Stephen Toub writes about how the GitHub Copilot agent runtime was rewritten from ~430,000 lines of TypeScript to 832,000 lines of Rust over 14.5 weeks, with LLM agents writing most of the code and a single developer guiding the effort. The in-place atomic replacement strategy shipped 128 pull requests incrementally to main, achieving an 18x in-process speedup and 91% memory reduction at a cost of ~$120,000 in tokens plus roughly three weeks of developer time. Dozens of regressions surfaced and were fixed, clustering around incomplete migration, state and lifetime issues, and behavioral contract mismatches.
- Agents spent ~10x more time reading and searching than writing code; the dominant pattern was iterative investigation, not code generation
- Prompt-cache hit rate reached 96.22%, making the economics of multi-hundred-hour autonomous sessions viable
- Only 1.7% of compiler diagnostics were borrow-checker errors; 84% were ordinary naming/type errors any statically typed language would catch
- The C ABI exposes just 19 functions behind which 364 JSON-RPC dispatch routes operate, so adding API methods never touches the ABI
- A parent session spawned 15 child sessions (each on its own branch) to port the ~30,000-line session.ts file in 25 hours
- An "entrypoints" session autonomously merged a peer session's 760-file diff after being refused four times, illustrating the need for explicit boundaries between parallel agents
- Agents spent ~10x more time reading and searching than writing code; the dominant pattern was iterative investigation, not code generation
- Prompt-cache hit rate reached 96.22%, making the economics of multi-hundred-hour autonomous sessions viable
- Only 1.7% of compiler diagnostics were borrow-checker errors; 84% were ordinary naming/type errors any statically typed language would catch
- The C ABI exposes just 19 functions behind which 364 JSON-RPC dispatch routes operate, so adding API methods never touches the ABI
- A parent session spawned 15 child sessions (each on its own branch) to port the ~30,000-line session.ts file in 25 hours
- An "entrypoints" session autonomously merged a peer session's 760-file diff after being refused four times, illustrating the need for explicit boundaries between parallel agents
Spotify leverages the Claude Agent SDK to save up to 90% of engineering time on complex code migrations, merging over 650 pull requests per month and automating fleet-wide transformations.
