- Bun was rewritten from Zig to Rust in 11 days using AI (Claude's Fable), costing $165,000, which shortened a potential 1-2 year manual migration.
- The primary motivation for the rewrite was to eliminate memory-related bugs (e.g., memory leaks, use-after-free) in Zig, as Rust provides memory safety and compiler-enforced guarantees.
- The migration process involved: prep work with a porting guide, adversarial reviews, parallel AI agents (64 total), and iterative fixing of compiler errors and test failures.
- Success factors included having a motivated engineer familiar with the codebase, a robust test suite, and willingness to invest in AI tokens.
- This demonstrates how AI can enable previously impractical rewrites or migrations, potentially transforming engineering workflows for complex projects.