Martin Fowler Defines Harness Engineering: Guides + Sensors Framework — LangChain Agent Jumps from Top 30 to Top 5 on TerminalBench by Changing Only the Harness

Martin Fowler·high signal

Martin Fowler published a full article on harness engineering for coding agents (April 2), formalizing the concept: Agent = Model + Harness, where the harness is everything except the model. The framework introduces guides (feedforward controls that steer before the agent acts) and sensors (feedback controls that observe and correct after). The proof point: LangChain's coding agent improved from 52.8% to 66.5% on TerminalBench 2.0 — jumping from Top 30 to Top 5 — by changing nothing about the model, only the harness. For builders: invest in your harness, not model upgrades.