I build Yonto Player, a media player for your own library on Android TV, phones and tablets, with AI coding agents. In my experience agents start every session knowing nothing about the repo, so the first thing they read decides a lot of what they do next. That first thing is one file at the root, AGENTS.md , and I have spent more time editing it than almost any other file.

The series' opening post showed a small sketch of it. This one goes deeper: what goes in, what stays out, and how it is written.

What the file is for

It is a contract between me and an agent that has never seen the project. It has to answer, quickly, the questions a new teammate would ask on day one: what is this, how is it laid out, how do I check my work, what must I never break, and where do I read more for the job I was given.

It does not try to be the documentation. The root file holds what nearly every job needs.…