Bun 1.4 Is Now Available in Vercel Functions
Quick Reads
- Vercel Functions now support Bun 1.4, a full rewrite of the runtime from Zig to Rust
- The update resolves over 2,900 issues and adds more than 1,500 new Node.js compatibility tests bun
- Bun 1.4 reduces idle CPU usage by 5x and memory by up to 35%, and starts 50% faster on Linux bun
- Bun on Vercel runs on Fluid compute with Active CPU pricing and supports WebSockets and
Bun.serve() - Upgrading requires an explicit opt-in, breaking changes apply
Vercel has officially added Bun 1.4 on Vercel Functions, marking a major step forward for developers building serverless applications with the JavaScript-native toolkit.
The release follows Bun’s biggest architectural shift yet, a full rewrite from Zig to Rust, and resolves more than 2,900 reported issues. For teams already running Bun on Vercel, this update isn’t automatic. It requires deliberate action.
Bun on Vercel Functions runs on Fluid compute with Active CPU pricing, supports WebSockets and Bun.serve(), and works with common frameworks such as Next.js, Elysia, and Hono.
The performance improvements are substantial. Bun 1.4 cuts idle CPU usage by 5x, reduces memory usage by up to 35%, and starts 50% faster on Linux. For serverless environments where cold starts matter, that’s a meaningful advantage.
Memory gains are particularly notable across common HTTP frameworks. Applications using HTTP servers with Bun can expect a 13% to 48% memory usage reduction, depending on the framework. Fastify drops from 233 MB to 120 MB at peak, while Express falls from 169 MB to 92 MB under load. Next.js App Router, meanwhile, settles at 238 MB, well below Node’s 410 MB on the same workload.
Beyond memory, the rewrite brings deeper Node.js compatibility. Bun 1.4 adds 1,517 tests from the Node.js test suite, the largest compatibility jump since Bun 1.0. Core modules including node:http, node:fs, node:cluster, and node:stream now pass 97% of Node’s own tests. Playwright, vitest, and OpenTelemetry also now run on Bun.
The release also introduces native tooling built directly into the runtime. Bun 1.4 ships Bun.Image for image processing, Bun.WebView for headless browser automation, Bun.cron() for scheduled jobs, and bun run --parallel for concurrent script execution, all without npm installs.
For Nigerian developers building serverless APIs, backend services, or full-stack apps deployed on Vercel, Bun 1.4 on Vercel Functions means lower compute bills through Active CPU pricing and faster response times from a leaner runtime.
To upgrade, set bunVersion to 1.4.x in your vercel.json file. However, Vercel is clear that this is not an automatic rollout. Upgrading is an explicit opt-in because Bun 1.4 includes several breaking changes. Teams should review the Bun 1.4 breaking changes and test their application before switching.
