bun vercel runtime websockets deployment

Bun 1.4 lands on Vercel Functions

Bun 1.4 runtime now available on Vercel Functions with Zig-to-Rust rewrite, requiring explicit opt-in via vercel.json bunVersion config.

Summary

Reduces cold-start friction for Bun workloads on Vercel and unlocks WebSocket support via Bun.serve(). Breaking changes require deliberate testing before upgrading existing deployments.

Why it matters

Reduces cold-start friction for Bun workloads on Vercel and unlocks WebSocket support via Bun.serve(). Breaking changes require deliberate testing before upgrading existing deployments.

Implementation verdict

Replaces previous Bun runtime on Vercel. Requires setting bunVersion: "1.4.x" in vercel.json and reviewing breaking changes before upgrading. Ready now for new projects; existing deployments need validation.

Sources

1.Bun 1.4 release, a rewrite of Bun from Zig to Rust, with over 2,900 issues resolved
2.Runs on Fluid compute with Active CPU pricing
3.Supports WebSockets and Bun.serve()
4.Upgrading is an explicit opt-in because Bun 1.4 has several breaking changes

Dev Signal

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