GitHub rewrote the runtime that powers Copilot's CLI, its desktop app and related Microsoft products from TypeScript on Node.js into Rust, using Copilot itself to generate most of the new code, the company said in a blog post.
The Node.js runtime required spawning a separate process per client with roughly 100MB of memory overhead each and communicating over JSON-RPC, limiting how deeply it could be embedded into other applications, GitHub said. Rust let the team embed the runtime directly in-process across six SDK language versions with minimal overhead.
The port ran incrementally from May through August 2026, replacing components in place across 128 pull requests while keeping the main branch shippable throughout, according to GitHub. Roughly 430,000 lines of TypeScript became 832,378 lines of production Rust, shipped across 135 releases, an average of 1.3 a day.
A single developer oversaw the effort, directing strategy and reviewing architecture while agents handled code generation, GitHub said. The result used only 158 unsafe code blocks, all at boundaries with external systems, and GitHub said runtime performance improved by orders of magnitude.
A four-month rewrite of 430,000 lines of production code with one engineer in the loop argues for what agentic coding actually changes: not that a person stops reviewing, but how much code one person can responsibly oversee.