The Anatomy of an Agent Harness
There's a 100-line Python agent that scores 74–76% on SWE-bench Verified — only 4–6 percentage points behind state-of-the-art systems built by well-funded teams. The execution loop itself isn't where the complexity lives. World-class teams invest six to twelve months building the infrastructure around that loop. That infrastructure has a name: the harness.
The formula is simple: Agent = Model + Harness. The model handles reasoning. The harness handles everything else — tool execution, context management, safety enforcement, error recovery, state persistence, and human-in-the-loop workflows. If you've been spending months optimizing prompts and model selection while shipping brittle agents, you've been optimizing the wrong thing.
