Anthropic just walked into Microsoft's house and asked for the keys. Cowork launched on macOS a month ago. Within weeks, The Information reported the launch "reverberated through Microsoft's ranks." Now Anthropic is bringing full feature parity to Windows, the platform Microsoft has spent $60M+ in TV ads and $37.5B per quarter in AI capex trying to own with Copilot. The math on Microsoft's side is brutal. 450 million Microsoft 365 paid seats, 15 million Copilot subscribers. That's a 3.3% conversion rate on their own customer base. Copilot's paid market share dropped from 18.8% to 11.5% in six months while the product was pre-installed on every Windows 11 machine on earth. They spent billions and 96.7% of users said no. The reason is structural. Copilot inherits Microsoft 365's file permission system through the Graph API. If your company has lax internal permissions (and most do, with 15%+ of business-critical files improperly accessible), Copilot surfaces sensitive data to anyone who asks. Enterprises are pausing deployments to run months-long data governance audits before they can turn the thing on. Cowork sidesteps this entirely. Sandboxed to a single folder you designate. No enterprise-wide permission architecture required. No Graph API inheritance. You point Claude at a directory and describe the outcome. Anthropic built it in a week and a half using Claude Code, which tells you everything about how they view the marginal cost of shipping new surfaces. This is the classic platform unbundling play: find the incumbent's structural weakness (Microsoft's permission nightmare), build a product that routes around it (folder-scoped agent), and land on their platform (Windows) before they can fix the root cause. Microsoft can't simplify Copilot's permission model without rebuilding how M365 handles file access across tenants. That takes years. Anthropic is converting Microsoft's demand generation into Claude subscriptions. At $100-200/month for Max, they only need a fraction of those 450 million seats to build a massive business on Microsoft's home turf.