x402 Preflight documentation

Checkable answers for x402 API developers. Every page states what the check observes, how to reproduce it, and what it does not prove.

How to check an x402 payment configuration before launch — A read-only check of the live HTTP 402 challenge: scheme, network, asset, amount, receiver, extensions, CORS and discovery metadata — without a wallet and without spending USDC.
x402 resource listed in the Bazaar but buyers cannot find or pay it — The usual causes: the resource returns 404/410 while still listed, the discovery metadata drifted from the live challenge, the method differs, or the discovery consumer never indexed the update.
x402 endpoint returns 200 without a payment challenge — If a plain GET returns the data instead of a 402, the listing cannot earn: any agent calling it gets the result for free. It is usually the payment middleware not mounted on that route, or a proxy answering first.
Discovery requirements for x402 catalogues (OpenAPI + x-payment-info) — Catalogue crawlers prefer OpenAPI-first discovery: each paid operation declares x-payment-info with protocols and a fixed or dynamic price, plus a 402 response; /.well-known/x402 remains the compatibility path.
What is x402? — An open, HTTP-native payment standard that lets a server charge per request: an unpaid call returns 402 with a payment challenge, the client pays in stablecoin (commonly USDC on Base), and the retried call is served — no account or key needed for that call.
How to add x402 to an Express API — Mount the payment middleware before the protected route, configure the accepted network/asset and receiver, return a decodable 402 challenge, and verify without a wallet.
x402 vs Stripe: per-call billing for APIs — They solve different problems: x402 is an open, crypto-settled, per-call rail for machine-to-machine payments; Stripe is a card/subscription stack for human checkout.

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The same notes as Markdown, for review and citation: github.com/chico10117/x402-preflight-docs