GitHub ports Copilot runtime to 800,000 lines of production Rust

The Copilot agent runtime was originally written in TypeScript on Node.js, backing the Copilot CLI, Copilot app, and cloud agent. That stack was fine for a console application, but as the runtime spread to VS Code, Visual Studio, Copilot Code Review, Copilot Cowork, Copilot Studio, and Office apps, its startup, memory, and throughput costs became harder to justify. The SDK had been layered on top of the CLI rather than the other way around, so every SDK consumer in C#, Python, Go, Java, or Rust spawned a subprocess and paid for a second language runtime it otherwise had no use for.

The port was done in place, component by component, with each pull request replacing a TypeScript implementation with a thin shim calling into Rust. That kept main shippable and let end-to-end tests run against the new code at every step. Early work began in early May 2026 with two pull requests setting up the Rust workspace, and progressed from pure-logic helpers inward to session orchestration, the most coupled part of the runtime.

What the outcome hinges on is confidence in sharing a process and a failure boundary with consuming applications. The completed Rust runtime supports both in-process and out-of-process hosting, but those in-process entry points remain opt-in for now, leaving the SDK's JSON-RPC path as the default while GitHub monitors feedback. For the six SDK language versions, the shift seems likely to reduce the overhead each consumer carries, though the timing of a full default change is not stated.