Microsoft has quietly begun replacing AI models from OpenAI and Anthropic with its self-developed "MAI" models in flagship office software such as Excel and Outlook, according to people familiar with the matter. Currently, tens of thousands of AI prompts are being processed by the in-house models each week, marking the first time the scale of Microsoft's MAI model deployment in office software has been disclosed. While MAI models still account for a relatively small portion of Microsoft's overall AI usage, the move signals progress in building low-cost, high-performance AI models. Following the news, Microsoft shares rose 2% in Tuesday trading. Mustafa Suleyman, head of Microsoft's AI models, stated publicly last month that the company is reducing its reliance on Anthropic, saying, "We pay Anthropic a significant amount of money annually, so our goal is to reduce, and eventually eliminate, that cost entirely." At the Build developer conference in June, Microsoft unveiled seven new AI models, with one reportedly achieving code generation capabilities comparable to Anthropic's previous-generation programming model Opus 4.6 at a lower cost. Beyond Excel and Outlook, MAI models have also been integrated into GitHub Copilot, and Suleyman revealed that in the coming months, Microsoft's in-house speech transcription models will be progressively applied to Teams video conferencing and other products.