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A valid signature alone does not prove that an x402 payment matches the price and terms the client intended to accept. Before signing, the client should validate the payment requirements; the scheme must bind the payment to those requirements; and the service must handle duplicate fulfillment separately from payment replay.
What happens in an x402 payment exchange?
HTTP 402 by itself does not define a payment protocol. RFC 7231, published in June 2014, says: “The 402 (Payment Required) status code is reserved for future use.” x402 uses that status as part of a later, protocol-specific request and response exchange; HTTP does not mandate the x402 handshake.
- Request: The client asks the resource server for a resource.
- Payment requirements: The server responds with HTTP 402 and acceptable payment requirements.
- Policy check: The client checks that the proposed terms are acceptable before creating a payment payload.
- Payment retry: The client selects a requirement, creates a scheme- and network-specific payload, and retries with it.
- Verification and fulfillment: The payment is verified and handled according to the service’s settlement and resource-delivery design.
In x402 v2’s HTTP transport specification, the response header PAYMENT-REQUIRED carries a base64-encoded PaymentRequired object. On retry, PAYMENT-SIGNATURE carries a base64-encoded PaymentPayload. These header names are version-specific; older deployments may use different names. Implementations may also allow a client that already knows the payment details to skip the discovery request.
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What does the quote contain, and what does the client sign?
The v2 format distinguishes the resource description and acceptable payment requirements from the payment the client selects and the scheme-specific payload it creates. A challenge therefore describes acceptable terms; it is not, by itself, proof that the client has paid or that the terms were signed by the server.
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Some EVM authorization examples include fields such as payer, recipient, amount, validity window, and nonce. Those are schema examples, not universal x402 fields or guarantees about every network’s cryptography. A client should determine what its chosen scheme actually commits to, then check that the proposed asset, network, recipient, amount, and relevant resource context match its own policy before authorizing payment.
Why must the payment be bound to the requirements?
A signature proves that particular data or an authorization was signed; it does not prove that the authorization corresponds to the quote the client meant to accept. The x402 Foundation’s exact scheme specifies: “A method MUST bind the payment to the requirements by one of: an instrument unique to this request; a server-issued nonce carried in the payment; a payer signature over the requirements; or a payee commitment to the requirements embedded in the instrument.”
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That is a requirement of the exact scheme, not a claim that every x402 deployment signs the HTTP 402 response. A signed offer extension is a separate feature. When reviewing a scheme, establish precisely which requirements its payment instrument or signature commits to and how the service associates that payment with the intended request.
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How are expiry, replay protection, and duplicate delivery different?
Expiry and replay protection
A validity window limits when an authorization can be used. A nonce or another scheme-defined replay primitive can prevent a consumed payment from being accepted again. The exact scheme requires a consumed replay primitive to fail, but the specific fields and mechanics vary by scheme and network. A signature should not be treated as a substitute for checking expiry or consuming the replay primitive correctly.
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Duplicate fulfillment
Preventing reuse of a payment does not automatically make resource delivery idempotent. A resubmission can be indistinguishable from an original attempt, and a network can report success to every caller. If the service verifies or settles once but delivers the resource multiple times, payment replay protection has not prevented duplicate fulfillment. The exact scheme warns that settlement handling may need to deduplicate atomically across processes.
What should a client do when a retry times out?
An uncertain result is not proof that the original payment failed. x402labs’ safe-retry guidance advises preserving the existing payment identifier for a recoverable retry and not creating a fresh authorization until the original attempt is known to be unrecoverable. Apply that as implementation-specific guidance: recovery depends on whether the service or facilitator supports lookup or reuse of that identifier.
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- Treat payment requirements as untrusted input; validate them against the client’s pricing and authorization policy.
- Enforce a maximum acceptable amount, and allow only expected assets, networks, and recipients.
- If a response is lost after submission, use the existing payment identifier for recovery where supported instead of immediately authorizing a second payment.
- Check the service’s response and settlement status before deciding whether a new payment attempt is necessary.
When should a service settle relative to resource execution?
The exact scheme distinguishes authorization flow—verify, execute the resource, then settle—from upfront settlement. These choices affect failure handling: with upfront settlement, a resource handler failure can leave payment committed without delivery, and the specification does not define a general refund. A service should make its execution and settlement ordering explicit and design recovery around the failure points it creates.
What does a gateway change?
Cloudflare’s documented gateway arrangement is one deployment example, not the general x402 wire protocol. Its gateway verifies client authorization and passes a signed PAYMENT-CONTEXT token to the origin; the origin must validate that token before serving the paid resource. For variable-price origins, the documentation describes returning the actual charge in PAYMENT-SETTLEMENT. It advises clients to inspect the status and x402 response before retrying if verification or settlement fails.
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Implementation review checklist
- Quote policy: Does the client verify the amount, asset, network, recipient, and resource context before signing?
- Binding: Does the scheme bind the payment to the relevant requirements using a request-specific instrument, nonce, payer signature, or payee commitment?
- Validity: Is the authorization time-limited, and are its expiry conditions enforced?
- Replay: Which nonce or other replay primitive is consumed, and does reuse fail?
- Recovery: Can the client preserve and reuse a payment identifier to resolve an uncertain attempt?
- Idempotency: Are verification, settlement, and fulfillment deduplicated atomically across concurrent requests?
- Failure ordering: If resource execution fails before or after settlement, what recovery path exists?
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