This extension provides an interface in GameMaker for installing UndertaleModTool projects as mods. It is currently Windows-only.
This extension is intended for use exclusively by modders. Decompilation isn't explicitly against GameMaker's current Terms of Service or End User Licence Agreement, but it has been in the past. If you're a developer, exposing yourself to even the most remote possibility of being hit with cease and desist order or having your license terminated by packaging a decompiler with your game is not worth it. There are also no protections against installing mods that execute malicious code! Anyone using this extension to create a mod management interface should advise users to only install mods from trusted sources.
Place UndertaleModTool's Underanalyzer.dll and UndertaleModLib.dll files in the ExtensionSource folder, then open the GameMakerModManager.sln solution in Visual Studio. Use the Release configuration and set the appropriate platform, then build ModManLib. Building the library will create the necessary files for the extension, and you can then open the ModManager project in GameMaker.
Make sure GameMaker is set to export in VM. Export the ModManager project and rename the game's data.win file to modman.win. Then, export the ModReceiver project and place its data.win file in the same folder as the ModManager game. Place the mods folder in %localappdata%/ModManager, then run the game. After you've pressed "Enter" enough times to step through the full mod installation process, the game should display "Inside successfully modded game!"
GameMaker Mod Manager is licensed under the GNU General Public License v3.0. It is based on UndertaleModTool, which is distributed under the same license.