CHAPTER 01 · What Is an Agent Harness? · 6 / 6

Key takeaways

A coding agent is a stateless model plus a stateful harness. The model reasons and asks; the harness gathers context, runs tools, and enforces rules.
The famous rule of thumb: building an agent is roughly 5% calling the model in a loop and 95% harness work (context, tools, sandboxing, error handling).
Improving the harness (tool descriptions, permissions, memory, context management) usually beats fiddling with the prompt.
Two useful maps: the six layers (input, knowledge, execution, integration, multi-agent, observability) and the seven pillars (context, memory, skills, contracts, tiered execution, deterministic computation, portability).
The central loop is "dumb on purpose." Intelligence comes from the model plus the quality of the surrounding layers.