Claude Code Introduces Automatic AGENTS.md Support, Announces Thariq Shihipar

On 18 September 2026, Anthropic released Claude Code version 2.1.277 with new logic for handling project instruction files.

What the Update Changes

The update adds a rule that, in the absence of a CLAUDE.md file, Claude Code will look for and apply an AGENTS.md file.

CLAUDE.md has been the standard file for defining how Claude Code should operate on a codebase.

AGENTS.md is introduced as an alternative instruction file that can convey similar configuration details.

Support for AGENTS.md is built on top of Claude Code’s mod system, which the team describes as an upcoming method for customizing the Claude Code harness.

The mod that enables AGENTS.md is included as a built‑in component of the new release.

Developers will also be able to create their own custom versions of project instructions by writing bespoke mods.

The source code for the built‑in AGENTS.md mod is publicly available via the link provided in the announcement.

“there are more mods here” said Thariq Shihipar in the quoted remark accompanying the release.

The comment indicates that additional mods beyond AGENTS.md are listed in the same repository.

Implications for Developers

Projects that previously lacked a CLAUDE.md file can now adopt AGENTS.md to guide Claude Code’s behavior without additional setup.

This flexibility reduces the friction of integrating Claude Code into existing repositories that may not follow the CLAUDE.md convention.

By leveraging the mod architecture, teams can tailor Claude Code’s instruction handling to match their specific workflow requirements.

The ability to supply custom mods suggests a path toward more granular control over LLM‑driven coding assistance.

Anthropic’s decision to ship the feature as part of a minor version bump signals an incremental approach to expanding Claude Code’s configurability.

The announcement was posted on Simon Willison’s Weblog, a platform that frequently shares updates about AI tooling.

Developers interested in the new behavior can simply add an AGENTS.md file to the root of their project and rely on the automatic fallback.

Because the fallback is triggered only when CLAUDE.md is missing, existing projects that already use CLAUDE.md will continue to operate as before.

This design preserves backward compatibility while offering an immediate upgrade path for new users.

The broader trend of exposing configuration files aligns with industry moves toward making LLM‑based tools more transparent and adaptable.

As more teams experiment with AI‑assisted coding, the availability of multiple instruction formats may encourage experimentation with different prompting strategies.

Anthropic’s emphasis on mods hints at a future where community‑contributed extensions could further enrich Claude Code’s capabilities.

Overall, the release expands the toolkit developers have for steering Claude Code without requiring deep changes to their codebases.

Why This Matters