Harness Engineering for AI Agents

Agent = Model + Harness. If you're not the model, you're the harness. If you have shipped an AI agent that worked brilliantly in a and then fell apart in production, you have already met the harness problem. The model was probably fine. The scaffolding around it - instructions, tools, loops, verification, logging - was not. This article synthesizes six landmark posts from early 2026 (LangChain, Martin Fowler, OpenAI, Addy Osmani) into one practical guide. No prior agent-building experience required. 1. Start Here New to AI agents? Read this section first.