Microsoft fully ported GitHub Copilot runtime from TypeScript to Rust using AI agents, costing $120,000 in tokens and three weeks of developer time. The migration boosted performance by 15.9x, reducing session throughput from 7.55 to 120 per second, and cut memory usage by 90 percent. The process revealed AI coding involves iterative investigation rather than rapid code generation. Despite passing compilation, dozens of regressions emerged, highlighting that compiler success does not guarantee semantic correctness. This case underscores the viability of AI-driven large-scale rewriting while emphasizing the need for rigorous validation beyond mere compilation.