x402 Doctor: why a payment failed, in plain words: the cause, whose side it is on (your client, your wallet, the network, the seller or the facilitator) and how to fix it, from over 40 named causes (x402 and Coinbase facilitator codes, EVM and Solana, HTTP and network errors). Send the seller 402 as challenge and the error as response, by POST JSON or GET query. Unknown failures are reviewed nightly.

Payment Options