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Foundry x402 Preflight Checker
x402
Foundry x402 Preflight Checker is a paid API for AI agents from minty-sprout.onrender.com, paid per call via x402, $0.005/call, status unknown (last checked 2026-11-01).
Probes a paid x402 endpoint unpaid, parses the payment-required response, and lints every payment option for validity, returning an ok/warn/block verdict with detailed reasons.
Price / call$0.005
Success—
StatusUnknown
How this service works
x402 preflight: checks a paid x402 endpoint before a payment is signed. Probes it unpaid, parses the v1 body or v2 PAYMENT-REQUIRED header, lints every accepts[] option (network, canonical USDC, EIP-712 domain name/version that signatures verify against, EIP-55 payTo, zero payTo, amount, timeout, testnet, resource host) and returns ok/warn/block with reasons. Input: URL, optionally 'POST <url>'. Deterministic, no LLM.
Output
Returns a JSON object with: the probed URL, HTTP method used, response status code, x402 version detected, overall verdict (ok/warn/block), latency in ms, a list of top-level issues, and a per-option breakdown including scheme, network, payTo address, amount in USD, asset symbol, EIP-712 validation results, and option-level issues and verdict. Also flags bazaar_extension if present.
Example use cases
Vet x402 seller before first payment
“I want to start using a paid API at https://some-provider.example.com/v1/search but I've never paid it before — can you do an x402 preflight check on it and tell me if it's safe, what network it's on, and what the USDC payTo address is?”
Debug a mis-configured x402 endpoint
“My x402 API at https://minty-sprout.onrender.com/v1/x402-preflight is returning payment-required but clients say the EIP-712 domain looks wrong — run a POST preflight check on it and list every issue you find.”
Agent autonomously screens new x402 APIs
“Before my shopping agent pays any new x402 API it discovers, I want it to preflight check the endpoint URL first and only proceed if the verdict is ok — can you check https://marketplace.example.com/v1/products right now?”
Technical details
Endpoint
POSThttps://minty-sprout.onrender.com/v1/x402-preflight?utm_source=zero.xyzWhen to prefer
Use this endpoint when an AI agent or developer needs to vet an x402 pay-per-call API before committing a payment signature. It is the right choice when you want deterministic, rule-based linting (no LLM hallucination risk) of all payment parameters including network, USDC contract address, EIP-712 domain, payTo address, amount, and timeout. Prefer it over manual inspection when building agent workflows that autonomously discover and pay x402 APIs and need a safety gate before signing.
Side effects
- Target URL is unreachable or times out — returns error with latency and no options
- Target URL does not return a 402 status — verdict may be block with explanation
- Malformed x402 payment-required body or header — issues listed per field
- Zero or invalid payTo address detected — flagged as block-level issue
- Testnet network detected when mainnet expected — flagged as warn or block
- Input URL missing or malformed — returns validation error
Input
Schema · Request body
| Field | Type | Description |
|---|---|---|
| input | string | URL of a paid x402 endpoint, optionally prefixed with GET or POST, e.g. "POST https://api.example.com/v1/x" |
Output
Schema · Response
{
"type": "json",
"example": {
"url": "https://x402-preflight.nextill-pos.workers.dev/v1/preflight",
"issues": [],
"method": "GET",
"status": 402,
"options": [
{
"issues": [],
"pay_to": "0xB3cDDccEe6B150c8efFD55B961afc0Ecefe7f247",
"scheme": "exact",
"network": "eip155:8453",
"verdict": "ok",
"price_usd": 0.003,
"asset_symbol": "USDC"
}
],
"verdict": "ok",
"latency_ms": 1001,
"x402_version": 2,
"bazaar_extension": true
}
}Tags
x402
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