Why did Stephen Toub rewrite the GitHub Copilot runtime from TypeScript to Rust?

Stephen Toub said the goal was to get away from Node.js and V8, not to adopt Rust for its own sake. The Copilot runtime powers the Copilot CLI, the GitHub app and Copilot Cloud Agent, and teams across GitHub and Microsoft wanted one shared agent harness they could embed in-process from any language. A Node-based core forced a separate process, a large working set and slow startup on every request.

• The runtime has six language SDKs that were thin bindings, and each spawned a separate process to host sessions.
• Toub said Node startup versus native code can cost hundreds of milliseconds to seconds per request, which dominates short agent turns.

Source: our own transcription of the episode. The full conversation is on Ryan Peterman (2026-11-02). Read the whole episode summary →

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