Cheat sheet

The harness

From lesson 0007. The loop is the agent. The box around it is what you ship.

1. AllowMay this slip run? Sandbox or a human stamp.
2. RunExecute. Paste errors. Retry.
3. StopDone, MAX, timeout, or pause.
4. ShowThe plan is visible.

Not

• Not a second model. Not a ChatGPT product named “Harness.”
• Not skills (catalog + chapter — that card), compaction, or graphs. Fat harnesses include those; we unpack the rest later.
• Cursor / Claude Code = an agent loop inside someone else’s harness.
• Harness MAX is a count. A goal stop is a checker. Loop engineering card.
• For a team lead: the harness is the Guard on the shared floor. Team floor card.
• Allow is also the security wall: the prompt is not. Agent security.

LangChain: core agent = LLM + tools in a loop; a harness wraps that loop. 12-factor: own the gap between writing a slip and running it.