UndertaleModTool

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.