Payment Header Check API: reads an x402 PAYMENT-SIGNATURE header before you send it - base64/UTF-8/JSON, version, authorization fields, clock window, who signed (lane a or b) and, with the seller PAYMENT-REQUIRED, payTo/amount/network/asset. Nothing is sent anywhere. $0.005 USDC per check. POST JSON {payment_signature, payment_required?}; free example at https://api.aiskills402.com/v1/run/payment-header-check/example

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