Paste a URL that's supposed to return an x402 payment challenge. This fetches it server-side, decodes the 402 payment requirements, and flags mistakes that will make payments fail — including the EIP-712 domain-name bug that we shipped and fixed ourselves.
Fetched server-side (SSRF-guarded — no private/internal URLs). Free, rate-limited per IP. Nothing you paste is stored.
If the URL returns HTTP 402, the inspector decodes every entry in its accepts[] array —
scheme, network, payTo, asset (and whether it's a recognized USDC contract),
maxAmountRequired in both atomic units and dollars, maxTimeoutSeconds, and the
EIP-712 signing domain from extra — and flags anything that looks wrong: a missing or
malformed address, an amount that isn't a valid integer, an unrecognized network, and — the one that
bit us — an extra.name that doesn't match what that network's real USDC contract expects
(Base mainnet USDC's on-chain name is "USD Coin", not "USDC"; get that wrong
and every buyer's signature verifies against the wrong EIP-712 domain and silently fails). If the URL
doesn't return 402 at all, it explains what it got instead.