Bun runtime for Vercel Functions now accepts Bun.serve as an entrypoint

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.

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Bun.serve({
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  routes: {
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    "/api/boolean": () => Response.json({ success: true }),
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    "/api/users/:id": (request) => Response.json({ user: request.params.id }),
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    "/*": () => new Response("api catch-all"),
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  },
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});

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.

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Bun.serve({
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  routes: {
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    "/health": Response.json({ status: "ok" }),
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  },
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  fetch(request, server) {
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    const { pathname } = new URL(request.url);
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    if (pathname === "/ws" && server.upgrade(request)) {
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      return;
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    }
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    return new Response("Not found", { status: 404 });
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  },
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  websocket: {
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    message(socket, message) {
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      socket.send(message);
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    },
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  },
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});

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.