{"ok":true,"result":{"ok":false,"error":"ISBN must be 10 or 13 digits"},"serviceId":"x402-isbn","free":true,"_register":"Get 500 free credits (no KYC): POST /api/v1/register-simple {name}","_trial":{"remaining":4,"max":5,"register":"POST /api/v1/register-simple {name} — 500 free credits, no KYC, auto-wallet"}}