What is x402?

x402 is an open protocol that lets software pay for a web resource per request by using the HTTP 402 Payment Required status code. A server answers an unpaid request with 402 and machine-readable payment terms; the client signs a stablecoin payment authorization and repeats the request; the server verifies and settles the payment and returns the result. No account, subscription or API key is needed to buy. On market.561.group there are 130 such offers, priced in USDC on Base. Browse them all.

How HTTP 402 works

HTTP reserved status 402 for payments but never defined how. x402 defines it: the 402 response carries a PAYMENT-REQUIRED header, a base64 JSON document listing what the server will accept for this exact request (scheme, network, asset, amount, payee, timeout). The client answers with a PAYMENT-SIGNATURE header, and a successful response carries a PAYMENT-RESPONSE header with the settlement receipt. The facilitator here is Coinbase's CDP.

A paid request, step by step

The example uses a real offer: The x402 Economy, a GET that costs $9.00 USDC.

Ask without paying.

curl --include https://market.561.group/v1/publications/the-x402-economy

Receive 402. The response status is 402. Decode the payment-required header (base64 JSON). Its accepts[0] reads scheme: "exact", network: "eip155:8453", amount: "9000000" (USDC has 6 decimals, so 9.00 USDC), the USDC contract as asset, a payTo address and maxTimeoutSeconds: 300.
Sign. Your wallet signs an EIP-3009 transferWithAuthorization for exactly those terms. The authorization is valid only for that payee, amount and time window.
Retry with payment. Repeat the identical request with the header PAYMENT-SIGNATURE: <base64 payload> (an x402 client library builds this).
Receive 200. The market verifies the authorization, settles it through the facilitator, and returns 200 with the purchased bytes and a PAYMENT-RESPONSE header.

These steps are visible in the live endpoints: /.well-known/x402 lists every payable resource, and any offer URL returns its terms on an unpaid request.

What is distinctive about this market

It refuses bad input before charging. Offers validate the request first; invalid input gets a 400 or 422, never a 402. Try RFC 8785 Canonical JSON with a duplicate JSON key: the response is 422 and no payment terms are issued. A valid request then receives the 402.
Repeat-purchase recovery. Recoverable offers (the books) accept an X-Payment-Recovery token, 32 random bytes as 64 hex characters that you save before paying. Repeating the same signed request with it returns the exact purchased response, so a dropped connection does not mean paying twice.
Buyer-side payee check. A payee address read from the seller's own 402 is not independent evidence of who gets paid. The repository's reference buyer takes a separately supplied X402_EXPECTED_PAY_TO and refuses a different payee.
Sign-in-with-X on the 402. The 402 response also carries a sign-in-with-x extension, so a wallet the operator has granted use of an offer can prove it owns the address instead of paying.
One catalog, many projections. The same offer list is served as x402 discovery, the JSON catalog, OpenAPI, an A2A agent card, read-only MCP discovery (POST /mcp), an ActivityPub actor, llms.txt and these pages. None of them keeps a second price list.
A four-valued reading of 402. The market's /v1/four/ offers implement Belnap's four-valued logic, where a point can be told true, told false, both, or neither. Read that way, a 402 is told "both": it answers (the request was understood and priced) and it refuses (the content is withheld until payment).

Buy a book about it

The x402 Economy ($9.00 USDC) covers HTTP 402's history, a paid request step by step, who governs x402, its rivals, market measurements, security and US law. It is delivered as an EPUB and PDF in one ZIP and is itself bought with x402.

Frequently asked questions

What is x402?

x402 is an open payment protocol that uses the HTTP 402 Payment Required status code so software can pay for a web resource per request. The server answers an unpaid request with 402 and machine-readable payment terms; the client signs a payment and retries; the server verifies, settles and returns the resource.

What is HTTP 402 Payment Required?

402 is a status code reserved in HTTP for payment. x402 gives it a concrete meaning: the response carries a PAYMENT-REQUIRED header describing the amount, asset, network and payee for that exact request.

Which network and currency does market.561.group use?

Payments settle in USDC on Base mainnet (CAIP-2 id eip155:8453) through the Coinbase CDP facilitator. Each offer's live 402 states the exact asset contract and amount.

How much does a call cost?

Prices are per request and range from $0.001 to $15 across 130 offers, according to /v1/catalog. Some offers price by input size and show a starting price; their 402 quotes the exact amount.

Do I need an account or API key?

The catalog describes none. A request is authorized by a signed payment for that request. Wallets that the operator has explicitly granted access can instead prove ownership with Sign-in-with-X, which is offered on the 402 response.

What happens if I send bad input?

The market validates input before it asks for money. Input that fails an offer's contract is refused with a 4xx status (for example 422 with a diagnostic) and no payment terms are issued.

What if my connection drops after I pay?

Recoverable offers, such as the books, accept an X-Payment-Recovery token that you generate before paying. Repeating the same signed request with the same token returns the exact purchased response again.

How do I know I am paying the right party?

The 402 names the payee, but a payee read from the seller proves nothing by itself. The reference buyer in this market's repository checks it against a payee you supply independently through the X402_EXPECTED_PAY_TO environment variable.

Where can I read about x402 in depth?

The book The x402 Economy covers the history of HTTP 402, a paid request step by step, who governs x402, its rivals, market measurements, security and US law. It is sold on this market.

Endpoints cited on this page

/.well-known/x402 · /v1/catalog · /openapi.json · /llms.txt · /.well-known/agent-card.json · /actors/market · /offers/
Publisher: Causeway Services, a 561 Group company.