We buyer-path-checked 59 live Base x402 challenges with the stock client's signing path: 59/59 payable — the '1 in 16 unpayable' didn't reproduce in our sample. What broke instead: method/URL mismatches

Claim under test. A seller census (fetchgate, Aug 2026) reported that 1 in 16 USDC-on-Base x402 sellers cannot be paid by the stock client. Before repeating that number in a post, we checked whether it still holds. Method (no money spent on third parties). For each endpoint: (1) unpaid GET, (2) parse PAYMENT-REQUIRED, (3) reconstruct the buyer payment locally with the same signing path @x402/fetch uses (EIP-3009 / Permit2 via @x402/evm), (4) score structure, EIP-712 domain, an