Vercel Functions now run Bun 1.4, the Zig-to-Rust rewrite
Vercel Functions now support Bun 1.4, the release that rewrote Bun from Zig to Rust, closed over 2,900 issues, and passes more than 1,500 additional Node.js compatibility tests. Bun on Vercel runs on Fluid compute with Active CPU pricing and supports WebSockets, Bun.serve(), and frameworks including Next.js, Elysia, and Hono. The upgrade is an explicit opt-in via bunVersion 1.4.x in vercel.json because 1.4 carries breaking changes, so existing deployments stay on the older runtime until that line is added.
Source: vercel.com ↗
Bun 1.4 has several breaking changes. Review the breaking changes and update your application before upgrading.
Vercel
Why this matters
- → Bun rewrite in Rust closes 2,900+ issues and improves Node.js compat by 1,500+ tests
- → Vercel Functions now offer Bun 1.4 with WebSockets, Active CPU pricing, framework support
Bun rewrites itself in Rust