Anthropic Adds AGENTS.md Fallback to Claude Code Without Replacing CLAUDE.md

Anthropic has added built-in AGENTS.md support to Claude Code, allowing the coding tool to use a shared project-instructions file when a repository has no CLAUDE.md of its own. The default is a compromise: Claude-specific instructions retain priority, while teams using AGENTS.md do not have to create a second file solely for Claude Code.

The feature ships as an agents-md plugin built into Claude Code. Its default setting, claude-md-or-agents-md, looks for AGENTS.md files only when the project lacks its own CLAUDE.md, .claude/CLAUDE.md, or CLAUDE.local.md on the path to the working directory. If one of those Claude-specific files exists, the plugin does not add an AGENTS.md fallback.

AGENTS.md is a lightweight Markdown convention for project guidance such as coding rules, build steps, and testing requirements. OpenAI said in December 2025 that the format had been adopted by more than 60,000 open-source projects and agent frameworks. Claude Code’s new fallback gives those repositories a route to reuse that guidance without silently replacing instructions written for Claude.

Fallback is only one available policy. Teams can use CLAUDE.md alone, load AGENTS.md alongside CLAUDE.md, or choose a managed-only mode. The combined setting avoids loading an AGENTS.md file twice when CLAUDE.md already imports it or links to it. Managed-only removes project, local, and user instruction files while retaining organization-managed instructions and the engine’s memory.

The choices in Claude Code

• CLAUDE.md only keeps the existing engine behavior.
• AGENTS.md fallback is the default when the project has no qualifying Claude file.
• Both formats loads AGENTS.md beside CLAUDE.md, with duplicate protection.
• Managed-only keeps centrally managed instructions while excluding project and personal files.

This is more than a filename check. AGENTS.md files added by the plugin are treated downstream as project instruction files, with the same placement in context, framing, announcements, and omission rules as CLAUDE.md. A Read action beneath the project root can also attach that subdirectory’s AGENTS.md, unless a CLAUDE.md there takes precedence.

Users can select the behavior through the Project instructions row in /config, user settings, --settings, or managed settings. A repository’s .claude/settings.json cannot set the plugin option. That makes instruction-loading policy a choice for the user or organization, rather than a setting imposed by checked-in project code.

Anthropic is also temporarily honoring the plugin’s earlier projectInstructions setting, mapping its old values to the new modes. For maintainers, the immediate decision is simpler: keep tool-specific guidance where it matters, or let one shared AGENTS.md file cover Claude Code too.