Agent = Model x Harness: Your Eval Layer Is Part of the Agent, Not a Tool Beside It (opens in new tab)

There's a formula I keep coming back to when people ask why their slick demo agent falls apart in production: Agent = Model × Harness The model is the raw reasoning — Claude, GPT, whatever. It's swappable, and it's getting better on a curve you don't control. The harness is everything else: the goals, the loops, the tools, the scheduler, the retry logic. Most of the engineering that matters lives in the harness, not the model. But here's the part most teams get wrong. They define the harness ...