Introduction to Harness Engineering: An AI Agent's Reliability Is Decided by the Design Around It
· 27 min read
"I upgraded the model to the latest version, but it still repeats the same mistakes." "No matter how I tune the prompt, I hit a wall somewhere." When you try to put an AI agent to work, you always run into this wall. In many cases this is not a problem with the model itself; it stems from the design quality of the environment surrounding the model (= the harness). In this post I organize the idea of harness engineering — which was rapidly systematized starting in February 2026 — together with configuration examples from my own ~/.claude/ directory.