The secret isn't the model. It's the harness.

Getting AI agents to write code is not new anymore. The real problem is not how smart the model is. The real problem is that agents do not have good environments to work in for a long time. Harness Engineering is the field that works on this problem. In November 2025, Anthropic ( https://www.anthropic.com/engineering/effective-harnesses-for-long-running-agents ) published a blog post about it. In February 2026, OpenAI ( https://openai.com/index/harness-engineering/ ) did the same. OpenAI said a team of 7 people made about 1 million lines of code in 1,500 PRs over 5 months. They wrote zero lines by hand (self-reported). On X, the post "the 10x skill of 2026 is Evaluation Engineering" went viral. The engineer's job is changing. From "writing code" to "building environments where agents write good code." Two Parts of Harness Engineering Agent Harness is the execution side. The setup that lets agents work well over long sessions. It automates environment setup, passes progress between sess