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Claude Code adopts OpenAI's AGENTS.md format

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Claude Code adopts OpenAI's AGENTS.md format

Anthropic is adding support for AGENTS.md, the markdown instructions format OpenAI created for AI coding agents, to its own Claude Code tool. Claude Code engineer Thariq Shihipar announced the change in a social media post on Friday, September 18, 2026: "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." Users can toggle this behavior with the /config command.

Until now, developers who used Claude Code alongside OpenAI Codex or other non-Anthropic tools had to maintain two sets of markdown instructions for their AI agents, CLAUDE.md and AGENTS.md, plus project-specific versions of each, and some resorted to workarounds such as symlinks to keep the two files in sync. The formats are similar but not identical: CLAUDE.md can carry Claude-specific instructions, while AGENTS.md is meant to be tool agnostic. Both kinds of file are read by coding agents on every request and lay out expected behavior, preferred tool usage and coding conventions.

OpenAI contributed AGENTS.md to the Agentic AI Foundation, under the Linux Foundation, last year in a push to build support for the standard. As of December 2025, more than 60,000 open source projects had implemented AGENTS.md. According to The Register, Claude Code's popularity had given Anthropic the room to keep insisting on its own format, but the company appears to have concluded there was nothing to gain from continuing that separate specification.

The change drew a public reaction from a rival: Thibault Sottiaux, OpenAI's head of core product, wrote "Yay! This is the way. Come to the light," and Shihipar answered with a handshake emoji. Other developers who follow both companies' tools also welcomed the move on social media.

Key facts

  • Starting in Claude Code version 2.1.277, Claude checks for and uses AGENTS.md when a folder has no CLAUDE.md; the behavior can be toggled with /config.
  • The change was announced by Claude Code engineer Thariq Shihipar in a social media post on Friday, September 18, 2026.
  • OpenAI contributed AGENTS.md to the Agentic AI Foundation, under the Linux Foundation, last year; more than 60,000 open source projects had implemented the format as of December 2025.
  • CLAUDE.md and AGENTS.md are similar but not identical: CLAUDE.md can hold Claude-specific instructions, while AGENTS.md is designed to be tool agnostic.
  • OpenAI's head of core product, Thibault Sottiaux, publicly welcomed the move; Shihipar replied with a handshake emoji.

Why it matters

Developers running Claude Code alongside OpenAI Codex or other non-Anthropic agents had been maintaining two separate markdown instruction files, CLAUDE.md and AGENTS.md, often with project-specific copies of each. By having Claude Code fall back to AGENTS.md when no CLAUDE.md exists, Anthropic removes a piece of duplicate upkeep for anyone working across both ecosystems. The Register's report frames this as Anthropic giving up a position of strength: Claude Code's popularity had let the company keep pushing its own format, but it apparently decided there was no longer anything to gain from that separation.

Who it affects

Anyone using Claude Code together with OpenAI Codex or other tools that read AGENTS.md, and anyone maintaining a project among the more than 60,000 open source repositories that had already adopted AGENTS.md as of December 2025. It also affects teams that had built workarounds, such as symlinks, to keep CLAUDE.md and AGENTS.md synchronized.

How to use it

Update to Claude Code version 2.1.277 or later. If a project folder has no CLAUDE.md, Claude Code will automatically look for and read AGENTS.md instead. The behavior can be turned on or off with the /config command. No pricing or licensing terms are involved; this is a change to which instruction file Claude Code reads, not a paid feature.

How solid is it

The claim rests on a named Claude Code engineer, Thariq Shihipar, stating a specific version number, 2.1.277, in a public social media post, plus a byline report from The Register. OpenAI's head of core product, Thibault Sottiaux, publicly responded and effectively confirmed the news, and other developers reacted to it online. There is no separate, formal Anthropic company statement beyond the engineer's post.

Risks and caveats

The only Anthropic voice on record is an engineer's social media post, not an official company announcement, and the article gives no figures on how many Claude Code users or projects this will actually change behavior for. The two formats also remain distinct rather than merged: CLAUDE.md can still carry Claude-specific instructions that AGENTS.md, being tool agnostic, is not designed to hold, so teams with Claude-specific setups may still need both files.

“Starting today in version 2.1.277, if there is no CLAUDE.md in a folder, Claude will check for and use AGENTS.md.”

— Thariq Shihipar, Claude Code engineer, in a social media post