Some briefs need to outlive the session they were typed in.
Spec
Also called Specification
A written description of what something must do, exact enough that two people, or two agents, would build the same behaviour from it. It says what and why, and usually leaves how to the builder.
A session forgets. A spec in the repo lets the next session, a reviewer and a second agent all hold the work to the same thing, and you can read it before any code exists. The price is that it is one more thing that can be wrong, and nothing tells you when it is.
Say it to your agent
“Write docs/specs/webhooks.md: what apply() must do when events repeat or arrive out of order, five example sequences with the result of each, and what is out of scope. No code yet. I will read it first.”