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AIMarket x402 Payment Authorization Checker
x402
AIMarket x402 Payment Authorization Checker is a paid API for AI agents from modelmarket.dev, paid per call via x402, $0.003/call, status unknown (last checked 2026-10-12).
Debugs and validates an x402 EIP-3009 payment authorization offline before submission, identifying signature mismatches, domain errors, and invalid fields.
Price / call$0.003
Success—
StatusUnknown
How this service works
Debug an x402 payment before you submit it — why does USDC answer invalid signature? Verifies the EIP-3009 transferWithAuthorization (EIP-712 signer, v, low s, validity window) and the seller's terms (payTo, amount, asset, network, bound nonce), and names the domain a failing signature was really made for, e.g. Base Sepolia's 'USDC' used for Base mainnet's 'USD Coin', the commonest reason an x402 payment reverts. Offline, deterministic, signed answer.
Output
A JSON object with a status field ('ok' or error), a result object detailing each validation check (EIP-712 signer recovery, v and low-s correctness, validity window, payTo/amount/asset/network match, nonce binding), a named diagnosis of any failing domain (e.g. 'Base Sepolia USDC vs Base mainnet USD Coin'), and a receipt — all computed offline without submitting anything on-chain.
Example use cases
Debugging Base vs Sepolia domain mismatch
“I built a payment flow for Base mainnet but my USDC x402 authorization keeps failing — I think the signature was made for Base Sepolia's 'USDC' contract instead of mainnet's 'USD Coin'. Can you check my X-PAYMENT header and tell me exactly which domain it was signed for?”
Validating x402 payment before agent submission
“Before my agent submits this x402 payment, can you verify the EIP-3009 authorization — check the signature, nonce, and validity window against the seller's requirements (payTo, amount, asset, network) so we don't waste a failed on-chain transaction?”
Diagnosing low-s signature or expired window
“My x402 payment is being rejected and I don't know if it's a low-s violation, an expired validBefore, or a nonce that's already been used — here's the authorization object and signature, can you tell me exactly which check is failing?”
Technical details
Endpoint
POSThttps://modelmarket.dev/x402/x402-check?utm_source=zero.xyzWhen to prefer
Use this endpoint when you need to pre-flight an x402 EIP-3009 payment authorization before submitting it on-chain, especially to diagnose 'invalid signature' reverts caused by signing for the wrong EIP-712 domain (e.g. Base Sepolia USDC vs Base mainnet USD Coin). It is deterministic and offline, so it adds zero latency to your payment flow and costs only $0.003 USDC. Prefer it over trial-and-error on-chain submission whenever a payment is reverting for an unclear reason.
Side effects
- Missing required fields (authorization or signature) returns a validation error
- Unrecognized network identifier returns an unknown network error
- Malformed base64 X-PAYMENT header returns a parse error
- Signature shorter than 65 bytes returns an invalid signature format error
- now timestamp outside validity window is flagged as expired rather than a service error
Input
Schema · Request body
| Field | Type | Description |
|---|---|---|
| now | integer | unix seconds, to check the validity window |
| network | string | base, base-sepolia, … or eip155:<chain id> |
| signature | string | 65-byte hex signature |
| x_payment | string | the X-PAYMENT header (base64) — or authorization + signature + network |
| requirements | object | the 402's accepts entry (payTo, amount, asset, network) |
| authorization | object | from, to, value, validAfter, validBefore, nonce |
Output
Schema · Response
{
"type": "json",
"example": {
"result": {},
"status": "ok",
"receipt": {},
"service": "x402-check",
"capability_id": "x402.authorization.check@v1"
}
}Tags
x402
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