{"slug": "claude-code-uses-bun-written-in-rust-now", "title": "Claude Code uses Bun written in Rust now", "summary": "Anthropic's Claude Code v2.1.181 now uses a Rust port of Bun, achieving 10% faster startup on Linux. Evidence from the binary shows Bun v1.4.0 and Rust source files, confirming the switch to a production-ready Rust-based runtime.", "body_md": "In [Rewriting Bun in Rust](https://bun.com/blog/bun-in-rust) Jarred Sumner made the following claim:\n\nClaude Code v2.1.181 (released June 17th) and later use the Rust port of Bun. Startup got 10% faster on Linux but otherwise, barely anyone noticed. Boring is good.\n\nI decided to have a poke at my own Claude Code installation to see if I could find evidence that it was using Bun written in Rust.\n\nI found these two commands convincing:\n\n```\nstrings ~/.local/bin/claude | grep -m1 'Bun v1'\n```\n\nFor me this outputs `Bun v1.4.0 (macOS arm64)`\n\n. The most recent release of [Bun on GitHub](https://github.com/oven-sh/bun/releases) is currently [v1.3.14](https://github.com/oven-sh/bun/releases/tag/bun-v1.3.14) from May 12th, so that v1.4.0 version number in Claude supports them shipping a preview of a not-yet-released Bun version.\n\n```\nstrings ~/.local/bin/claude | grep -Eo 'src/[[:alnum:]_./-]+\\.rs'\n```\n\nThis outputs a list of [563 filenames](https://gist.github.com/simonw/c92fb0f67b114ac26e3b95a09ddccfdc), starting with these:\n\n```\nsrc/runtime/bake/dev_server/mod.rs\nsrc/runtime/bake/production.rs\nsrc/bundler/bundle_v2.rs\n```\n\nIt looks like Bun in Rust is indeed being run in production across millions of different devices. Like Jarred said, \"Boring is good\".\n\nTags: [bun](https://simonwillison.net/tags/bun), [rust](https://simonwillison.net/tags/rust), [anthropic](https://simonwillison.net/tags/anthropic), [claude-code](https://simonwillison.net/tags/claude-code), [jarred-sumner](https://simonwillison.net/tags/jarred-sumner)", "url": "https://wpnews.pro/news/claude-code-uses-bun-written-in-rust-now", "canonical_source": "https://simonwillison.net/2026/Jul/19/claude-code-in-bun-in-rust/#atom-everything", "published_at": "2026-07-19 03:54:09+00:00", "updated_at": "2026-07-19 04:24:42.953169+00:00", "lang": "en", "topics": ["developer-tools", "ai-tools"], "entities": ["Claude Code", "Bun", "Rust", "Anthropic", "Jarred Sumner"], "alternates": {"html": "https://wpnews.pro/news/claude-code-uses-bun-written-in-rust-now", "markdown": "https://wpnews.pro/news/claude-code-uses-bun-written-in-rust-now.md", "text": "https://wpnews.pro/news/claude-code-uses-bun-written-in-rust-now.txt", "jsonld": "https://wpnews.pro/news/claude-code-uses-bun-written-in-rust-now.jsonld"}}