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Vercel — AI · · 2 min read

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

Mirrored from Vercel — AI for archival readability. Support the source by reading on the original site.

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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.

Contributors

Eric Dodds

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