Anthropic described how its engineers use Claude Code to execute large-scale codebase migrations, citing examples such as Labs co-lead Mike Krieger migrating a Python codebase to 165,000 lines of TypeScript over a weekend using hundreds of agents, eight phase gates, and adversarial review rounds. Bun co-founder Jarred Sumner similarly used Claude Code to migrate Bun from Zig to Rust, producing a million lines of code in under two weeks while passing 100% of Bun’s existing test suite in CI before merge. Anthropic attributes the approach’s success to the parallel, independently-verifiable nature of migration work and argues the core insight is fixing the process, or loop, that produces the code rather than fixing the code directly.
