x402 Seller Kit — sell your API to AI agents per call

Plan (7 steps, ~2 hours, ~$7/month hosting)

1. Catalog (src/x402/catalog.ts)

export const ENDPOINTS = [
  {
    key: "health-ping", method: "GET", path: "/v1/ping", kind: "query", price: "$0.005",
    summary: "Cheap liveness + capability probe for agents",
    inputSchema: { type: "object", properties: {} },
    outputExample: { ok: true, endpoints: 2 },
  },
  {
    key: "address-normalize", method: "POST", path: "/v1/address/normalize", kind: "body", price: "$0.02",
    summary: "Normalize a Japanese address into pref/city/town/block + WGS84 point",
    input: { address: "東京都千代田区丸の内１－１－１" },
    inputSchema: { type: "object", required: ["address"], properties: { address: { type: "string" } } },
    outputExample: { pref: "東京都", city: "千代田区", town: "丸の内一丁目", addr: "1-1", level: 8, point: { lat: 35.6812, lng: 139.7671 } },
  },
] as const;

Everything else (402 headers, /openapi.json with x-payment-info, /.well-known/x402, /llms.txt, Bazaar discovery extension) is generated from this one file.

2. Local testnet check

$ npm run dev
x402 api listening on :4021 (network=base-sepolia, facilitator=https://x402.org/facilitator)

$ curl -i -X POST localhost:4021/v1/address/normalize -H 'Content-Type: application/json' -d '{"address":"東京都千代田区丸の内1-1-1"}'
HTTP/1.1 402 Payment Required
PAYMENT-REQUIRED: eyJ4NDAyVmVyc2lvbiI6MiwiYWNjZXB0cyI6W3sic2NoZW1lIjoiZXhhY3QiLCJuZXR3b3JrIjoiZWlwMTU1Ojg0NTMyIiwiYW1vdW50IjoiMjAwMDAiLCJhc3NldCI6IjB4MDM2Q2JENTNhM...

$ npm run doctor
✔ GET /openapi.json            200, 2 paid routes, contact email present
✔ GET /.well-known/x402        200, 2 resources
✔ GET /.well-known/ai.txt      200
✔ GET /llms.txt                200
✔ GET /favicon.ico             200
✔ GET /health                  200
✔ POST /v1/address/normalize   402 + PAYMENT-REQUIRED (exact, USDC, 20000 atomic)
✔ GET /v1/ping                 402 + PAYMENT-REQUIRED
✔ free tier gated by X-Free-Tier header (crawlers still see 402)
9/9 passed

3. What I need from you before mainnet

1. A

fresh receive-only wallet address (MetaMask → new account). Paste only the 0x… address into PAY_TO_ADDRESS. The private key never goes on the server.
2. A

Coinbase CDP Secret API key (portal.cdp.coinbase.com → API keys → tick "Opt-out of IP allowlisting"). Put CDP_API_KEY_ID / CDP_API_KEY_SECRET in Render's environment tab, not in git.
3. Set X402_NETWORK=base and PUBLIC_BASE_URL=https://<your-service>.onrender.com.

4. After deploy — discovery checklist (this is where revenue comes from)

• npm run doctor -- https://<your-service>.onrender.com → 9/9
• x402scan → Resources → Register → paste origin → fix any warnings
• Circle Agent Marketplace interest form (endpoints, prices, docs URL)
• Make the GitHub repo public (x402Relay auto-indexes /.well-known/ai.txt)
• Optional: MCP wrapper → npm (mcpName) → mcp-publisher publish

Expectations, honestly

Median x402 endpoint price is $0.02; ~79% of listed routes are dead, so staying alive puts you in the top 20%. The first weeks of traffic will be registry probes (hourly 402s, no payments). A real customer shows on /stats as a 402 → paid 200 pair plus a USDC transfer on BaseScan.