Claude Code now supports AGENTS.md files

Summary

Claude Code v2.1.277 falls back to reading the AGENTS.md file if a CLAUDE.md file is not present in a directory. This feature was implemented as a built-in 'mode' within Anthropic's upcoming customization system.

Why it matters

AGENTS.md is emerging as a new cross-vendor standard for defining instructions for coding agents. By adopting this filename as a fallback, Anthropic reduces friction for teams that standardize on this name across multiple AI tools. This move also indicates that the 'Claude Code mods' architecture has matured enough to be released as an extensible plugin layer for users. This could reshape how project-level agent configuration works.

Highlights

• Fallback only: The AGENTS.md file is read only if CLAUDE.md is absent. This acts more as a compatibility layer than a full equivalence.
• Built on the unreleased 'Claude Code mods' system. Consequently, the underlying plugin API is still subject to change.
• The source code for the built-in mode is public. This allows the community to fork it and create custom project instruction modes.

Critical take

This 'article' lacks independent testing. Edge cases (such as what happens when both files exist in nested directories) are not discussed. There is also no comparison of how other agents handle the AGENTS.md file. This single-vendor quote, relayed via Simon Willison’s blog, reads more like a product announcement than a comprehensive technical review.