Check an x402 endpoint before paying it: fetch the unpaid 402 challenge of any x402 URL and verify it is payable and safe - HTTP 402, a parseable x402 v1/v2 challenge, network, asset, payTo, amount, whether the advertised resource URL is the URL you actually called, manifest consistency (/.well-known/x402.json), header-size overflow risk for Node clients, and a signed pay / caution / avoid verdict. Read-only: we never send a payment.