Migrating the GitHub Copilot runtime to Rust, using Copilot
Blog post from GitHub
GitHub and Microsoft rewrote the shared Copilot agent runtime from TypeScript/Node.js to more than 832,000 lines of production Rust over roughly 14.5 weeks, using AI agents for most implementation work while shipping incremental changes through 128 pull requests and maintaining active product development. The runtime powers the Copilot CLI, Copilot app, SDKs, VS Code, Visual Studio, Copilot Code Review, Microsoft 365 integrations, and other products, and the migration aimed to eliminate Node/V8 startup, memory, process-management, and cross-process JSON-RPC costs while enabling native in-process embedding across TypeScript, Python, Go, C#, Java, and Rust SDKs through a C ABI. Rather than conducting a high-risk cutover, the team replaced components atomically in place, validating every step with end-to-end tests, staged releases, code review, agent-driven merge automation, and human oversight focused on architecture, constraints, and final decisions. Session-log analysis showed that agents spent far more effort reading, searching, testing, coordinating, and reviewing than editing code, while prompt caching kept token costs manageable despite approximately 136 billion tokens and a reported $120,000 model bill. The port exposed limitations of compilation and static typing as correctness guarantees, with regressions arising mainly from behavioral differences, omitted features, lifecycle changes, library substitutions, and rebasing drift, reinforcing the importance of protected end-to-end tests and explicit agent instructions. Benchmark results indicated substantial gains, including an 18-fold improvement for an in-process client/session/turn scenario and sharply reduced CPU and memory use, while the authors emphasize that further architectural cleanup, concurrency improvements, and performance optimization remain ahead.
| Trend | Post Mentions | Total Month Mentions | Posts | Companies | MoM |
|---|---|---|---|---|---|
| AI Coding Assistant | 46 | 341 | 115 | 55 | -77% |
| Subagents | 14 | 15 | 10 | 7 | -95% |
| AI Agents | 5 | 931 | 231 | 103 | -84% |
| MCP | 4 | 2,241 | 148 | 72 | -74% |
| Real-time | 3 | 649 | 155 | 80 | -85% |
| Developer Experience | 1 | 131 | 58 | 24 | -72% |
| LLM | 1 | 747 | 162 | 79 | -85% |
| OpenTelemetry | 1 | 125 | 18 | 15 | -83% |
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