The Bun runtime for Vercel Functions now supports Bun.serve() as a function entrypoint, including WebSocket handlers. The server you run locally with Bun deploys as-is, without being wrapped in a framework.
Enable the runtime by setting "bunVersion": "1.x" in vercel.json.
Copy link to headingDeploy a routes-based server
Create a server with a routes map in server.ts at the project root.
Bun.serve({
routes: {
"/api/boolean": () => Response.json({ success: true }),
"/api/users/:id": (request) => Response.json({ user: request.params.id }),
"/*": () => new Response("api catch-all"),
},
});
A Bun.serve server deployed to Vercel with static, dynamic, and wildcard routes
Copy link to headingAccept WebSocket connections
Add a websocket handler and call server.upgrade(request) in fetch to upgrade matching requests. The rest of the server stays the same.
Bun.serve({
routes: {
"/health": Response.json({ status: "ok" }),
},
fetch(request, server) {
const { pathname } = new URL(request.url);
if (pathname === "/ws" && server.upgrade(request)) {
return;
}
return new Response("Not found", { status: 404 });
},
websocket: {
message(socket, message) {
socket.send(message);
},
},
});
A websocket handler echoes messages back, and fetch upgrades requests to /ws and falls back to a 404
WebSocket connections run on Fluid compute with Active CPU pricing, so you pay only for time spent processing messages, not idle connection time. A connection is pinned to one function instance for its lifetime, and a single instance can handle multiple concurrent connections. Use an external data store to coordinate messages across instances.
Read the documentation to get started.