GitHub has replaced the TypeScript runtime at the heart of many Copilot products with more than 800,000 lines of production Rust. The company says AI coding agents wrote most of the new code, while one primary developer guided a migration completed over several months.
The shared runtime powers the Copilot CLI, app and SDK, along with integrations in products including Visual Studio Code, Visual Studio and Microsoft Office. Its old architecture grew from a command-line application: programs using the SDK had to start a separate Node.js process and communicate with it through JSON-RPC. That brought startup, memory and reliability costs to every integration.
GitHub split the rewrite into 128 pull requests that reached the main branch and shipped incrementally. The company says this avoided a single risky cutover and allowed regressions to be found and repaired as the work progressed. It also reports orders-of-magnitude performance improvements, though the article does not provide a single benchmark applicable to every host product.
The project is a large real-world example of agent-assisted maintenance rather than a claim that human review is unnecessary. Architecture decisions, tests and staged deployment remained central to making the generated code usable in production.