Use when an agent is about to install or recommend a package an LLM suggested: send up to 10 names, each with its registry (npm, pypi or crates), and get back whether the registry lists each name (exists true/false), latest version, licence, links, creation time and flags (deprecated, yanked, created_recently, name_differs), under one signed receipt, so a made-up package name is caught before install. Not a security verdict. A batch where every lookup fails gets 502 and is not charged. Costs US$0.005 per call in USDC via x402 (Base), charged only when the call succeeds; a bad request is refused unpaid. Pay by putting an x402 payment payload in _meta["x402/payment"]. Without one it uses the free trial (5 calls a day per address), then returns the payment requirements. Docs: https://vehcdj664efetfrsolne5umanq.srv.us/docs/check-packages