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If an x402 client keeps receiving 402 Payment Required after sending PAYMENT-SIGNATURE, the header alone does not prove the payment matches the server’s challenge. Start with the server’s JSON error field, then compare the signed payload, required amount, scheme and network, payer balance, and facilitator configuration.
What a 402 means in an x402 payment flow
A 402 is normally the payment challenge, not by itself proof that the integration is broken. In the typical flow documented by the x402 protocol repository, a client requests a protected resource and the server responds with status 402 and payment requirements. The client selects a requirement the server accepts, builds and signs a payload for its scheme and network, then retries with the payment payload in PAYMENT-SIGNATURE. The server validates the payment locally or asks a facilitator to verify it. After valid verification, the server fulfills the request; the server or facilitator settles on-chain, and the successful response includes the resource and settlement response.
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The initial request and challenge can be skipped when the client already knows the accepted terms. Either way, the seller’s requirements and the client’s signed payment must agree.
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The x402 FAQ’s troubleshooting guidance is to check the error field in the server’s JSON response. It identifies several possible causes; the response error is a better starting point than assuming every repeated 402 has the same fix.
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- Invalid signature or mismatched payload: The signature may be invalid, or the signed payload may contain the wrong chain ID or other required fields.
- Payment below the requirement: The submitted amount may not meet the amount specified by the challenge.
- Insufficient USDC: The payer may not have enough of the required asset to cover the payment.
- KYT screening: A Know Your Transaction screening flag may prevent acceptance even when a signature is present.
These are documented possibilities, not evidence of one particular seller’s incident. A repeated 402 should be diagnosed from the actual response and configured requirement rather than attributed to an invented single bug.
Check the scheme and network across all three participants
x402 requires explicit support for a (scheme, network) pair. Check the pair configured by the seller against the capabilities of both the client and the facilitator. If one participant does not support that pair, the signed payload and server configuration can be individually plausible yet still fail to work together.
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In the seller guide’s Express quickstart, the seller configures an exact scheme, a price (the example uses $0.001), a network identifier, and a recipient wallet, then applies payment middleware to a route. The price is an example configuration value, not a universal x402 fee.
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1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minute| Example deployment | Network identifier | Configuration detail |
|---|---|---|
| Base mainnet | eip155:8453 |
Use a Base-format payout address and a production-compatible facilitator. |
| Base Sepolia | eip155:84532 |
The quickstart uses this test network in its example setup. |
| Solana mainnet | solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp |
Use a Solana-format payout address. |
These are the identifiers and setup examples in the seller guide; confirm current package, network, and facilitator support in the documentation for the versions you deploy.
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A practical debugging order
The following sequence combines the causes and configuration requirements documented by the x402 FAQ, protocol repository, and seller guide. It is an operational workflow, not a formally published vendor checklist.
- Read the server response. Inspect the JSON
errorfield for the specific rejection detail. - Compare the signed payload with the challenge. Check the required amount, chain or network, and payload fields against what the client actually signed.
- Verify pair support. Confirm that the selected scheme and network are supported by the client and by the facilitator as well as configured by the seller.
- Check the payer’s funds and eligibility. Confirm there is enough of the required asset for the challenged amount, and account for any KYT screening result.
- Check environment alignment. Make sure the server’s network identifier, facilitator endpoint, recipient address, and client setup all target the intended testnet or mainnet.
Keep test and production settings separate
The seller guide documents test facilitator setup for testing and directs production deployments to a CDP facilitator endpoint. Keep testnet and mainnet values distinct: changing networks can require a different network identifier, facilitator configuration, and payout address format. Do not infer that a working test setup is ready for mainnet simply because the route returns a successful test response.
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The guide recommends testing on testnet before production, checking that funds reach the configured wallet, monitoring the facilitator, and starting with small mainnet payments. It warns that “Mainnet transactions involve real money.”
Plan for settlement, refunds, and support
The FAQ says buyers sign locally, so 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 the seller to the buyer, not a reversal or chargeback built into exact settlement. Decide how customer support will verify a complaint, authorize a new transfer, and communicate refund timing before accepting production payments.
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Facilitator trust is part of the seller’s risk model
A facilitator verifies and settles payments across services, so its operation is a material part of the integration’s trust and availability picture. A preprint dated July 21, 2026, by Qinying Wang, Yong Yang, Yuan Chen, Shouling Ji, and Mathias Payer reports that its authors evaluated 15 facilitators and found rule violations in all of those evaluated. The authors say they disclosed the findings and that affected parties adopted mitigations, including changes by Coinbase. The study also measured more than 119 million Base and Solana transactions; that figure describes the study’s measurement, not a general adoption count.
The authors discuss shared facilitator infrastructure as a concentration of trust. Their report and stated mitigations do not establish that any particular current deployment remains exposed. For a seller, the practical implication is to monitor the facilitator, understand which service verifies and settles payments, and follow current security guidance for the endpoint in use.
SDK and configuration details to verify before deployment
The seller guide says its Python SDK still uses v1 patterns, while its TypeScript and Go examples use current v2 patterns. Because SDK generations and supported networks can change, verify that your code follows the documentation for the installed package version rather than copying configuration from a different language or generation. Review protected routes, price, payout address, scheme, network, and facilitator together; a mismatch at any of these boundaries can make payment troubleshooting harder.
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