Bun 1.4 is available in Vercel Functions, opt-in because of breaking changes
Vercel Changelog·medium signal
Vercel added Bun 1.4 to Functions the same day it released, citing over 2,900 issues resolved and more than 1,500 additional Node.js compatibility tests passing after the Zig-to-Rust rewrite. It supports WebSockets and Bun.serve() plus Next.js, Elysia and Hono, running on Fluid compute with Active CPU pricing. You opt in by setting bunVersion to 1.4.x in vercel.json; Vercel is explicit that this is manual because 1.4 has several breaking changes.