The scale gets your attention
GitHub says its Copilot agent runtime has been rewritten from TypeScript into more than 800,000 lines of production Rust. In Stephen Toub’s account, AI agents wrote most of the code and the work landed through 128 pull requests, primarily led by one developer over a few months.
The runtime supports several Copilot products. This was an ongoing production migration, not a toy application generated from a blank prompt.
The method is the part worth borrowing
The team moved components incrementally rather than waiting for one enormous replacement. Existing end-to-end tests exercised the new implementation as work progressed, and the account describes regressions and fixes.
For a smaller codebase, the useful lesson is to define a narrow boundary and a way to check behavior before asking an agent to replace it. A successful first component is stronger evidence than a huge diff that nobody has run.
Line count is not a quality score
Rust fit the team’s embedding and resource requirements. The post does not argue that every TypeScript application should be rewritten, and the graph includes work added during the migration.
If you try a similar approach, compare startup behavior, memory, compatibility and failing cases. Keep reviewable checkpoints so a bad change can be isolated. The striking number is the hook; the ability to ship and verify each step is the more transferable idea.
Source published 2026-09-16. Coverage is based on the maker’s announcement and demonstration.