The Halo Editing Kit, often abbreviated as HEK, was released by Gearbox Software along with the Halo Custom Edition. It includes, among other things, basic "tags" for bitmaps, vehicles, weapons, bipeds, and other necessary elements of a Halo map.
The kit comes with three programs designed to assist mappers.
The first program is Tool. Tool is both the backbone and the pain of CE. Although it is capable of everything from bitmap creation to the final map-building, its old-style command prompt methods have driven at least a few mappers insane.
The second program, known as Guerilla and labeled with a gorilla (pun intended), opens tags and edits them. Tags are files that make up everything in a Halo map: weapons, vehicles, etc. Guerilla can edit them so that mappers can alter what a specific item, what it does and how it looks (such as change a vehicle's color). Guerilla tends to crash wehn it encounters corrupted tags.
The third program is Sapien, continues the primate naming scheme. Sapien opens scenario tags and creates a visual representation of the final map. Mappers can insert vehicles and weapons, place spawn points, and make camera points for cutscenes. Everything that can be positioned in the map physically is done with Sapien. This is widely considered the easiest program of the three, and is comparable to Halo 3's Forge. Sapien is slow-loading, and crashes often; its debug.txt file is useful when attempting to diagnose such problems.
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The Halo Editing Kit also includes a tutorial in the form of a step-by-step walkthrough of the making of the accompanying tutorial map. The tags that come with the kit are almost singularly made for this small map and include select bitmaps from the Campaign and Multiplayer maps. The combination of the tags, programs, and tutorial gives players a glimpse into the unique possibilities of the Halo Custom Edition.
Also, there are some programs that prove to be very useful such as Gmax/3DS Max for 3d modelling, and Photoshop for creating TIF images for creating bitmaps.