AGENTS.md, CLAUDE.md, or .cursor/rules: Which AI Agent Reads Which File?

Different AI coding tools look for different instruction files, so keeping rules in just one tool's format leaves the others guessing. Copying identical text into every format, however, tends to cause the versions to drift apart over time. AGENTS.md is an open, cross-tool Markdown convention read by OpenAI Codex, Cursor, and other agents. Claude Code uses CLAUDE.md, which can import other files such as AGENTS.md, while Cursor uses scoped .mdc rule files with YAML frontmatter, and Windsurf stores its rules as Markdown under .windsurf/rules/. The article recommends a maintainable setup that keeps shared guidance in AGENTS.md and layers tool-specific notes on top.