Halo 2 and Halo 3 introduced its official mod tools for PC
Halo: The Master Chief Collection is now onto Season 8 (which will be the final one), and with it comes the long-awaited release of new mod tools for Halo 2 and Halo 3 (plus Halo: Combat Evolved has had its tools updated).
Halo: Combat Evolved got mod support in June, of course, and the fresh update on Steam applies some changes to “support both classic and remastered HUD elements”, but existing mods “should remain compatible”. Some bug fixes were applied here, too.
Note that unlike the Halo: CE tools, which can be snapped up by anyone no matter what games they own in the Master Chief Collection, for access to the Halo 2 and 3 tools you must have a license for the base game within Steam.