UndertaleModTool is a comprehensive, open-source modding and reverse-engineering tool for GameMaker games, specifically Undertale and Deltarune. It provides a full suite of functionality: reading/writing GameMaker data files (.win, .unx, etc.) with byte-for-byte accuracy, decompiling GML VM bytecode back into human-readable GameMaker Language (GML), compiling modified GML back into bytecode, and editing all game assets (sprites, sounds, rooms, code, textures, shaders, fonts, etc.) through a Windows-only WPF GUI or a cross-platform CLI. The tool also includes a project system for exporting/importing assets as JSON and PNG, a built-in C# scripting engine for automation, and game-specific definitions that tailor decompilation and modding to Undertale and Deltarune. The architecture is a classic library-plus-front-end pattern: the core library `UndertaleModLib` handles all binary serialization, decompilation, compilation, and project management. The GUI application `UndertaleModTool` and CLI `UndertaleModCli` are thin layers that use this library. The library itself is heavily version-aware, supporting multiple GameMaker versions (GMS1, GMS2, 2022-2024) with hundreds of conditional checks. The decompiler integrates with the external `Underanalyzer` submodule for advanced GML compilation and decompilation. A large collection of built-in C# scripts covers common modding tasks (resource export/import, technical utilities, game-specific modifications). While primarily an application, the repository also serves as a living document of GameMaker's internal data format—a significant reverse-engineering effort. The code is rich with comments explaining undocumented structures, version differences, and workarounds for engine bugs. This hybrid nature makes it both a practical modding tool and a valuable research resource for understanding GameMaker internals.