GitHub Copilot ported its agent runtime from TypeScript to Rust: 800K lines, 14 weeks
vykthur · x · 2026-09-17
Stephen Toub details porting the GitHub Copilot agent runtime (the shared engine behind the Copilot CLI, app, and SDKs) from TypeScript to Rust: 800K+ lines of production Rust, completed in 14 weeks with one primary DRI driving a fleet of agents and a small support team.
The post covers rationale, design choices (incremental in-place porting vs. a long-lived branch with a single cutover), performance gains, regressions, and cost. As vykthur notes, amid the AI software-factory hype this is a useful close-up: such projects still involve heavy human steering, layered review, strong E2E tests as the oracle, and regressions you have to hunt down.
Related event: GitHub Rewrote Its Copilot Runtime in 800K Lines of Rust Using Copilot(10 posts)→
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