Microsoft's flagship office suite is now running on Anthropic's model by default. As of May 4, an internal Message Center notice (MC1269241) took effect that flips Anthropic Claude on as the default model in Microsoft 365 Copilot for Excel and PowerPoint, with Word scheduled to follow in the summer. For most commercial tenants, the change happened automatically.
This is a meaningful shift. Copilot has been a primarily OpenAI-powered product since its launch. Microsoft owns about half of OpenAI's commercial entity and embedded its models everywhere from Bing to GitHub to Windows. Multi-model availability was announced last year, but turning Claude on by default in Excel and PowerPoint signals something different. Microsoft is hedging.
The geography is uneven. For UK and EU/EFTA tenants the toggle exists but is set to off, because Microsoft has not extended its EU Data Boundary commitment to cover prompt routing through Claude. A pricing model an analyst writes in London on May 5 was, by default, processed by a different model than the same workbook open on a colleague's desk in Texas. The discrepancy lives at Copilot, Settings, View all, AI providers operating as Microsoft subprocessors, and is governed by a new AI Administrator role.
For Anthropic, the rollout is the most consequential distribution win of its corporate life. Claude is now plumbed into the most widely used productivity software in the world. The company is also separately raising at a $900 billion valuation and paying $1.25 billion a month to xAI for Colossus compute. The Microsoft default is the kind of distribution outcome that makes those numbers easier to underwrite.
What the default actually changes for a daily Copilot user is harder to say. Microsoft has not published benchmarks comparing Claude-routed and OpenAI-routed outputs inside Excel formulas or PowerPoint deck generation, and the routing decision itself is somewhat opaque. Power users will notice differences in tone and formatting; admins inherit a new compliance surface. For most users, the change is invisible.
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