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To fix an x402 payment failure, first identify whether your client, resource server and facilitator use x402 v1 or v2. Then inspect the raw payment challenge, validate it against that version’s schema, and compare the offered terms with the fields the client signs and the server verifies. The exact error code helps distinguish a malformed payload from a mismatch in amount, recipient, network, scheme or version.
Start with the deployed x402 version and flow
Do not rename fields until you know which protocol version each component expects. The x402 project publishes separate v1 and v2 specifications. V2 uses amount and a resource-level PaymentRequired structure; v1 includes legacy naming such as maxAmountRequired. A field that is correct for one version may be wrong for the other.
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Record the client SDK and version, server middleware and version, facilitator, transport, scheme and network involved. Check each component’s documented support rather than assuming they all accept the same version or fields. For example, Cloudflare’s current gateway documentation describes its use of x402 v2: Cloudflare x402 documentation.
The typical documented flow is: a client requests a resource, the server responds with HTTP 402 and a payment challenge, the client selects an accepted requirement and creates a scheme- and network-specific payment payload, then retries with a payment signature. The server verifies the payment locally or through a facilitator. Implementations can differ, so use this as a diagnostic outline, not a guarantee that every integration has identical steps. See the x402 flow documentation.
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Capture what actually crossed the HTTP boundary
Inspect the response and retry request as received, not just the objects your application intended to send. In the v2 HTTP transport, protocol data is carried in PAYMENT-REQUIRED, PAYMENT-SIGNATURE and PAYMENT-RESPONSE headers. Save the status and headers, decode the v2 PAYMENT-REQUIRED value, and retain its JSON keys and values without normalizing or renaming them. For v1, inspect the legacy challenge shape and transport conventions instead. The v2 HTTP transport specification documents the header conventions.
Keep a copy of the challenge, the selected requirement, the signed payload, the server’s verification input and the facilitator response. Comparing these artifacts makes it possible to see where a value first diverges.
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Validate the challenge against its versioned schema
For a v2 challenge
The v2 PaymentRequired structure includes x402Version, resource and accepts. Parse the JSON, confirm the version, and check that accepts is present and has the expected shape. Validate the selected requirement against the v2 schema, including field names and types.
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Check the requirement’s scheme, CAIP-2 network, amount, asset and payTo, as well as maxTimeoutSeconds and relevant extra values. V2 specifies amount as a string in atomic token units—not a human-readable decimal amount. Use the actual versioned schema in the v2 specification to verify required fields and types.
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For a v1 challenge
Validate against the v1 schema and preserve its legacy names, including maxAmountRequired where applicable. Do not mechanically convert a v1 challenge to v2 field names, or vice versa; check what the deployed client and server actually support in the v1 specification.
Compare the payment terms from challenge to verification
After confirming the schema, trace the selected requirement through signing and verification. The client should select a requirement the server offered; the signed authorization and the values passed to server-side or facilitator verification should correspond to that same requirement. Compare:
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- Scheme: The selected scheme’s spelling and value should match the offered requirement and the verifier’s supported scheme.
- Network: Check the exact network identifier, including its CAIP-2 form where required, and confirm the server and facilitator support it.
- Asset and amount: Confirm the asset is the expected one and that the amount uses atomic units. Check token decimals or configuration when translating a displayed amount into atomic units.
- Recipient: Compare the requirement’s
payTowith the recipient encoded in the signed authorization. - Timeout and metadata: Check
maxTimeoutSecondsand any scheme-specificextravalues against what the payload and verifier expect.
This cross-flow comparison follows the documented client selection and verification flow; it is a practical debugging method, not a claim that every implementation uses identical internal interfaces.
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Use the returned error code to choose the next check
The x402 v2 specification says standard error codes may be returned by facilitators or resource servers. Capture the exact code and use it to narrow the likely layer rather than treating every response as a generic payment failure. The checks below follow the distinctions in the v2 specification.
| Error | First checks |
|---|---|
invalid_payload |
Check JSON parsing, payload shape, protocol version and scheme-specific fields. |
invalid_payment_requirements |
Validate the challenge or requirements object: required fields, types and server-generated values. |
| Amount mismatch | Compare the signed amount with the offered amount; verify atomic units and token-decimal configuration. |
| Recipient mismatch | Compare payTo in the offered requirement with the recipient encoded in the authorization. |
invalid_network |
Check the network identifier and whether the server and facilitator support that network. |
invalid_scheme or unsupported_scheme |
Check the scheme value and confirm the facilitator supports that scheme. |
invalid_x402_version |
Check version compatibility across client, server and facilitator; verify that you are not mixing v1 field names and v2 expectations. |
settlement_pending |
Use the returned transaction hash and network to reconcile transaction status before deciding whether to retry. |
Handle pending settlement before retrying
settlement_pending is non-terminal in the v2 specification. The response carries a transaction hash and network so the caller can reconcile on-chain. Preserve both values and check the transaction’s status on the stated network before sending another payment attempt. An uncertain confirmation is not, by itself, evidence that the payment failed.
Quick Recap
Retest the corrected path
- Fix the specific malformed field or configuration identified by schema validation and the error code.
- Repeat the request through the same client, server, facilitator, version, scheme and network path that failed.
- Log the new challenge, selected requirement, verification result and settlement status so you can confirm that the corrected values remain aligned across the flow.
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