As promised, 343 Industries has released the official modding tools for Halo: Combat Evolved. What’s really cool here is that everyone can gain access to these mod tools. And yes, you can use them even if you don’t own Halo: The Master Chief Collection.
These modding tools for Halo: Combat Evolved feature Guerilla, Sapien, Tool and Standalone.
Firstly, Guerilla is a program that allows you to edit the values of game content, which are files collectively called tags. There are tags for AI, bipeds, textures (aka, bitmaps), and more.
On the other hand, Sapien is the world or level editor for the game. This program allows you to populate data related to the level itself including things such as weapon placement, AI squads, and more.
With Tool, modders can perform all the tasks related to importing source content into tag files or even to building map files.
Lastly, Standalone is basically a development build of the game itself. It runs directly off tag files instead of cache files. This is largely meant to help improve iteration times for people who are building and modding content. According to the devs, this program is only for advanced mods. Additionally, 343 Industries suggest using it for non-multiplayer development only.
You can download the mod tools for Halo: Combat Evolved from here.
Have fun modders!