UndertaleModTool

UndertaleModTool is a modding tool for GameMaker games, like Undertale and Deltarune.

Main Features

Can read every single byte from the data file for lastest version of Undertale, Deltarune, and most other GameMaker: Studio games (GM:S 1.4 and GMS2 bytecode versions 13 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 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!