Microsoft 365 Copilot has switched from Anthropic’ s Claude to OpenAI's new GPT-5.6, the company's latest flagship model family, as the preferred AI model across Word, Excel, PowerPoint, Copilot Chat and Cowork. The change comes two months after Microsoft made Anthropic's Claude the default model in several of the same applications, and marks the latest shift in an ongoing contest between the two AI labs for position inside Microsoft's productivity platform.
GPT-5.6, which OpenAI describes as a flagship series that "delivers more useful work from every token, with stronger performance per dollar and on demand capability for the most complex tasks," launched July 9 in three variants - Sol, Terra and Luna.
The top-tier Sol model, which GitHub describes as suited to "complex reasoning over large codebases and demanding, long-running agentic work," became available in GitHub Copilot the same day after the Trump administration approved a broad release.
"We can't wait for customers to see what GPT-5.6 in Microsoft 365 will do, enabling them to work even more effectively with AI in the tools they use every day," said Nitin Agrawal, President, Copilot & Agents Core at Microsoft, in the announcement.
"We're excited to continue building with OpenAI to bring more powerful AI experiences to people and organizations around the world."
"Microsoft 365 is where millions of people write, analyse, create, and collaborate every day," said Nikunj Handa, OpenAI's Head of API Product.
"By bringing GPT-5.6 to Microsoft 365 Copilot through the OpenAI API, we're helping organizations get more useful work from every token, and more value from AI in the tools they already use."
The announcement states that in addition to serving the models natively, Microsoft "will also access OpenAI models directly through the API." Under the partners' April 2026 amended agreement, Microsoft remains OpenAI's primary cloud provider and OpenAI products ship first on Azure, but OpenAI may now serve customers on any cloud.
How Claude gained ground in Copilot
The new preferred-model designation follows a period in which Anthropic's models took on a progressively larger role inside Copilot. Claude models first arrived in Microsoft 365 Copilot in September 2025, powering the Researcher agent and Copilot Studio. In January 2026, Anthropic became a formal Microsoft subprocessor, with Claude enabled by default in commercial tenants outside the EU, EFTA and the UK.
In May 2026, Claude became the default model for Copilot in Excel and PowerPoint, with Word scheduled to follow later in 2026.
Microsoft has presented the strategy as a response to the pace of model releases.
"Every 60 days at least, there's a new king of the hill," Jared Spataro, Microsoft's chief marketing officer for AI at Work, told Fortune in March.
"There's so much demand for a platform that doesn't feel like, 'I have to skip over to the next vendor.'"
Cowork: OpenAI's model on Anthropic's architecture
Among the applications where GPT-5.6 takes preferred status is Cowork, Microsoft's agentic workspace for long-running, multistep tasks. Copilot Cowork was built in close collaboration with Anthropic, and at launch used Claude as its reasoning model along with the same agentic harness as Anthropic's Claude Cowork desktop product.
Microsoft has positioned Copilot Cowork as a direct competitor to Anthropic's product.
"OpenAI can't afford to be constrained to a single cloud while rivals like Anthropic are aggressively expanding through multicloud and direct enterprise channels," Dion Hinchcliffe, VP and practice lead at The Futurum Group, told CIO Dive.
Microsoft has not said whether the May Claude defaults in Excel and PowerPoint will be reversed, or how the preferred-model designation will interact with the existing model picker in Copilot Chat.