Bun's 535,000-line Zig codebase is ported to Rust in 11 days by up to 64 parallel Claude Code agents
Jarred Sumner rewrote the Bun JavaScript runtime from Zig to Rust as a mechanical port, keeping the architecture and using Bun's TypeScript test suite as a language-independent conformance suite. He ran about 50 dynamic workflows in Claude Code over 11 days, with each file written by one agent, checked by two adversarial reviewer agents and then fixed, peaking at 4 workflows of 16 agents each. The rewrite was merged after 100% of the test suite passed in CI on Linux, macOS and Windows with no tests skipped or deleted, and the Rust build has shipped inside Claude Code since v2.1.181.
The numbers (as reported)
| Source size | 535,496 lines of Zig (excluding comments), 1,448 .zig files |
|---|---|
| Time | 11 days (May 3 to merged May 14, 2026) |
| Commits | 6,778 |
| Parallel agents | About 64 at peak (4 workflows x 16) |
| Token cost | Around $165,000 at API pricing (5.9B uncached input, 690M output, 72B cached input tokens) |
| Tests | 100% of suite passing in CI on all platforms, 0 skipped or deleted (60,624 tests on Debian x64) |
| Estimate without AI | 3 engineers about a year (author's estimate) |
| Bugs fixed vs. Zig version | 128 bugs that reproduce in v1.3.14 fixed in v1.4.0 |
Facts
- Company / subject
- Bun (Oven / Anthropic)
- Tools
- Claude Code
- Codebase
- brownfield
- Work type
- Language port
- Stack
- Zig, Rust, JavaScriptCore
- Published
- 2026-07-08
- Last checked
- 2026-10-20
Caveats
Bun was acquired by Anthropic in December 2025 and the author works at Anthropic, so this is close to vendor-published. Used a pre-release Anthropic model (Claude Fable 5); tokens were not paid for. The time-without-AI figure is the author's own estimate. Output is a faithful port, not idiomatic Rust (about 4% of code sits inside unsafe blocks).
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