Nine engineers, 1,211 PRs in a day: Lessons from building Copilot Cowork

Microsoft's Command Line blog describes how nine engineers took Copilot Cowork from an empty repo on January 13 to general availability on June 16, with agents writing most of the code, then replaced the live Python runtime with Rust by October 10. It reports 1,211 merged PRs in one day and median time to first token falling from 7.9 s to 2.7 s, partly because the agent loop also moved. Practices include specs first, replayable fixtures, and checks that must fail on purpose. All figures are self-reported.