Harness

An Introduction to Agent Engineering: What 400,000 Sessions Showed About Designing How You Delegate

Anthropic analysed about 400,000 sessions and found people still make about 70% of the planning decisions, while AI makes about 80% of the execution decisions. People with deeper expertise produce five times the output per session. The length of task you can leave alone keeps doubling, and the gap comes from the environment around the model, not the model itself.