Stephen Toub
Distinguished Engineer at Microsoft, previously known for two decades of .NET performance work. On 2026-09-16 the GitHub Blog published his first-person account, “Migrating the GitHub Copilot runtime to Rust, using Copilot” https://github.blog/ai-and-ml/github-copilot/migrating-the-github-copilot-runtime-to-rust-using-copilot/. Its subject is the agent runtime behind the Copilot CLI, which the Copilot app and the Copilot SDK share. The rewrite moved it from TypeScript on Node.js to roughly 800,000 lines of production Rust, carried out by one engineer leading a fleet of Copilot agents: 14.5 weeks, 128 porting pull requests, replacement component-by-component, agents handling about 61% of tool calls, and token spend reported around $120,000 by The Register https://www.theregister.com/devops/2026/09/18/microsoft-agentically-ports-copilot-runtime-to-rust-for-120k/5297549. In-process embedding cut a measured session lifecycle from 5.25 seconds to 292 milliseconds https://www.infoq.com/news/2026/10/github-copilot-rust-migration.
This page exists because of a fact-check. 2026-10-23-youtube-lets-get-rusty-rust-perfect-language relayed “Microsoft recently used Copilot to rewrite the GitHub Copilot runtime from TypeScript to Rust” and the 2026-10-25 ingest could not find a primary, so the source page recorded the claim as the presenter’s assertion. The primary turned up on 2026-11-09, and it is stronger than the relay. This was not a Copilot side-project but a named engineer’s first-person port, published by GitHub itself. See rust-for-agents.
He is also the subject of a 2026-11-02 developing.dev podcast interview on the port’s how and why, where the interviewee counts the codebase at 400,000 lines against the blog’s 800,000-plus for the new Rust https://www.developing.dev/p/e073-stephen-toub. The two figures measure different things, a TypeScript count against the Rust that replaced it. Each figure needs its source quoted with it.
Sources
- GitHub Blog, 2026-09-16: https://github.blog/ai-and-ml/github-copilot/migrating-the-github-copilot-runtime-to-rust-using-copilot/
- InfoQ summary, 2026-10-09: https://www.infoq.com/news/2026/10/github-copilot-rust-migration
- The Register, 2026-09-18: https://www.theregister.com/devops/2026/09/18/microsoft-agentically-ports-copilot-runtime-to-rust-for-120k/5297549
- Interview: https://www.developing.dev/p/e073-stephen-toub