Anthropic Engineers Complete Million-Line Code Migrations in Days Using AI Agents, Slashing Costs from Years to Weekends

Anthropic engineers are using Claude Code and AI agents to complete massive code migrations in days, including a million-line Zig-to-Rust port finished in under two weeks, slashing what once took years and millions of dollars down to weekend-length projects costing as little as $165,000 in API tokens.

• Anthropic engineers are completing massive code migrations in days using Claude Code and AI agents, including a million-line Zig-to-Rust port of Bun completed in under two weeks and a Python-to-165,000-line TypeScript migration completed over a single weekend.
• The AI-powered migration process follows a six-step framework — building a rulebook and dependency map, stress-testing rules, translating code in parallel with multi-agent loops, then compiling, smoke testing, and verifying behavioral parity — with adversarial review agents and mechanical verification replacing human judgment at each stage.
• While costs remain significant — the Bun migration consumed roughly $165,000 in API tokens — AI is fundamentally changing the economics of code migrations, reducing what were once multi-year, multi-million dollar projects into days-long efforts where the worst-case outcome is simply deleting the branch and starting over.