fastify.md

@arcp/fastify

Fastify integration: attach the ARCP WebSocket upgrade to the underlying app.server.

Install

pnpm add @arcp/fastify @arcp/runtime

Use

import Fastify from "fastify";
import { ARCPServer } from "@arcp/runtime";
import { attachArcpToFastify } from "@arcp/fastify";

const app = Fastify({ logger: true });
const arcp = new ARCPServer({
  /* … */
});

app.get("/healthz", async () => "ok");

await app.listen({ host: "0.0.0.0", port: 3000 });

attachArcpToFastify(app, {
  path: "/arcp",
  allowedHosts: ["arcp.example.com"],
  onTransport: (t) => arcp.accept(t),
});

Note the call order: attachArcpToFastify reads app.server, which is only populated after app.listen().

API

attachArcpToFastify(app, options)

function attachArcpToFastify(
  app: FastifyInstance,
  options: AttachArcpUpgradeOptions,
): ArcpUpgradeHandle;

Delegates to attachArcpUpgrade(app.server, options). Options match @arcp/node:

Field	Notes
path?: string	Defaults to "/arcp".
allowedHosts?: readonly string[]	Validate Host on the upgrade.
onTransport: (transport, req) => void	Pair with server.accept.

Returns an ArcpUpgradeHandle.

Lifecycle

Detach on shutdown to avoid leaking upgrade listeners during HMR or hot-reload:

const handle = attachArcpToFastify(app, {
  /* … */
});
app.addHook("onClose", async () => {
  await handle.close();
});

Source

packages/middleware/fastify/src/.

Runnable example

examples/fastify/.