GitHub says it moved the Copilot agent runtime to roughly 800,000 lines of Rust, using Copilot throughout the migration. The project does not show an agent replacing a team; it shows automation changing the economics of a rewrite that was previously hard to justify.
Quick answer
| Challenge | GitHub approach |
|---|---|
| Scale | Progressive migration of a large runtime. |
| Tool | Copilot agents for translation, testing and fixes. |
| Control | Human review, tests and behavioral comparison. |
Why Rust
An agent runtime handles concurrency, tools, events and isolation across platforms. Rust offers strong memory control and native distribution, with a learning curve and demanding builds.
What agents accelerate
They can translate repetitive patterns, prepare tests and process batches in parallel. Engineers remain accountable for architecture, invariants and behavioral differences that function signatures do not reveal.
The superficial-parity trap
A compiling port is not necessarily equivalent. Teams must compare traces, errors, performance and recovery. Changes from parallel agents also need a merge strategy and clear file ownership.
The practical lesson
Start with a measurable component, keep a reference implementation and automate differential tests. AI makes migration more affordable but does not reduce the need for specifications. It makes acceptance criteria more important.
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