x402 Seller MCP server

cryptomonnaie — pay-per-call API over x402

An Express server that sells paid API endpoints over the x402 protocol (USDC payments on Base), built to be consumed by AI agents.

A client (human or agent) calls a paid endpoint → the server replies 402 Payment Required with the payment requirements → the client signs a USDC payment and replays the request with the PAYMENT header → a facilitator verifies and settles the payment on-chain → the server serves the response. No blockchain key management server-side: it only holds the receiving address.

Available endpoints

All /api/* routes are paid (x402 payment required), except /health, /stats, and /.well-known/x402.json, which are free. Every response is clean JSON — never a raw 500, always {error: "..."} with the right HTTP status code on any problem (validation, upstream source down, etc.).

Replace $URL with the server's URL (http://localhost:4021 locally, the Render URL in production) in the examples below.

Crypto prices & gas (dedicated routes, optimized for agent search)

These are thin, single-purpose wrappers around the same sources as /api/defi/price and /api/chain/gas below — kept as separate routes (with narrow, intent-matching descriptions) so an agent searching for e.g. "ETH price USD" or "gas price Base" finds and calls them directly, instead of having to first discover the generic parameterized endpoint.

Crypto / DeFi data (source DefiLlama, free and open)

⚠️ License note: DefiLlama's terms of service restrict their free API

to personal, non-commercial use and prohibit commercial exploitation of

the data without prior written agreement (defillama.com/terms, clauses 7

and 8.10). These endpoints (plus the 6 /api/price/ and /api/gas/

ones above, and the 4 /api/defi/yields/* sub-routes below, all of

which reuse the same DefiLlama sources) are built on it anyway, on the

explicit and informed decision of this service's operator (compliance

risk accepted) — to be revisited if DefiLlama raises the issue, or by

moving to their paid Pro API (pro-api.llama.fi) if needed.

Shortened. The full README is on GitHub.