Deploy a Bun application on Vercel

Vercel is a cloud platform for building, deploying, and scaling apps. Vercel Functions can run on the Bun runtime, either behind a framework that Vercel supports or as a Bun.serve() server.

Automatic source maps, bytecode caching, and request metrics for node:http and node:https are not supported on the Bun runtime (request metrics for fetch are). See feature support in the Vercel documentation.

Configure Bun in vercel.json

To run your Functions on Bun, add a bunVersion field to your vercel.json file:

vercel.json

{
	"bunVersion": "1.4.x" 
}

Vercel manages the patch version.

For best results, match your local Bun version with the version Vercel uses.

Add a server

Choose how requests reach your code.

Vercel's Bun framework preset sends every request for the deployment to a single Bun.serve() server. Vercel uses the preset when the project sets bunVersion, has a bun.lock file, and has a server entrypoint at one of these paths:

server.{js,cjs,mjs,ts,cts,mts}
src/server.{js,cjs,mjs,ts,cts,mts}

bun install creates bun.lock on Bun 1.2 or later. On older versions, run bun install --save-text-lockfile. The preset does not detect the binary bun.lockb format.

Call Bun.serve() once while the module loads. Vercel detects that call and routes incoming requests to it. Vercel supports the fetch, routes, error, and websocket options:

server.ts

Bun.serve({
	routes: {
		"/health": () => Response.json({ status: "ok" }),
	},
	fetch() {
		return new Response("Hello from Bun on Vercel");
	},
});

A minimal project is package.json, bun.lock, server.ts, and the vercel.json from the previous step. It doesn't need an api/ directory or any routing configuration.

port and hostname only apply when you run the server locally; they don't configure the deployed endpoint. Unix sockets and HTML imports in routes are not supported on Vercel.

To serve WebSocket connections, see the Bun example in Vercel's WebSockets documentation.

To add a Bun server to a project that also has a frontend, create api/server.ts and call Bun.serve() once while the module loads. Vercel deploys it as a single Function at /api/server. Unlike the framework preset, only requests for /api/server reach this server.

api/server.ts

Bun.serve({
	fetch(request) {
		const url = new URL(request.url);

		return Response.json({
			message: "Hello from Bun on Vercel",
			pathname: url.pathname,
		});
	},
});

This setup only needs the bunVersion setting from the previous step; it doesn't use the framework preset or require a bun.lock file. To send other paths to this server, add route overrides to vercel.json. Each override must use the full request path, including the /api/server prefix. See the Vercel Bun runtime documentation for details.

Frameworks that Vercel supports, such as Next.js, Express, Hono, and Nitro, run on Bun once you set bunVersion.

If you're deploying a Next.js project (including ISR), also update the package.json scripts so the Next.js CLI runs under Bun:

package.json

{
	"scripts": {
		"dev": "bun --bun next dev", 
		"build": "bun --bun next build" 
	}
}

The --bun flag runs the Next.js CLI under Bun. Bundling (with Turbopack or Webpack) is unchanged.

Deploy your app

Connect your repository to Vercel, or deploy from the CLI:

terminal

# Using bunx (no global install)
bunx vercel login
bunx vercel deploy

Or install the Vercel CLI globally:

terminal

bun i -g vercel
vercel login
vercel deploy

Learn more in the Vercel Deploy CLI documentation →

Verify the runtime

To confirm your deployment uses Bun, log the Bun version:

server.ts

console.log("runtime", process.versions.bun);

runtime 1.4.0

See the Vercel Bun Runtime documentation for feature support →

Fluid compute: Both Bun and Node.js runtimes run on Fluid compute and support the same core Vercel Functions features.
Middleware: To run Routing Middleware with Bun, set the runtime to nodejs:

middleware.ts

export const config = { runtime: "nodejs" };