Bun Rewrites 535K Lines of Zig into Rust in Four Months

Bun creator Jarred Sumner ported the JavaScript/TypeScript runtime from Zig to Rust in four months instead of a year, using about 50 workflows with Claude Fable 5. The rewrite cost $165K in tokens, fixed 128 old bugs and memory leaks, with Bun v1.4.0 released in August 2026.

• 535,496 lines of Zig converted to Rust in 4 months instead of a year
• Usage: 5.9B input and 690M output tokens, about $165K
• 64 parallel Claude agents produced up to 1,300 lines of code per minute
• Rust version fixed 128 bugs and memory leaks; HTTP throughput rose 2–5%