Anthropic Publishes the Full Cost Ledger for Its Agentic Migrations: 5.9B Input Tokens, $165K, 1,448 Files

Anthropic / Hacker News·high signal

Anthropic's engineering post details two large-scale Claude Code migrations with unusually specific numbers. Bun's Zig-to-Rust port produced a million lines in under two weeks across a 1,448-file PR with 100% of the existing test suite passing before merge, consuming 5.9 billion uncached input tokens and 690 million output tokens (~$165,000 at API pricing); 19 regressions surfaced post-merge, all fixed, and the result is 19% smaller binaries with 2–5% better throughput and one benchmark dropping from 6,745 MB to 609 MB. A separate Python-to-TypeScript migration hit 165,000 lines over a weekend using hundreds of agents behind eight phase gates and three adversarial review rounds, for 27 million tokens.