff in GML2 file format so that editors can be updated
• Add missing chunk editors for Timelines and Extensions
• Finish writing the decompiler:
  • add support for resolving resource types (requires propagating information from builtin function calls backwards, possibly across different code fragments)
  • decompile other loop types than just while(true)
  • decompile compound if expressions (things like a || b are decomposed into multiple ifs internally because GML has short-circuit evaluation)
  • make sure the output syntax is fully GML-compatible
• Write a matching GML compiler, so that edits of high-level code are possible
• Add support for other bytecode versions, at least for bytecode 14 so that Undertale 1.00 can be opened as well
• Add support for YYC games - you won't be able to edit code, but everything else should technically work
• Maybe try to write a GameMaker Project output module, should be technically possible after decompiler is done but I'm not sure I really want to do that for legal reasons...

Interested in the file and instruction format research I've done while working on this? Check out these:

• https://github.com/krzys-h/UndertaleModTool/wiki/Corrections-to-Game-Maker:-Studio-1.4-data.win-format-and-VM-bytecode,-.yydebug-format-and-debugger-instructions
• https://github.com/krzys-h/UndertaleModTool/wiki/Changes-in-Game-Maker:-Studio-2
• https://github.com/krzys-h/UndertaleModTool/wiki/Extensions,-Shaders,-Timelines-format
• https://github.com/krzys-h/UndertaleModTool/wiki/Bytecode-version-differences
• https://github.com/krzys-h/UndertaleModTool/wiki/YYC-games

Undertale has a special thanks section so I will have one to! No minigame here, though.

Special thanks to everybody who did previous research on unpacking and decompiling Undertale, it was a really huge help:

• https://pcy.ulyssis.be/undertale/
• https://github.com/donkeybonks/acolyte/wiki/Bytecode
• https://github.com/PoroCYon/Altar.NET
• https://github.com/WarlockD/GMdsam
• @NarryG for helping me figure out the missing stuff for GMS2 and Nintendo Switch release
• @colinator27 for figuring out Extensions, Shaders and a bunch of other stuff

And of course, special thanks to Toby Fox and the whole Undertale team for making the game ;)

How about a random screenshot because I'm bad at writing READMEs? NYEH HEH HEH HEH!