Glonce

Claude Code 2.1.277 adds AGENTS.md fallback support

toolsThe Glonce editorsSimon Willison🔥55Glonce rating55Impact40Novelty55Relevance85Surprise25Players60Credibility78

Anthropic has added support for AGENTS.md as a fallback file for project instructions in Claude Code. Starting in version 2.1.277, if a folder does not contain a CLAUDE.md file, Claude Code will check for an AGENTS.md file and use it instead.

The feature is built on Claude Code mods, described as an upcoming system for customizing the Claude Code harness. The AGENTS.md fallback ships as a built-in mod, and users will be able to build their own custom versions of project instructions themselves.

The announcement was quoted on Simon Willison's blog and attributed to Thariq Shihipar, who also pointed to the source code for the mod and to a page listing further available mods. The post does not state Shihipar's role or affiliation, only that he is the person quoted. It also does not say whether AGENTS.md is read only when CLAUDE.md is absent or whether the two can be combined, beyond stating that AGENTS.md is checked when there is no CLAUDE.md; nor does it give a timeline for when Claude Code mods will become generally available, beyond calling them 'upcoming'.

Key facts

  • Claude Code version 2.1.277 adds AGENTS.md as a fallback for project instructions.
  • AGENTS.md is checked and used only when a folder has no CLAUDE.md file.
  • The fallback ships as a built-in mod on top of Claude Code mods, an upcoming system for customizing the harness.
  • Users will be able to build their own custom versions of project instructions using the same mod mechanism.
  • The announcement, quoted on Simon Willison's blog, is attributed to Thariq Shihipar, who linked to the mod's source code.

Why it matters

Until version 2.1.277, Claude Code read project instructions only from a CLAUDE.md file. The update adds a second path: when a folder has no CLAUDE.md, Claude Code now checks for an AGENTS.md file and uses that instead. It is also the first public example of a 'Claude Code mod,' a customization mechanism the announcement calls upcoming rather than already generally available.

Who it affects

This changes behavior for any project or folder that has an AGENTS.md file but no CLAUDE.md; Claude Code will now pick up instructions from it automatically. Projects that already have a CLAUDE.md are unaffected, since the source states AGENTS.md is checked only in its absence.

How to use it

Update to Claude Code 2.1.277 or later. If a folder has no CLAUDE.md, place project instructions in an AGENTS.md file and Claude Code will find and use them without further configuration. The fallback itself is a built-in mod, and its source code is public; the announcement says users will also be able to build their own custom mods for project instructions, though no instructions for doing so are given in this post.

How solid is it

The claim comes from a direct quote, attributed by name to Thariq Shihipar, published on Simon Willison's blog together with a link to the mod's source code. There is no independent report beyond this quoted announcement, and the post gives no title or role for Shihipar to weigh the claim against.

Risks and caveats

The source does not explain what a 'mod' is beyond calling it a way to customize the Claude Code harness, and gives no release date for Claude Code mods becoming generally available. Details of how custom mods for project instructions would be built are not covered in this announcement.

“We're adding support for AGENTS.md to Claude Code. Starting today in version 2.1.277, if there is no CLAUDE.md in a folder, Claude will check for and use AGENTS.md. AGENTS.md support is built off of Claude Code mods, our upcoming way to customize the Claude Code harness. This is a built-in mod, but you'll be able to build custom versions of project instructions yourself as you'd like too.”

— Thariq Shihipar