If your team already standardizes on AGENTS.md for agent instructions across tools like Cursor, Codex, or Copilot, you no longer need a separate CLAUDE.md file for Claude Code. Version 2.1.277, released September 18, 2026, adds AGENTS.md support: in a project with no CLAUDE.md, Claude Code reads AGENTS.md instead.

That single line in the changelog removes one of the most annoying friction points in multi-agent setups. Previously you had to keep two instruction files in sync — one for Claude, one for the rest of the agent ecosystem — and every tool had a slightly different opinion about which file wins. Now Claude Code falls back to AGENTS.md automatically, and you can switch which file it treats as project instructions under "Project instructions" in /config.

What the changelog actually says

The exact change, in version 2.1.277 of the Claude Code changelog:

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)

The caveat at the end matters more than it looks: this applys to the CLI and to sessions talking to the Anthropic API. If you run Claude Code against Bedrock, Vertex, or Foundry, you won't see AGENTS.md fallback yet — those platforms still expect CLAUDE.md. I'd hold off deleting your CLAUDE.md until the fallback lands there, or keep a thin CLAUDE.md that points at AGENTS.md for those environments.

Why this is a bigger deal than a one-line release note

AGENTS.md became a de-facto standard across coding agents in 2026. Cursor, OpenAI Codex, and several CLI agents all read it. A company-wide or repo-wide AGENTS.md is how you encode build commands, test conventions, directory layout, and "never touch the generated folder" rules once and have every agent respect them.

Before 2.1.277, Claude Code ignored that file entirely. Teams either forked their instructions into CLAUDE.md and risked drift, or wrote AGENTS.md and silently lost all that context when someone opened Claude Code in the same repo. The new fallback means the same file that tunes Cursor will tune Claude Code — no migration, no duplication.

If you already maintain AGENTS.md, here's the practical checklist:

  • Drop CLAUDE.md if it only duplicated AGENTS.md content.
  • If the two files diverge, keep CLAUDE.md — it still wins as the primary source.
  • On Bedrock/Vertex/Foundry, keep CLAUDE.md until Anthropic backports the fallback.
  • Check /config → "Project instructions" to see which file the current session is using.

The rest of 2.1.277 worth knowing

The same release ships a few items that matter beyond the headline feature:

  • Auto mode server classifier: 2.1.278 (September 19) defaults auto mode for Claude API and Enterprise users, and on Bedrock, Vertex, Foundry and gateways, to the server-side classifier, which does not charge for classifier overhead. You opt out with CLAUDE_CODE_AUTO_MODE_SERVER=0 on those platforms, and /status now shows an Auto mode server row telling you where the classifier runs.
  • Anti-supply-chain hardening: plugins installed from an npm source are now fetched with npm pack --ignore-scripts and integrity-verified, so a package's install scripts no longer run. That's a real reduction in the attack surface I've flagged before around third-party agent plugins.
  • Prompt sanitization: invisible Unicode formatting and tag characters in a prompt are removed, and the cleaned prompt is shown for review before it is sent. Good for defenders, mildly annoying if you relied on zero-width tricks to steer the model.

Version 2.1.277 also removes the deprecated TaskOutput tool, replacing it with plain Read, and fixes a long list of crash and hang bugs in /plugin, /mcp, and fullscreen resume. If you've been hitting any of those, the update is worth taking now.

Bottom line

One less per-tool instruction file, one less drift source. Standardize on AGENTS.md, delete the Claude-specific duplicate, and your instructions follow you across agents. Just don't retire CLAUDE.md yet if your team uses Bedrock, Vertex, or Foundry.

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