GitHub rewrote its Copilot agent runtime into 800k lines of Rust, mostly with Copilot itself
mariorod1 · x · 2026-09-17
GitHub's official blog reveals that the agent runtime backing the Copilot CLI, app and SDK — originally TypeScript on Node.js — has been completely rewritten into 800,000+ lines of production Rust. Most of the code was written by AI agents, landing across 128 PRs that shipped incrementally rather than waiting for a single cutover.
- Performance improved by orders of magnitude; the few regressions were found and fixed quickly.
- A project that would have taken a team one to two years was completed primarily by a single developer in a few months while the rest of the team kept expanding the runtime.
- Stephen Toub shares the approach and learnings in a 65-minute write-up, noting a rewrite this size was unaffordable before agents.
Related event: GitHub Rewrote Its Copilot Runtime in 800K Lines of Rust Using Copilot(10 posts)→
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