Check your rule file against the agents that read it.
Paste a CLAUDE.md, AGENTS.md, Cursor rule or Copilot instructions file. See which agents load it from where it sits, what gets cut, and which lines ask for something only a hook can enforce. Every verdict links to the note where we tested it.
Checked in your browser. Nothing you paste leaves this page.
What it checks
- Who loads it, from where it sitsClaude Code, Codex, Cursor, three Copilot surfaces and Antigravity, at the root, in a subfolder or in your home folder.Which tools read which file
- What gets cutCodex keeps 32 KiB of AGENTS.md and cuts the rest without a word. Anthropic suggests 200 lines; Cursor, 500 a rule.Why Codex ignores AGENTS.md
- Lines no agent enforcesA "never" in a rule file is a request. It finds the commands a deny rule or a hook could refuse instead.Claude Code hooks
- Frontmatter that decides loadingCursor’s four load modes, Copilot’s applyTo, and the paths that scope a Claude rule.Cursor rules
- Imports and pointersOnly Claude Code resolves an @ import. A sentence saying "read AGENTS.md" is one Claude may never act on.Does Claude Code read AGENTS.md?
- What should not be thereSecrets, repeated lines, directory trees, and lines that were only true "for now".Context rot and stale rules
A checker reads the file. It can't see what your agent was handed in a session, or which rules it cited. context-tax shows what Claude Code sends on every turn before you type. Harbor counts, for each rule, how often it was served and how often an agent cited it.
Decide once.
Every agent knows.
Nothing to migrate and nothing to write. Two weeks in, you see which rules your agents were given and which ones they cited. Leave an address and the people who build it will be in touch.