Every AI coding tool now has a "project rules" file. Cursor has .cursor/rules, Claude Code has CLAUDE.md, OpenAI Codex has AGENTS.md. Teams write them once, watch the agent ignore half of it, and conclude the feature is broken.
Most of the time the feature isn't broken. The rules file is written for a human reader, and each tool consumes it differently. Here's how the three tools actually load rules as of mid-2026, and the writing habits that transfer across all of them.
Cursor: rules are conditionally attached, not always read
Modern Cursor rules live in .cursor/rules/ as .mdc files with frontmatter (the old single-file .cursorrules has been dropped from the current docs entirely — treat it as deprecated and migrate). The frontmatter decides when the rule enters context:
description: "Conventions for API route handlers"
