How Cursor, Claude Code, and Codex actually load your project rules (and why yours get ignored)

In the realm of AI coding tools, the concept of "project rules" files has become a standard, but users often find that their specified rules are ignored. Cursor, Claude Code, and Codex each have their own formats for these files—.cursor/rules, CLAUDE.md, and AGENTS.md respectively. The issue lies not in the tools' functionality but in the human-readable format of these files, which each tool interprets differently. Understanding how these tools actually parse the rules can help developers avoid frustration and ensure their project guidelines are correctly applied.