# Bun 1\.4 is now available in Vercel Functions

**Published:** August 20, 2026 | **Authors:** Florentin Eckl, Tom Lienard

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Vercel Functions now support the new [Bun 1.4 release](https://bun.com/blog/bun-v1.4), a rewrite of Bun from Zig to Rust, with over 2,900 issues resolved, more than 1,500 additional Node.js compatibility tests passing, and new features and performance improvements.

Bun on Vercel Functions:

- Runs on Fluid compute with [Active CPU pricing](https://vercel.com/docs/functions/usage-and-pricing)
- Supports [WebSockets](https://vercel.com/docs/functions/websockets#bun) and `Bun.serve()`
- Supports common frameworks such as Next.js, Elysia, and Hono

To update from the previous Bun version, set `bunVersion` to `1.4.x` in `vercel.json`.

**vercel.json**
```json
{
  "bunVersion": "1.4.x"
}
```

Upgrading is an explicit opt-in because Bun 1.4 has several [breaking changes](https://bun.com/blog/bun-v1.4#upgrading-to-1-4). Review the breaking changes and update your application before upgrading.

Learn more about using Bun on Vercel in the [runtime documentation](https://vercel.com/docs/functions/runtimes/bun).

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📚 **More updates:** [View all changelog entries](/changelog/sitemap.md) | [Blog](/blog/sitemap.md)