x402 Doctor
Check x402 and MPP paid APIs before your agent pays: free endpoint check, $0.001 go/caution/no_go preflight, and fixes for sellers.
Hosted MCP Server
npx add-mcp 'https://x402-doctor.fizzl.eu/mcp'Installs into Claude Code, Codex, Cursor and more
Documentation
Check any x402 endpoint the way a paying agent would.
Paste a paid endpoint URL: x402 (HTTP or MCP), MPP from Stripe and Tempo, and L402 (Lightning) too. Read-only diagnostics: no wallet, no payment, no risk. Try an example.
Free web check $0.001 preflight $0.01 diagnosis $0.05 fix code $0.25 delivery check
See it in 70 seconds
Before your agent pays
Preflight
Before paying an endpoint you haven't used: go, caution or no_go, the payment option to use, and why. A seller whose payout address is on the OFAC sanctions list is no_go.
GET /api/v1/preflight?url=…&max_usd=0.05
Batch
Weighing several services? One verdict per endpoint and the totals, in one call.
GET /api/v1/preflight/batch?url=…&url=…
Deep
Before an expensive call: the verdict plus the full diagnosis, 30 days of daily scans, the seller's other endpoints and domain moves.
GET /api/v1/preflight/deep?url=…
Delivery check
Not a reading but a purchase: we buy the endpoint once (at most $0.10) with its declared example input and publish whether it was paid and delivered, with the charge measured on chain.
GET /api/v1/delivery-check?url=…
Paid per call in USDC via x402 or MPP on Base, Solana, the XRP Ledger, Algorand or Stellar (RLUSD on the XRP Ledger), with a signed receipt. They never pay the endpoint. Also as MCP tools at /mcp; details in the OpenAPI.
Questions
How do I check an x402 endpoint before my AI agent pays it?
Call x402 Doctor's preflight: GET https://x402-doctor.fizzl.eu/api/v1/preflight?url=<endpoint>&max_usd=0.05. For $0.001 it answers go, caution or no_go, with the reasons and the cheapest option that will settle. It checks the 402 the way a paying agent would: the price against the listing, whether the payout address can receive, the OFAC sanctions list, testnet-only offers and the endpoint's 30-day record. The MCP tool x402_quick_check gives a free pass, warn or fail.
Why can't agents pay my x402 endpoint?
Most often the 402 names another URL than the one called (the resource field), the listed URL doesn't answer 402 on the listed method, only a testnet is offered, or the 402 asks more than the listing says. Paste your endpoint above for a free diagnosis with the fix for each problem, or run npx github:Fizzl13/x402-doctor <url> in CI.
Is there a free x402 endpoint validator?
Yes. The web check on this page is free: paste a URL and it decodes the 402 challenge and runs every check without paying. In CI, use npx github:Fizzl13/x402-doctor <url> or the GitHub Action Fizzl13/x402-doctor-action. Agents can use the free MCP tool x402_quick_check (10 calls an hour).
Which x402 endpoints actually settle and deliver?
The x402 Trust Index at https://x402-doctor.fizzl.eu/trust checks about 50,000 paid x402 and MPP endpoints every day and keeps a 30-day record per endpoint; GET https://x402-doctor.fizzl.eu/api/trust?url=<endpoint> returns it for free. Every week x402 Doctor also buys about 30 random payable endpoints and publishes what came back.
Does x402 Doctor check Solana, the XRP Ledger, Algorand and Stellar?
Yes. It checks offers on Base and other EVM chains, Solana, the XRP Ledger, Algorand, Stellar and Circle's Arc, and MPP challenges. Its own paid calls accept USDC on Base, Solana, Algorand or Stellar, RLUSD on the XRP Ledger, or MPP.
How do I add x402 Doctor to Claude or Cursor?
Add the remote MCP server https://x402-doctor.fizzl.eu/mcp. In Claude Code: claude mcp add --transport http x402-doctor https://x402-doctor.fizzl.eu/mcp. Agent skills: npx skills add Fizzl13/x402-doctor, or in Claude Code /plugin marketplace add Fizzl13/x402-doctor and then /plugin install x402-doctor@fizzl.