Use when an agent is about to give a user links or citations: send up to 10 URLs, and get back for each one the status, redirect chain, final URL, content_kind, title and hash, so a dead, moved or blocked link is caught before it is shown. A URL whose domain does not resolve is reported as dead and charged; a batch where none resolves gets 422 and is not charged. A redirect to another site that errors is off_site with off_site_error: true. 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-links