Scale and Maintain in Harness Engineering

Lessons in this section

1. Subagents and MCP

   Why big tool output pushes your rules out of the agent's context, and why an agent without a way to look things up guesses. You add a log-searching subagent and an MCP server, in Claude Code, Codex and Cursor, so the rules stay in view and the agent calls the real API.

2. Architecture Tests

   Why an agent takes a shortcut through your layers while every test stays green, how an architecture test turns a layer rule into a check that teaches the fix, and how a doc-gardening skill finds stale docs on a schedule without deleting them.

3. Continuous Improvement

   How to turn every agent mistake into one new check in the right layer, and how ablation (removing one part at a time and re-running every check) shows which parts of your harness carry weight and which only cost context.

4. Capstone Project

   Put the whole course into one harness. It must pass every part of the capstone check: single-session jobs, two five-session builds, the architecture test, the subagent and MCP server, and the harness lint. Then export it for Claude Code, Codex or Cursor and try it in your own repo.