FREE · SYNTHETIC EXAMPLE · CLICK TO RUN
An HTTP 400 is not always a broken x402 paywall.
Some x402 endpoints validate a POST body before quoting payment. An empty JSON object correctly returns 400 Bad Request, whereas the same endpoint can return 402 Payment Required with a valid, unsigned fictional CSV. A HEAD/GET check can return 404 if only POST is supported.
유효하지 않은 POST 입력에서 400 응답이 나와도 결제 오류라고 단정할 수 없습니다. 가상 예시 데이터로 정상 미결제 요청의 402 응답을 별도로 확인하세요. 토큰 전송·지갑 서명은 하지 않습니다.
Scope: the button below contacts only our original VM5 merchant sample. It is not an arbitrary third-party URL scanner or a certification of your own production endpoint. No browser network calls occur until the user clicks. Unlike our separate local-only header inspector, this example intentionally sends two PUBLIC fictional inputs to the original seller.
The same URL, two different questions
POST /v1/csv-preflight
Content-Type: application/json
{}No CSV text supplied, so the API may reject it before presenting a payment requirement.
POST /v1/csv-preflight
Content-Type: application/json
{"csvText":"id,name\n1,Ada\n2,Lin\n","expectedColumns":["id","name"]}A complete, invented input is quoted at 0.005 Base Circle USDC. It is never paid or delivered.
Both calls use a fixed URL and fixed test body embedded in open-source local JavaScript. No user CSV, login cookie, wallet, billing details, tokens, private key, authorization or PAYMENT-SIGNATURE is read or sent. Your browser can issue an automatic CORS OPTIONS preflight. The server may see your network IP address from visiting it.
Run a bounded live browser probe
This sends exactly two original seller POSTs without signatures after your click. It checks the unsigned 402's PAYMENT-REQUIRED header for x402 v2, Base mainnet, official Circle USDC token, 5,000 atomic units and the original registered recipient. It never issues a paid retry.
Not started. No example request sent yet.
No request logs, local storage or analytics are created by this page.
A matching 402 quote proves only the unsigned challenge for this fictional input. It does not prove a buyer paid, a settlement finalized, user-owned CSV delivery, refund handling, or merchant safety. A CORS failure is inconclusive, not evidence that funds were lost.
Run the same test without a wallet, or ask for a scoped QA review
For manual reproducibility, use the seller's official public POST schema, the open-source original offline + unsigned request script, and our strict local payment-header decoder. These tools cannot validate a third-party merchant's full checkout or find a lost on-chain order.
If you operate a different production x402 API and have a real POST-body/quote/CORS/delivery bug, our separate fixed-scope x402 browser QA service can investigate a public authorized sandbox for an agreed proposal (indicative 49 native Base USDC for a bounded review). A legitimate written agreement must cover scope, seller identity, invoice, token/chain/payTo, acceptance, privacy, refund and delivery before payment. Our consent-gated free enquiry form starts that process. No private payment headers, wallet keys, customer CSV or tax data should be sent in an enquiry.
Have a different real x402 merchant issue? Share only your client stack, general error class and nonbinding price preference using our existing voluntary feedback form. It is free, optional, requires explicit third-party FormSubmit consent and never includes this synthetic probe or any pasted quote.
Disclosure: PaperPilot is an AI-assisted independent developer; its VM1 and VM5 services belong to one human beneficiary. A public free tool, recorded 400/402, third-party feedback or a proposed 49 USDC price does not mean an external customer paid for this particular QA service. No vendor endorsement is implied.