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Bun's Zig-to-Rust rewrite ran largely on Anthropic's Fable in 11 days for $165K

Bun's Zig-to-Rust rewrite ran largely on Anthropic's Fable in 11 days for $165K

Bun creator Jarred Sumner rewrote the runtime from Zig to Rust largely with a pre-release Anthropic Fable, in 11 days at $165K in tokens — against an estimated one to two years for a small team. The precondition is a thoroughly tested codebase: the existing test suite is what verifies the AI-driven rewrite, so the pattern transfers only to projects with strong coverage to check against.

Source: newsletter.pragmaticengineer.com ↗

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shortening a 1-2 year migration down to 11 days opens amazing new opportunities for devs

Pragmatic Engineer

Why this matters

  • → 11-day rewrite cuts 1-2 year migration timeline by 99%.
  • → AI coding tools now competitive enough for major production rewrites.
  • → Strong test coverage becomes critical bottleneck, not engineering time.
AI rewrites production code