One rules file for every coding agent: AGENTS.md plus a one-line import
Teams that mix Claude Code, Codex, Cursor and Copilot end up with drifting instruction files. Keep the rules in AGENTS.md and have CLAUDE.md import it.
The problem
Codex, Cursor and Copilot read AGENTS.md. Claude Code reads CLAUDE.md. Recent Claude Code versions can read AGENTS.md directly, but only when no CLAUDE.md exists. Once a repo has both, Claude reads only CLAUDE.md by default. Teams then copy rules between the two files, and within a month they disagree on the test command.
The tip
Make AGENTS.md the single source of truth for build commands, conventions and project layout. Put an @AGENTS.md import on the first line of CLAUDE.md. Claude Code expands @path imports when it loads the file, so it sees the same rules every other agent sees. Anything that only applies to Claude (plan-mode rules, Claude-specific skills) goes below the import.
A symlink (ln -s AGENTS.md CLAUDE.md) also works, but the import is safer: Git on Windows often checks symlinks out as plain text files, and an import leaves room for Claude-only notes.
Example
<!-- CLAUDE.md -->
@AGENTS.md
## Claude Code only
- Use plan mode for any change under `src/billing/`.
- Run `/context` after editing this file to confirm both files loaded.When NOT to use it
If your team only uses Claude Code, skip the indirection and keep one CLAUDE.md. Imports also don’t reduce context cost, so a bloated AGENTS.md is still bloated.