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Rewriting Bun in Rust
12 days ago
- Bun has been rewritten from Zig to Rust using AI-powered coding agents, significantly reducing memory-related bugs.
- The rewrite was enabled by a comprehensive TypeScript test suite that served as a conformance benchmark for automated porting.
- Key engineering techniques included dynamic workflows, trial runs, adversarial review, and process-based fixes rather than manual code editing.
- Jarred Sumner praised Zig for Bun's initial development but chose Rust for its memory safety features like RAII and compiler checks for use-after-free errors.
- The rewrite took about 11 days of intensive AI-assisted work, with an estimated cost of $165,000 in API tokens, funded by Anthropic.
- The new Rust version has been live in Claude Code for nearly a month, improving startup speed by 10% on Linux with minimal user impact.
- The project demonstrates how advanced language models can enable ambitious, large-scale software rewrites previously considered infeasible.
My Thoughts on the Bun Rust Rewrite
11 days ago
- Jarred initially had 'beginner energy' which was healthy for learning but led to mediocre engineering outcomes.
- After Bun took venture capital and became a startup, Jarred's management style caused issues: poor communication, unrealistic expectations, and low empathy.
- The Zig team found Bun's codebase problematic with hacks, abuse of assertions, and reckless feature development, leading to social distance.
- The relationship between Zig and Bun deteriorated, culminating in relief when Bun was acquired by Anthropic and stopped donations.
- The Rust rewrite announcement was welcomed by the Zig team to distance themselves from Bun's engineering practices.
- The blog post about the rewrite is criticized for misrepresenting bug elimination, test suite effectiveness, and performance improvements.
- Zig's compiler performance is highlighted as efficient compared to Bun's unmentioned compilation speed post-rewrite.
- Despite criticisms, there is gratitude for Bun's donations to Zig and no personal ill will towards Jarred, but happiness that business interests are no longer intertwined.
Claude Code uses Bun written in Rust now
a day ago
- Jarred Sumner announced Claude Code v2.1.181+ uses Rust port of Bun, speeding up Linux startup by 10% with little user notice.
- Evidence found in Claude Code installation shows Bun v1.4.0 (macOS arm64), a version not yet released on GitHub as of the text's date.
- Rust version of Bun is available as a canary release, and 563 Rust source files in Claude indicate it's running in production.

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Rewriting Bun in Rust

My Thoughts on the Bun Rust Rewrite

Claude Code uses Bun written in Rust now