Agent = Model + Harness. What Scales This Agent?

2024 was the year of prompt engineering. 2025 was the year of context engineering. 2026 has been the year of the harness. You can’t read an AI engineering thread without tripping over the word. My problem with how it gets explained: until you’ve actually built one, the definitions never stick. The fastest way to understand what a “harness” actually is, is to build one. So that is what we are going to do. In twenty lines of Python, using one API call, one tool, and one loop, we’ll have a working coding agent.