Claude Code switches to Bun (written in Rust): what this means for performance

Claude Code, the official CLI from Anthropic, now uses Bun as its JavaScript runtime—Bun itself is primarily written in Zig (with some Rust components). A technical choice that highlights the shift of the JS stack toward high-performance runtimes.

The concrete case

Discussed on Hacker News: Claude Code - Anthropic's CLI for developers - now uses Bun as its JavaScript runtime, replacing Node.js. Bun itself is written in Zig (and uses Rust bindings for some components), making it a concrete example of the shift toward native-compiled JS runtimes.

This is an interesting signal: a tool used by critical developers (those coding with AI) is adopting a runtime that isn’t yet the industry standard. Anthropic is making a deliberate choice for performance.

Why Bun?

Bun (https://bun.sh) was launched in 2022 by Jarred Sumner as an alternative to Node.js and Deno. Its main arguments:

• Ultra-fast startup: Bun starts 4× faster than Node on average — critical for a CLI launched dozens of times a day
• Runtime + bundler + test runner in a single binary
• Node.js compatibility: most npm packages work without modification
• I/O performance: Bun outperforms Node on file and network operations in many benchmarks

For a CLI tool like Claude Code, startup time is the key metric. No one wants to wait 500ms on every invocation.

What this illustrates: the JS runtime wars

Since 2020, the JavaScript ecosystem has seen a diversification of its runtimes:

• Node.js (2009) — the historical standard, Google’s V8
• Deno (2018) — Ryan Dahl starts from scratch, native TypeScript, default security
• Bun (2022) — JavaScriptCore (WebKit) + Zig, maximum performance
• WinterJS (2023) — Rust + SpiderMonkey (Mozilla)

Each runtime makes different bets. Bun bets on raw speed and npm compatibility.