GitHub rewrote the Copilot agent runtime into more than 800,000 lines of Rust, with agents writing most of the code

Sources: primary, discussion
Summary: GitHub describes migrating the Copilot agent runtime to Rust, stating the result exceeds 800,000 lines, that agents produced most of the code, and that the work landed as 128 pull requests merged into main and shipped incrementally by primarily one developer over a few months. The runtime backs the Copilot CLI, the app and the SDK, and the agent loops inside VS Code, Visual Studio and Office. Every performance figure in the post is GitHub's own and none is independently reproduced.
Why it matters: This is a named production migration with counts attached, which is a firmer datapoint on agent-assisted work than a benchmark score.
Follow-up: Track whether GitHub publishes defect or incident data for the Rust runtime against the implementation it replaced.