Deploy a Bun application on Vercel

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:

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.

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.

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:

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