The Model Is the Engine; the Harness Is the Car

When an agent makes a mistake, the reflex is to switch models. Almost always, that’s the wrong diagnosis.

The model is the engine. What turns an engine into a vehicle is everything else: chassis, steering, brakes, dashboard. In an agent, that “everything else” is called the harness: the system prompt, the tools, the permissions, the limits, the checkpoints.

An agent that invents facts needs a tool that checks the source and a verification step that rejects the deliverable when a claim has no backing. A smarter model, without that, invents better-written facts.

Practical test: swap your agent’s model for another one. If the behavior changes a lot, too much behavior was hanging off the model and too little was written into the harness.