Microsoft built Copilot Cowork, an agentic execution layer for multi-step workflows, taking it from an empty repo to worldwide GA in about four and a half months before moving its runtime from Python to Rust while live. On October 21, nine engineers merged 1,211 pull requests into the main branch of the Copilot Cowork backend. Agents wrote most of the code, but engineers defined the work and judged results, writing clearer specs, setting architecture and security boundaries, and focusing review on higher-risk changes, production impact, and disagreements among checks or reviewers. Automated checks and agent reviewers examined every change; agents could propose changes but not decide what was allowed. The team adopted a rule of no code without a markdown spec written first, covering what the change does, how success will be known, and what it must not break. Six practices emerged covering specifying work, running agents within clear boundaries, testing checks, preserving team learning, shaping the codebase for parallel work, and limiting automated loops.