Harness

The orchestration layer around the model: protocols like MCP, a skills system, tools, state management, and the loop that makes the agent execute, evaluate, and decide the next step. I think of three cumulative layers — Weights (training), Context (RAG, prompting), and Harness (orchestration).

It's the part that turns a model that only generates text into a system that acts in the world. Much of the differentiation in production today lives here, not in the model weights.