--- title: Using the Bun Runtime with Vercel Functions product: vercel url: /docs/functions/runtimes/bun canonical_url: "https://vercel.com/docs/functions/runtimes/bun" last_updated: 2026-08-24 type: reference prerequisites: - /docs/functions/runtimes - /docs/functions related: - /docs/project-configuration/vercel-json - /docs/functions/websockets - /docs/incremental-static-regeneration - /docs/routing-middleware - /docs/functions/limitations summary: Learn how to use the Bun runtime with Vercel Functions to create fast, efficient functions. install_vercel_plugin: npx plugins add vercel/vercel-plugin --- # Using the Bun Runtime with Vercel Functions > **🔒 Permissions Required**: The Bun runtime ## Related pages > **For AI agents:** Follow these links to understand how this page connects to the rest of the Vercel ecosystem. For the full cross-link map (inbound, outbound, prerequisites, and semantic neighbors), see the .graph.md link below. - [Bun runtime on Vercel Functions](https://vercel.com/blog/bun-runtime-on-vercel-functions?from=related&source_path=%2Fdocs%2Ffunctions%2Fruntimes%2Fbun&source_site=vercel-docs&relationship=related) - [Bun runtime now in Public Beta for Vercel Functions](https://vercel.com/changelog/bun-runtime-now-in-public-beta-for-vercel-functions?from=related&source_path=%2Fdocs%2Ffunctions%2Fruntimes%2Fbun&source_site=vercel-docs&relationship=related) - [Bun runtime for Vercel Functions now accepts Bun.serve as an entrypoint](https://vercel.com/changelog/bun-serve-entrypoint-for-vercel-functions?from=related&source_path=%2Fdocs%2Ffunctions%2Fruntimes%2Fbun&source_site=vercel-docs&relationship=related) - [Bun 1.4 is now available in Vercel Functions](https://vercel.com/changelog/bun-1-4-is-now-available-in-vercel-functions?from=related&source_path=%2Fdocs%2Ffunctions%2Fruntimes%2Fbun&source_site=vercel-docs&relationship=related) - [Bun runtime now supports large functions and extended max duration](https://vercel.com/changelog/bun-runtime-now-supports-large-functions-and-extended-max-duration?from=related&source_path=%2Fdocs%2Ffunctions%2Fruntimes%2Fbun&source_site=vercel-docs&relationship=related) - [Elysia can now be automatically deployed on Vercel](https://vercel.com/changelog/support-for-elysia?from=related&source_path=%2Fdocs%2Ffunctions%2Fruntimes%2Fbun&source_site=vercel-docs&relationship=related) - [Build with an Express starter template](https://vercel.com/kb/guide/build-with-a-express-starter-template?from=related&source_path=%2Fdocs%2Ffunctions%2Fruntimes%2Fbun&source_site=vercel-docs&relationship=related) — Deploy an Express app to Vercel from a template. Browse Express starters from Vercel and the community, then run them lo - [How to ship an Elysia app on Vercel](https://vercel.com/kb/guide/ship-a-elysia-app-on-vercel?from=related&source_path=%2Fdocs%2Ffunctions%2Fruntimes%2Fbun&source_site=vercel-docs&relationship=related) — Deploy a Elysia app to Vercel with zero configuration. Learn how to ship from a template, the Vercel CLI, or Git, and co - [How to ship a Nitro app on Vercel](https://vercel.com/kb/guide/ship-a-nitro-app-on-vercel?from=related&source_path=%2Fdocs%2Ffunctions%2Fruntimes%2Fbun&source_site=vercel-docs&relationship=related) — Deploy a Nitro app to Vercel with zero configuration. Learn how to ship from a template, the Vercel CLI, or Git, and con - [How to stop Vercel Functions from timing out](https://vercel.com/kb/guide/what-can-i-do-about-vercel-serverless-functions-timing-out?from=related&source_path=%2Fdocs%2Ffunctions%2Fruntimes%2Fbun&source_site=vercel-docs&relationship=related) — Vercel Functions that time out usually trace back to a few causes. Learn how Fluid Compute fixes most of them and how to - [Frameworks on Vercel](https://vercel.com/docs/frameworks?from=related&source_path=%2Fdocs%2Ffunctions%2Fruntimes%2Fbun&source_site=vercel-docs&relationship=related) — Vercel supports a wide range of the most popular frameworks, optimizing how your application builds and runs no matter w - [Elysia on Vercel](https://vercel.com/docs/frameworks/backend/elysia?from=related&source_path=%2Fdocs%2Ffunctions%2Fruntimes%2Fbun&source_site=vercel-docs&relationship=related) — Build fast TypeScript backends with Elysia and deploy to Vercel. Learn the project structure, plugins, middleware, and h Full cross-link map for this page: [/docs/functions/runtimes/bun.graph.md](/docs/functions/runtimes/bun.graph.md?from=related&source_path=%2Fdocs%2Ffunctions%2Fruntimes%2Fbun&source_site=vercel-docs&relationship=graph) Bun is a fast, all-in-one JavaScript runtime that serves as an alternative to Node.js. Bun provides Node.js API compatibility and is generally faster than Node.js for CPU-bound tasks. It includes a bundler, test runner, and package manager. ## Configuring the runtime For all frameworks, including Next.js, you can configure the runtime in your `vercel.json` file using the [`bunVersion`](/docs/project-configuration/vercel-json#bunversion) property. Once you configure the runtime version, Vercel manages the patch versions automatically. Currently, `"1.4.x"` and `"1.x"` are the only valid versions: - `"1.4.x"` selects the [complete rewrite of Bun from Zig to Rust](https://bun.com/blog/bun-v1.4), which contains several [breaking changes](https://bun.com/blog/bun-v1.4#upgrading-to-1-4). Set this version explicitly once you have migrated your application. - `"1.x"` selects the previous latest Bun version (currently `1.3.14`). ```json filename="vercel.json" { "$schema": "https://openapi.vercel.sh/vercel.json", "bunVersion": "1.4.x" } ``` > **💡 Note:** Vercel manages the Bun minor versions automatically. `1.4.x` and `1.x` are the > only valid values currently. ## Deploy with the Bun framework preset Use the Bun framework preset when you want one `Bun.serve()` server to route requests for your application. After you configure `bunVersion` in `vercel.json`, add a `bun.lock` file and a `server` entrypoint in the project root or the `src/` directory: - `server.{js,cjs,mjs,ts,cts,mts}` - `src/server.{js,cjs,mjs,ts,cts,mts}` With Bun 1.2 or later, run `bun install` to create `bun.lock`. For older versions, run `bun install --save-text-lockfile`. The preset doesn't detect the legacy `bun.lockb` format. A minimal project contains `package.json`, `bun.lock`, `server.ts`, and the `vercel.json` configuration shown above. You don't need an `/api` directory or routing configuration. Call `Bun.serve()` once during module startup. Vercel uses that call to detect the server, then routes incoming requests through a Vercel Function. You can use the `fetch`, `routes`, `error`, and `websocket` options: ```ts filename="server.ts" Bun.serve({ routes: { '/health': () => Response.json({ status: 'ok' }), }, fetch() { return new Response('Hello from Bun on Vercel'); }, }); ``` The `port` and `hostname` options only apply when you run the server locally. They don't configure the public endpoint on Vercel. Unix sockets and HTML imports in `routes` are not supported. To serve WebSocket connections with `Bun.serve()`, see the [Bun example in the WebSockets documentation](/docs/functions/websockets#bun). ## Deploy a Bun server from `/api` Create `api/server.ts` to deploy a native Bun server as one Vercel Function. Vercel serves the function at `/api/server`, so you can add it to a project that also contains a frontend. > **💡 Note:** To use custom routing with an `/api` server, configure route overrides in > `vercel.json`. Each override must use the full request path, including the > `/api/server` prefix. Call `Bun.serve()` once during module startup: ```ts filename="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 deployment model only requires the `bunVersion` configuration shown above. It doesn't use the Bun framework preset or require `bun.lock`. Unlike the preset, it only sends requests for `/api/server` to this server. ## Framework-specific considerations ### Next.js When using Next.js, and [ISR](/docs/incremental-static-regeneration), you must change your `build` and `dev` commands in your package.json file to use the Bun runtime: **Before:** ```json filename="package.json" { "scripts": { "dev": "next dev", "build": "next build" } } ``` **After:** ```json filename="package.json" { "scripts": { "dev": "bun run --bun next dev", "build": "bun run --bun next build" } } ``` ### Routing Middleware The Bun runtime works with [Routing Middleware](/docs/routing-middleware) the same way as the Node.js runtime once you set the `bunVersion` in your `vercel.json` file. Note that you'll also have to set the runtime config to `nodejs` in your file. ## Feature support The Bun runtime on Vercel supports most Node.js features. The main differences relate to automatic source maps, bytecode caching, and request metrics on the `node:http` and `node:https` modules. Request metrics using `fetch` work with both runtimes. Vercel Functions using the Bun runtime support [large functions](/docs/functions/limitations#large-functions-beta) with uncompressed bundles up to 5 GB and [extended max duration](/docs/functions/configuring-functions/duration#extended-max-duration-beta) up to 30 minutes. Both features are in beta. See the table below for a detailed comparison: ## Supported APIs Vercel Functions using the Bun runtime support [most Node.js APIs](https://bun.sh/docs/runtime/nodejs-apis), including standard Web APIs such as the [Request and Response Objects](/docs/functions/runtimes/node-js#node.js-request-and-response-objects). ## Performance considerations Bun is generally faster than Node.js, especially for CPU-bound tasks. Performance varies by workload, and in some cases Node.js may be faster depending on the specific operations your function performs. ## When to use Bun Bun is best suited for new workloads where you want a fast, all-in-one toolkit with built-in support for TypeScript, JSX, and modern JavaScript features. Consider using Bun when: - You want faster execution for CPU-bound tasks - You prefer zero-config TypeScript and JSX support - You're starting a new project and want to use modern tooling Consider using Node.js instead if: - Node.js is already installed on your project and is working for you - You need automatic source maps for debugging - You need request metrics on the `node:http` or `node:https` modules Both runtimes run on [Fluid compute](/docs/fluid-compute) and support the same core Vercel Functions features. --- [View full sitemap](/docs/sitemap)