Harness Engineering - Part 5: Context Engineering

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: Part 1: The Raw Model Problem Part 2: Defining the Harness — The Six Components Part 3: The Control Loop Part 4: The Tool Layer Context Engineering ← You are here Part 6: The Filesystem & Environment Part 7: The Memory Layer Part 8: Observability Part 9: The Harness Architecture Part 10: Decomposing Claude Code By the end of this article, you'll know what the Context actually is, why every turn forces you to answer "what should the model know right now?" from scratch, and the three moving pieces (system prompt, history, retrieval) that make up a well-designed context. Let's get started. 📚 Want to go deeper than the articles? While you follow along with this series, I've put together two hands-on resources that go further than any single article can: Build a Harness from Scratch — Udemy Course — A self-paced course where I walk you through building a production-grade agentic harness from the ground up, in code. Harness Engineering for AI Agents — Live Maven Workshop — A live, cohort-based workshop for builders who want direct feedback, Q&A, a