Claude Code reads AGENTS.md now, but only when CLAUDE.md is missing

Claude Code shipped AGENTS.md support on September 18, 2026. The changelog line for 2.1.277 is one sentence, and every word in it matters: Added AGENTS.md support: in a project with no CLAUDE.md, Claude Code reads AGENTS.md instead; change it under “Project instructions” in /config (not yet on Bedrock, Vertex or Foundry) // Detect dark theme var iframe = document.getElementById('tweet-2101023103962718457-239'); if (document.body.className.includes('dark-theme')) { iframe.src = "https://platform.twitter.com/embed/Tweet.html?id=2101023103962718457&theme=dark" } Read that again: in a project with no CLAUDE.md. This is a fallback, not a merge. If your repo already has a CLAUDE.md, nothing changed for you. Claude Code keeps reading CLAUDE.md and keeps ignoring AGENTS.md, exactly as before. What 2.1.277 actually changed Three things, and only three. When Claude Code finds no CLAUDE.md, it reads AGENTS.md instead of loading nothing. The behavior is togglable under Project instructions in /config. It is not available yet on Amazon Bedrock, Google Vertex AI, or Microsoft Foundry. There is no documented settings.json key and no environment variable for the toggle. The only documented surface is the /config menu. If you deploy Claude Code through managed settings, that matters: you cannot pin this behavior across a fleet today. One more thing worth knowing before you build on it. The memory documentation page has not caught up. At the time of writing it still says, in plain text, that Claude Code reads CLAUDE.md and not AGENTS.md. The changelog is ahead of the docs here, so trust the release notes and verify in your own session. The fallback almost never fires in a real repo Claude Code does not look for one instructions file. It walks a tree. /