GitHub rewrote Copilot's agent runtime into 800K lines of Rust, mostly written by AI agents
martinwoodward · x · 2026-09-17
GitHub's engineering blog details how the team used the Copilot app and CLI to completely rewrite the Copilot agent runtime from TypeScript/Node.js into over 800,000 lines of production Rust, with AI agents writing most of the code across 128 PRs that shipped incrementally.
- A port that would have taken a full team one to two years was done primarily by a single developer in a few months.
- Existing end-to-end tests ran against the new code at every step; regressions were caught and fixed quickly.
- Runtime performance improved by orders of magnitude.
- The runtime backs Copilot CLI, the Copilot app, the SDK and multiple Microsoft/GitHub products.
Related event: GitHub Rewrote Its Copilot Runtime in 800K Lines of Rust Using Copilot(10 posts)→
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