Harness

Plain English. The software wrapped around a model that turns it into a working tool: the loop that feeds it files, runs its commands, checks its output and manages what it can see. The model supplies raw capability; the harness decides how that capability gets applied to a job. Coding tools like Claude Code and Codex are harnesses. A harness is not an agent — an agent is what you get when a model runs inside one, pursuing a goal.

Why it moves money. The same model in different harnesses produces different results at wildly different costs, which means the harness — replaceable, largely open-source, mostly ordinary software — now moves outcomes as much as the billion-dollar model underneath. If capability lives partly in the wrapper, the model layer's pricing power is smaller than its training bill implies, and value migrates to whoever owns the wrapper and the workflow around it.

What to watch. Controlled comparisons that hold the model constant and swap the harness. When the spread across harnesses exceeds the spread across model generations, differentiation is being priced at the wrong layer.

From the signals. One benchmark ran the same model through nine harnesses and found a 17x spread in median cost per task. Harness choice moved an ARC-AGI-3 score by roughly 36 points in a controlled comparison. A working developer's account of why the harness, not the model, is now the product.