Anthropic's Claude Code Migrates 1M Lines of Code to a New Language in Two Weeks

Anthropic reports that using Claude Code and models like Claude Fable 5 and Claude Opus 4.8, developers can migrate large codebases to a new language in days or weeks instead of years. Two examples are highlighted: Jarred Sumner, co-founder of Bun, used Claude Code to migrate Bun from Zig to Rust, producing a million lines of code in less than two weeks, with only 19 regressions found after the merge, all since fixed. Mike Krieger, an Anthropic Labs leader, migrated a Python codebase to 165,000 lines of TypeScript over a weekend, using hundreds of agents, eight checkpoints, three rounds of adversarial review, and a final parity check. The article outlines a six-step process for such migrations, starting with establishing a reliable arbiter (test suite), then creating a rules document, dependency map, and divergence registry, followed by parallel agent-driven translation, compilation, and testing. It notes that the Bun migration cost about 5.9 billion uncached input tokens and 690 million output tokens, roughly $165,000, while Mike's migration used 27 million tokens. The economics have changed, making migrations feasible for smaller benefits, such as reducing compile times from eight minutes to two seconds.