Hook

A shell command the harness runs at a fixed point in the agent's loop — not a request to the model. Exit codes are the interface: a non-zero exit can block the transition the hook fired on. It is the only lever on this page the model cannot argue with, which is also why an untrusted repository that can write a hook has execution on your machine. The distinction worth holding: an instruction is obeyed by disposition, a hook is enforced by the harness.