I use AGENTS.md to make sure my agents loop effectively (tests, quality, etc). Not to describe the code / architecture.
To me it is! I thought if an AGENTS.md is researched with much effort once, next time reading the brief conclusion would save on effort and time and cost. Would have seemed logical to me. Like a glossary, looking up a manual, getting briefed before starting.
But I accept the research findings, and I have even had bad experiences with an outdated AGENTS.md derailing the agent.
So now I'm trying to keep to the basics, e.g. just a sentence or two saying "use bun instead of npm" only because I've had to manually prompt that too many times.
It's remarkable how good agents are at exploring, and at treating relatively sparse code reads and get a good picture nevertheless.
This way it can refer to the docs only when relevant instead of dragging around the full knowledge of the front-end structure while it's working on the backend.