A harness is made up of several components. The instructions tell AI how to behave and what personality to have. The safety filters control what AI can and cannot say. The tools give AI abilities like searching the web, running code, or reading files. The memory decides whether AI remembers you between conversations. The context management controls how much information AI can hold at once and what gets dropped when space runs out.
Think of the AI model as an engine. The harness is the steering wheel, the brakes, the dashboard, and the road rules all combined. The same engine inside two different cars drives very differently. Same with AI. The same model inside two different apps behaves like two completely different products because the harness is different.
