Microsoft is swapping the AI inside Excel. You didn't pick it.

The AI answering your Excel formulas and summarizing your Outlook threads changed this week — and there was no announcement, no toggle, and no way for you to opt out. Microsoft has quietly started routing Microsoft 365 Copilot prompts through its own in-house MAI models instead of OpenAI's, and the reason is the one that should get every operator's attention: cost.

What actually happened

Per reporting from Bloomberg on July 7, 2026 (summarized by The Decoder), Microsoft has begun serving tens of thousands of weekly Copilot prompts in Excel and Outlook from its homegrown MAI model family — the first disclosed production-scale use of its own AI inside 365 Copilot. Routine, high-volume tasks like structured-data manipulation in Excel and email summarization in Outlook are being offloaded to MAI, while more open-ended work can still route to OpenAI or Anthropic.

The motive isn't subtle. Microsoft AI chief Mustafa Suleyman said the quiet part in June: "We pay a lot of money to Anthropic — so our goal is to reduce and ultimately eliminate that cost." This is a server-side change. There's no user setting, no version note, and no way to tell which model handled any given prompt. The interface is identical; the engine underneath is not.

Why it matters for your business

You are not the customer for a decision like this — you're the surface it happens on. When AI is bundled into a SaaS product you rent, the vendor controls which model runs, when it changes, and what tradeoff between quality and margin they're comfortable making on your behalf. Today it's a cost cut Microsoft is confident won't be noticed. It might be right. But "trust us, you won't notice" is not a quality guarantee you can put in front of your own customers.

The takeaway isn't "distrust Copilot." It's that anywhere AI output actually matters to your business — the numbers in a client report, the tone of a customer email, the logic in an automation — you want to own the model choice, not inherit it silently. Bundled AI is fine for convenience work. For anything you'd stake your name on, build it where you decide what's answering, and can prove it hasn't changed under you.

Sources: The Decoder — Copilot goes cheap as Microsoft phases out OpenAI and Anthropic models, Winbuzzer — Microsoft shifts AI workloads to MAI models.