What shipped? Vercel Functions now support Bun 1.4 — the first stable release of the Bun rewrite from Zig to Rust.
What changed? Bun 1.4 is a ground-up reimplementation: the Zig runtime was replaced with a Rust codebase, resolving over 2,900 issues and adding more than 1,500 additional Node.js API compatibilities. The Vercel integration works automatically — set "bunVersion": "1.4" in your project config and deploy. Existing Bun 1.3 projects on Vercel will continue to work, but the 1.4 runtime is available immediately for new deployments. The changelog highlights major improvements to node:crypto, node:buffer, and node:stream compat, plus faster package installs from the rewritten module resolver.
Why does a builder care? Bun went from Zig to Rust mid-stream and shipped it without breaking the platform — that's the story. If you've been holding off on Bun because of Node.js edge cases or missing APIs, 1.4 closes most of those gaps. The Rust rewrite also means better long-term maintainability and performance headroom. For Vercel users: this is the fastest server-side runtime on the platform getting faster, with no config change beyond a version bump.