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A 402 Payment Required is x402’s payment challenge, not proof that the request is broken. If your API keeps returning 402 after the client sends PAYMENT-SIGNATURE, start with the response’s JSON error field. The x402 FAQ identifies invalid signatures or payloads, an amount below the requirement, insufficient USDC, and KYT screening as possible causes. Then check that the signed payment, server configuration, client, and facilitator agree on the required scheme and network.
What the seller-side x402 flow does
The protocol’s usual flow separates the payment challenge, verification, delivery, and settlement. The first request and challenge can be skipped when the client already knows the accepted terms.
- The client requests a protected resource.
- If payment is required, the server returns
402 Payment Requiredwith one or more payment requirements. - The client selects a requirement it supports and builds a payment payload for its scheme and network.
- The client signs the payload and retries the request with
PAYMENT-SIGNATURE. - The server validates the payment locally or asks a facilitator to verify it.
- After valid verification, the server fulfills the resource; the server or facilitator settles the payment on-chain, and the successful response includes the resource and a settlement response.
This sequence matters when debugging: the signature header is only one part of the exchange. A request can carry that header and still fail verification if its payload does not satisfy the challenge.
Configuration sellers need to align
The seller setup defines what the API accepts and where payment goes. The official seller quickstart’s Express example sets a scheme, a price such as $0.001, a network identifier, and a recipient wallet, then applies middleware to a protected route. It also describes test facilitator setup and directs production deployments to a CDP facilitator endpoint.
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- Protected route: Apply payment middleware to the endpoint that should require payment; confirm you are testing that route.
- Price: Set the amount required by the route and compare it with the amount in the challenge and signed payload.
- Payout address: Use an address valid for the chosen network and verify that it is the intended recipient.
- Scheme and network: Select a pair supported by the seller integration, client, and facilitator.
- Facilitator: Use the test setup for testnet work and the intended production endpoint for mainnet.
Network identifiers are not interchangeable. The seller guide gives eip155:84532 for Base Sepolia and eip155:8453 for Base mainnet. Its Solana mainnet example uses solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp and a Solana-format payout address. Use the identifier and address format for the actual deployment rather than copying a testnet value into mainnet configuration.
Why a request can keep returning 402
When a client reports that it keeps getting 402 despite attaching PAYMENT-SIGNATURE, the x402 FAQ’s guidance is direct: “Check the error field in the server’s JSON response for details.” Use that error as the starting point, then investigate the matching condition.
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- Invalid signature or payload: The signature may be invalid, or the signed payload may contain the wrong chain ID or other fields. Compare the payload with the requirements returned in the challenge; do not assume that a correctly named header proves the payment is valid.
- Amount below the requirement: The signed payment must meet the route’s required amount. Check for a mismatch between the server’s configured price, the challenge, and what the client signed.
- Insufficient USDC: The FAQ names insufficient USDC as a possible reason payment is not accepted. Confirm the payer has the required asset and enough of it for the requested payment.
- KYT screening: A KYT flag can also prevent acceptance. A client-side signature change will not resolve a screening decision; inspect the server’s error and the facilitator’s applicable response.
A practical debugging order
The following sequence combines the documented failure causes with the seller configuration requirements. It is a useful diagnostic workflow, not a formally published vendor checklist.
- Read the JSON error. Capture the server response for the failed retry and inspect its
errorfield before changing configuration. - Compare the signed payload with the challenge. Check the required amount, chain or network, and payload fields against what the client actually signed.
- Check support for the scheme-network pair. The protocol requires clients and facilitators to explicitly support a
(scheme, network)pair. Confirm the pair advertised by the seller is supported by both the client and the facilitator. - Check the payer’s asset and balance. Verify the payer has enough USDC for the required amount, and account for any KYT screening result indicated by the error.
- Verify the deployment environment. Confirm the seller is using the intended testnet or mainnet network identifier, facilitator endpoint, and payout address. Testnet and mainnet require different configuration.
- Retest the protected route. Make a fresh request and follow the challenge-response exchange so you are comparing the latest server requirement with the new signed payload.
Test carefully before accepting real payments
The seller guide recommends exercising the integration on testnet before production, checking that funds reach the configured wallet, monitoring the facilitator, and beginning with small mainnet payments. Its warning is explicit: “Mainnet transactions involve real money.” Treat a successful verification response and a received payout as separate operational checks.
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Exact payments, refunds, and facilitator trust
The x402 FAQ says buyers sign locally; sellers do not hold buyer keys. It describes the current exact scheme as an irreversible push payment. A refund is therefore a separate transfer from seller to buyer, not a built-in reversal or chargeback feature of exact settlement. Define the refund decision, recipient verification, support workflow, and accounting process as seller policy before relying on the integration for customer payments.
Facilitators are also a meaningful trust and operational dependency because they verify and settle payments across services. In a preprint dated July 21, 2026, Qinying Wang, Yong Yang, Yuan Chen, Shouling Ji, and Mathias Payer report evaluating 15 facilitators and finding rule violations in all facilitators in their evaluated set. The authors say they disclosed the findings and that affected parties adopted mitigations, including changes by Coinbase. Their study highlights concentration of trust in shared facilitator infrastructure; it does not establish that any particular current deployment remains exposed after those mitigations.
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