How it works: two mental maps

Different sources slice the harness in different ways. Two slicings are worth keeping in your head as a map for the rest of the guide.

Map A: the six layers (from the Claude Code architecture breakdown)

This view puts a simple loop in the middle and wraps six concentric layers around it.

Layer	What it does	Where in this guide
1. Input	Session boundary, trust, what the agent is allowed to do	Ch 7
2. Knowledge	Persistent instructions and context that survive long sessions	Ch 9
3. Execution	Dispatching tools and running the loop	Ch 2, 6
4. Integration	External systems and packaged extensions (MCP, plugins)	Ch 10
5. Multi-agent	Delegated work that does not blow up the main context	Ch 11
6. Observability	Deterministic control and audit points (hooks, checkpoints)	Ch 12

The key phrase from that breakdown is that the loop in the center is "the dumb part on purpose." The intelligence is not in some clever orchestration engine. It is in the model's reasoning plus the quality of the layers around it.

Map B: the seven pillars (from the "Agentic Harness Architecture" breakdown)

This view is more opinionated and more builder-focused. It lists seven things a production harness gets right, and for each one it separates what the platform gives you for free from what you have to architect yourself.

Pillar	The core idea	Where in this guide
1. Context engineering	Decide what enters the context window, and what stays out	Ch 3, 4, 5
2. Memory management	Knowledge that lives outside the model's weights	Ch 9
3. Skills as workflow encoding	Package a repeatable procedure the agent loads on demand	Ch 10
4. Agent contracts and orchestration	Give each subagent a clear input/output contract	Ch 11
5. Tiered execution	Answer cheaply when you can, escalate only when you must	Ch 5, 11
6. Deterministic computation	Push exact work into code, not the model's head	Ch 10
7. Portability and shareability	The whole setup lives in the repo, so a teammate can clone it	Ch 9, 16

The seven pillars breakdown adds one more idea that deserves a spot of its own: the anti-hallucination contract. The most important rule for any agent handling real data is that if a tool can fetch the truth, the agent must not be allowed to make the answer up. We will return to this in Chapters 10 and 13.

Do not memorize either table. The point is that "harness" is not one blob. It is a set of distinct problems (context, memory, tools, safety, delegation, observability) that you can attack one at a time. That is exactly how the rest of this guide is organized.