x402 payments for Fastify

Register the plugin

Both styles are supported; mix them freely per route.

import Fastify from "fastify";
import paygate from "paygate/fastify";

const app = Fastify();
await app.register(paygate, { merchantId: process.env.PAYGATE_MERCHANT_ID });

// Declarative: paywall from route config.
app.get(
  "/api/data",
  { config: { paygate: { priceCents: 10 } } },
  async (req, reply) => reply.send({ data: "premium" }),
);

// Imperative: decide inside the handler.
app.get("/api/premium", async (req, reply) => {
  const { paid } = await req.paygatePay({ priceCents: 25 });
  if (!paid) return; // 402 challenge already sent
  reply.send({ data: "…" });
});

await app.listen({ port: 3000 });

Verification and settlement

The plugin performs the same verify-then-settle cycle as the Express middleware: challenge on missing payment, facilitator verification before the handler, settlement after success, receipt attached to `req.paygate`.

On the buyer side, any AI agent can pay your endpoint with `npx payagent pay <url>` or the payagent SDK — the 402 challenge your route now returns is the whole integration contract.

Scaffold a new seller

npx paygate init --framework fastify --wallet 0xYourAddress --price 10

Pricing guidance

• Integer cents only; 1 cent is the minimum price. Use dynamic `priceCents` functions to price by payload size or model depth rather than inventing tiers.
• Fastify's route-config style keeps prices visible next to the route definition — reviewers see the price where they see the handler.

FAQ

Can I run paygate on only some routes?

Yes. Routes without a `paygate` config entry (and without an imperative paygatePay call) are untouched — the plugin only intervenes where a price is declared.

What happens on facilitator downtime?

Verification failure returns 402 to the buyer rather than serving unpaid content — the middleware fails closed.