Hook
A command the tool runs by itself at a fixed moment — before a file is written, after a task ends — so a rule holds without anyone remembering it.
The difference from an instruction is who enforces it. A sentence in a prompt asks the model to run the formatter; a hook runs the formatter. One is followed most of the time, the other every time, and for anything that must not be skipped that gap is the whole point.
It can also refuse. A hook that returns a failure before a write blocks the write, which turns a house rule into something closer to a lint gate — the model sees the refusal as feedback and adjusts instead of proceeding.
The usual mistake is to reach for one too early. A hook that fires on every step and prints nothing useful becomes noise the model has to read on each turn, and noise costs context.