A synthesis of hard-won lessons from Claude Code, OpenHands, SWE-agent, GoClaw, Nanobot, PicoClaw, and the emerging discipline of harness engineering . This is the guide we wish existed when we started building agents. The goal: an AI agent that is fast, scalable, capable, reliable, efficient, and secure — not by accident, but by design. 📖 How to read this guide Read top-to-bottom for the full mental model. Each section builds on the previous one. Skim the boxes if you only want the takeaways — every section ends with an Actionable rules box. Jump to Part 14 — The Build-Your-Own Roadmap if you already know the theory and want a sequenced plan. Bookmark Part 15 — Anti-Patterns for design reviews.…