Claude Code Adds AGENTS.md Support via New Harness Customization Mods
Anthropic's Claude Code version 2.1.277 now automatically checks for and uses an `AGENTS.md` file whenever a `CLAUDE.md` file is missing in a directory. This interoperability feature is implemented using "Claude Code mods," a new framework designed to customize the tool's harness. Adopting `AGENTS.md` improves interoperability across AI coding tools by recognizing an established open format used by tens of thousands of repositories. Additionally, the new mods system enables developers to build custom harness extensions tailored to their specific project instruction requirements. The `AGENTS.md` integration is shipped as a built-in mod, and Anthropic has made its source code available on GitHub as an example. Developers will be able to author their own custom mods to personalize project instructions and harness workflows.
## BACKGROUND
`CLAUDE.md` and `AGENTS.md` are plain markdown files placed at the root of project repositories to guide AI coding agents on project structure, build steps, and coding conventions. While `CLAUDE.md` is specific to Anthropic's Claude Code, `AGENTS.md` is an open standard designed to serve as a universal README for AI agents across different development tools.