GitHub ported the Copilot runtime to 800,000 lines of Rust in 14.5 weeks with one developer and agents writing most of the code

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GitHub replaced roughly 430,000 lines of TypeScript/Node.js in the Copilot agent runtime with more than 800,000 lines of production Rust across 128 incremental pull requests and 135 releases between May and August 2026, with a single primary developer and agents authoring most of the code. The old design forced SDK consumers to spawn a separate CLI process, adding over 100 MB of memory per client and out-of-process JSON-RPC. GitHub logged 12.76 million session events during the port and reports a 96.22% prompt-cache hit rate, which is what made long autonomous sessions affordable.