The format is open and described at agents.md; several agents read the file directly and others have an equivalent of their own, such as CLAUDE.md or copilot-instructions. The content is the same: what you would otherwise tell a new colleague on day one, on a single page. The more precise the file, the less the agent guesses — and the fewer pull requests that do the right thing the wrong way.
We built the AGENTS.md generator, which drafts one from a few questions about the repo, and described the thinking in AGENTS.md-generatorn: sidan kodagenten läser först (in Swedish). Repo X-ray checks whether the file exists.