Verify any third-party x402 endpoint is alive and correctly configured before paying it: a bounded GET/402 preflight fetches the payment contract, checks reachability and response latency, validates accepts, network, price and expiry against the x402 spec, and returns a signed observation receipt with an observed_at timestamp you can cite — protection against ghost endpoints, dead quotes and wasted USDC. Limitation: no payment is made, one endpoint per call, and a passing probe proves the contract at observation time — not future uptime or provider honesty — $0.005 USDC per paid call through x402.