GitHub rewrote the Copilot runtime in 800,000 lines of Rust, mostly by AI agents
DanWahlin · x · 2026-09-18
GitHub's engineering blog details how the team used Copilot itself (Copilot app + CLI) to fully rewrite the Copilot agent runtime from TypeScript/Node.js into 800,000+ lines of production Rust, with AI agents writing most of the code:
- The rewrite landed via 128 PRs merged incrementally rather than a single cutover; the few regressions were caught and fixed quickly.
- Runtime performance improved by orders of magnitude. The runtime powers Copilot CLI, the Copilot app, the SDK, and other Microsoft/GitHub agentic products.
- A rewrite that would have taken a whole team 1–2 years was done primarily by one developer in a few months—"not affordable before agents."
Related event: GitHub Rewrote Its Copilot Runtime in 800K Lines of Rust Using Copilot(10 posts)→
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