Github rewrote its copilot agent runtime into 800,000 lines of production rust, and the engineers…
github rewrote its copilot agent runtime into 800,000 lines of production rust, and the engineers say a rewrite this size was not affordable before agents. september 16, the change went out with the claim that agents finally make large-scale rewrites cheap.
the economics of maintenance just flipped.
Context
GitHub's blog post of 16 September 2026 by Stephen Toub says a rewrite this size was not affordable before agents, that the Copilot runtime migrated to more than 800,000 lines of production Rust, and that AI agents wrote most of the code across 128 pull requests.
More than 800,000 is a lower bound, and secondary posts cite 832,378 lines, a rounding difference and not a conflict. The agents making large rewrites cheap is GitHub's own framing. A dollar token cost figure from The Register is secondary and not in the GitHub post. The economics of maintenance flipping is the author's opinion.
Related work
- Earlier note on the same rewrite ↗Same GitHub post.
Watch next
- Any dated GitHub follow-up on the migration.
Sources
Provenance
The note above is reproduced unedited from the original post, first published on Threads on 19 September 2026 at 01:47 IST. Sources are the papers and datasets the note draws on.
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