Harness
The layer that sits between the language model and the world.
A language model on its own only produces text. The harness decides what it can do with that text: which tools it sees, what it runs without asking, what survives from one session to the next, what fires automatically after every change.
The word means the gear you strap onto an animal in order to steer it. It is a good image, because a harness does not make the horse stronger — it makes the horse's strength usable.
Changing models changes what the agent knows. Changing the harness changes what it can do, how expensive its mistakes are, and what is left once the session ends.