Bun moved from Zig to Rust with Claude Code: a million lines in under two weeks for ~$165,000

What the agent did

• Produced about a million lines of Rust in less than two weeks, with 100% of Bun's existing tests passing in CI before merge
• Ran a three-file stress test of the translation rules first, which caught two critical issues before the work fanned out to 1,448 files
• Had two adversarial reviewer agents check every translation in separate contexts, with a third agent settling disagreements
• Used 5.9 billion uncached input tokens and 690 million output tokens; 19 regressions surfaced after merge and were all fixed

What it means for you

• Build the judge first: a test suite that runs against both the old and the new code, and that you have checked fails on broken code.
• Fix the rules, not the files: when reviewers keep finding the same mistake, add a line to the rulebook and regenerate that batch.
• Anthropic's code-migration-kit-with-claude-code on GitHub is a general template of this process; it is not what the Bun port ran on.

Why it matters

Anthropic says million-line migrations used to cost $3 to $4 million in engineering over about four years; this one cost about $165,000 in tokens, as reported. The new Bun also uses less memory: one benchmark of 2,000 repeated builds fell from 6,745 MB to 609 MB, and the binary is 19% smaller on Linux and Windows.

Figures as reported by Anthropic, which owns Bun; the token cost leaves out engineer time, and about 4% of the Rust code still sits in unsafe blocks.