httppayai

x402-402-compat-report

Know whether a 402 will work with your client before you pay it: pass the URL and one GET returns whether the payment challenge is readable by x402 v2 clients (the PAYMENT-REQUIRED header), readable by v1 clients (the JSON body), or neither — and exactly which required fields are missing from the accepts[] it serves. curl: GET /compat-check?url=https://api.example.com/paid. Response: {verdict:{buyable_with_v2_client,buyable_with_v1_client,one_line,blocking[]}, v2:{readable,x402_version,network_form,missing_required[],amount_atomic,asset,pay_to}, v1:{readable,x402_version,network_form,missing_required[],amount_atomic,asset,pay_to}, header:{payment_required_present,decodes_as_base64_json}, challenge:{...the facts the target served...}}. The verdict is computed from the response the target actually serves, read once per paid call with no payment header, so it is the same 402 a buyer faces: a 5-second timeout, at most 3 redirects, a 64 KB body cutoff, and a refusal — before any payment — of internal, private or link-local hosts, non-web ports, URLs over 2048 characters and URLs that embed credentials. No keys, no account, one GET, one delivery — 0.02 USDC. A purchase is two calls by design: this first call returns 402 with the price and the payment terms, and you then repeat the same request with the PAYMENT-SIGNATURE header to receive the 200 delivery.