Vendor-published Language migration
GitHub ports the Copilot agent runtime from TypeScript to 800K+ lines of Rust, mostly by one engineer
Stephen Toub used the GitHub Copilot app and CLI to port the Copilot agent runtime in place, component by component, over roughly 14.5 weeks. 128 port PRs landed and main shipped 135 releases during the window. Dozens of regressions were traced and fixed; the post documents failure patterns in detail.
The numbers (as reported)
| Production Rust written | 832,378 lines |
|---|---|
| TypeScript passed through the port (author estimate) | ~430,000 lines |
| Porting window | ~14.5 weeks |
| Port pull requests | 128 |
| Token spend | ~$120,000 (~136.3B tokens) |
| Developer time (author estimate) | ~3 weeks |
| Prompt-cache hit rate | 96.22% |
| One-turn session, in-process vs before | 5.25 s → 292 ms |
Facts
- Company / subject
- GitHub (Microsoft)
- Tools
- GitHub Copilot app, GitHub Copilot CLI
- Codebase
- brownfield
- Work type
- Language migration
- Stack
- TypeScript, Node.js, Rust
- Published
- 2026-09-16
- Last checked
- 2026-10-17
Caveats
Published by GitHub about its own product. The author notes team members also contributed (bindings, packaging, reviews), and that performance numbers include other changes from the same period.