GitHub rewrote 800,000 lines of Rust using Copilot agents — one engineer in months
marlene_zw · x · 2026-09-23
What happened
GitHub published a detailed blog post on rewriting the Copilot agent runtime from TypeScript/Node.js into 800,000+ lines of production Rust, with AI agents writing most of the code.
Key facts
- Primarily one engineer + a team of agents, completed in a few months — work that previously would have taken a whole team one to two years
- Shipped incrementally via 128 PRs merged to main rather than a single cutover; regressions were caught and fixed quickly along the way
- Runtime performance improved by orders of magnitude
- The runtime backs Copilot CLI, Copilot App, Copilot SDK, and the cloud agent
Why it matters
The authors note a rewrite of this scale was "not affordable" before agents — a concrete proof point for how coding agents reshape engineering economics.
Related event: Solo engineer rewrites Copilot runtime in Rust with AI agents, 800k lines(2 posts)→
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