Welcome back to the Harness Engineering series — a 10-part journey from raw language model to production-ready agentic system. Made by builders. For builders.
In Part 4, we looked at the Tools — the set of functions the model can call. But there's still one big open question hanging over every turn of the Loop: what does the model actually see when the Loop calls it?
The answer is: whatever the harness put in the payload. That payload — the entire package of instructions, history, retrieved documents, tool definitions, and everything else — is called the Context.
Some readers know this territory under an older name: prompt engineering. That name isn't wrong, but it's narrow. A prompt sounds like something you write once and ship. The reality of running an agent is that the payload changes every turn, and designing what goes in it is an ongoing discipline. Hence the newer, more accurate term: context engineering.
What's ahead:
