Q: What is harness engineering?

The harness is all the software you build around an LLM to turn it into a working, reliable system.

The model itself runs on the servers of frontier labs (OpenAI, Anthropic, Google). You access it through API calls. On its own, an API call gives you one thing: text in, text out. Everything else has to be engineered around it: the loop that lets an agent take multiple steps, the tools it can call, how context and memory are managed, retrieval (RAG), guardrails, and evals that tell you whether the system actually works.

That surrounding layer is the harness. Harness engineering is the discipline of building it well, and it's where most of the real work in AI engineering lives. The model is a commodity you rent. The harness is what you build, and it's what separates a demo from a production system.

If you strip the buzzword away: harness engineering is most of what an AI Engineer does all day.