Anthropic just adopted OpenAI's standard, a year after refusing the most-requested feature in Claude Code's repo. OpenAI already runs its agents on Anthropic's protocol. AI's two biggest rivals each won exactly one standards war, and both wars were decided by the same force. It wasn't model quality. AGENTS.md started as an OpenAI project in August 2025. A plain markdown file that tells any coding agent how to build and test your repo. Codex read it, then Copilot, Cursor, Gemini CLI, Jules,…