Copilot rewrote its own runtime into 800,000 lines of Rust

“A rewrite this size wasn’t affordable before agents,” writes Microsoft engineer Stephen Toub. The Copilot agent runtime now runs on more than 800,000 lines of production Rust, written mostly by the Copilot app and the Copilot CLI.

The port shipped in pieces, 128 pull requests merged incrementally, while the rest of the team kept expanding the runtime.

Bun, the JavaScript runtime Anthropic bought in December, took the opposite route. Founder Jarred Sumner ran multiple Claude Code agents in parallel over 535,496 lines of Zig, and 11 days later the Rust version passed Bun’s existing tests.

Both firms sell the agents they credit. The New Stack cautions against reading 11 days against 14-and-a-half weeks as a benchmark: Bun translated Zig, GitHub moved a TypeScript and Node.js runtime while shipping the product.