This guide will give you a high level view of how Halo 1 modding works using the official editing kits (HEK and H1A-EK), which are intended for creating custom maps. We will not be going into community-made tools or unofficial modding workflows.

Basic workflow

Playable levels in Halo are represented as map files. The ultimate goal in modding is to create new custom maps with your desired changes or custom content. The official asset pipeline to do that looks like this:

The resources

• Data files are your raw source assets like textures (.tif files), level scripts (.hsc), models (JMS), and animations. They can be found within the data folder of your mod tools installation. The game itself does not use these files. You create these files using external software, for example Blender for 3D assets.
• Tags are files which represent each type of object and asset in the game world. You will find these files in your editing kit's tags folder. There are many types of tags and most modding is the creation and editing of tags. Depending on the type, tags reference other tags as dependencies. The editing kit/mod tools come with a base tag set but you can modify them or create your own. Halo does not use these files directly.
• Map files are like archives of all the tags needed for a playable level. Maps are loaded by the game and the tags within them are copied into memory to drive the game systems.

The tools

• Tool is a command-line utility mainly used to create tags from data files (like models, sounds, and textures) and maps from tags.
• Sapien can be thought of like Forge from later Halos. It is an interactive editor used to populate levels with objects, place multiplayer objectives, set up AI encounters, and assign environmental effects like weather. Sapien edits a level's tags, specifically its scenario and scenario_structure_bsp.
• Guerilla is a general-purpose tag editor. It is used to create and edit all other types of tags like weapons, vehicles, and particles.