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 demo 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. Skip ahead if you already know what an LLM, tool call, or agent loop is.
