(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 👈👈👈

Have questions? Want to talk? Join us on Discord! https://discord.gg/RxXpdwJ

• Can read every single byte from the data file for lastest version of Undertale, Deltarune, and probably other GameMaker: Studio games (GM:S 1.4 and GMS2 bytecode versions 14 to 17 are currently supported) 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.
• Includes a (very) simple room/level editor.
• Allows for code disassembly and editing. This means you can add any custom code to the game, either using the built-in GML compiler or GML assembly.
• Experimental high-level decompiler. The output is accurate (except for the latest GameMaker versions), but it could use some more cleaning up of the high-level structures.
• Support for running scripts that automatically modify your data file (or perform other nefarious tasks) - this is the way to distribute mods, but creating them is a 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! (see here)

Included are some 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.
  • TheWholeWorldRevolving: The world is spinning, spinning

Additionally, included are some community-made scripts, "experimental scripts." For more information, consult the README file inside that directory.

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 a current list of stuff that needs to be worked on:

• Add missing chunk editors for Timelines and Extensions
• Add support for YYC games - you won't be able to edit code, but everything else should technically work
• Add support for latest versions of GameMaker (notably, 2.3) - decompiler cannot function in most cases currently

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

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 lots of things, including the gml compiler, Shaders and a bunch of other stuff
• @Kneesnap for improving the decompiler a bunch

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

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