- I bootstrap AGENTS.md with my basic way of working and occasionally one or two project specific pieces
- I then write a DESIGN.md. How detailed or well specified it is varies from project to project: the other day I wrote a very complete DESIGN.md for a time tracking, invoice management and accounting system I wanted for my freelance biz. Because it was quite complete, the agent almost one-shot the whole thing
- I often also write a TECHNICAL-SPEC.md of some kind. Again how detailed varies.
- Finally I link to those two from the AGENTS. I also usually put in AGENTS that the agent should maintain the docs and keep them in sync with newer decisions I make along the way.
This system works well for me, but it's still very ad hoc and definitely doesn't follow any kind of formally defined spec standard. And I don't think it should, really? IMO, technically strict specs should be in your automated tests not your design docs.