Microsoft files amicus brief supporting Anthropic in Pentagon fight

The technology giant Microsoft has filed an amicus brief in Anthropic’s case against the Trump administration, urging the court to temporarily block the implementation of the Pentagon labelling the Claude-maker as a supply chain risk.  

The filing is a significant development for Anthropic, which first filed a complaint against the administration on Monday over the Pentagon’s determination and President Trump’s recent order for federal agencies to cease use of Anthropic’s AI products after negotiations over safety guardrails fell apart.  

A Microsoft spokesperson told The Hill on Tuesday that the company believes “everyone involved shares common goals and we need time and a process to find common ground.”

“The Department of War needs reliable access to the country’s best technology,” the spokesperson said. “And everyone wants to ensure AI is not used for mass domestic surveillance or to start a war without human control. The government, the entire tech sector, and the American public need a path to achieve all these goals together.”  

The filing comes just hours before a federal judge in northern California will take up Anthropic’s request for a temporary block of the designation. Anthropic also filed suit in the D.C. federal appeals court, requesting a review of the Pentagon’s determination.  

Microsoft’s amicus brief, shared with The Hill Tuesday, argues a pause in the designation will help avoid “disrupting” the U.S. military’s ongoing uses of advanced AI. The company, which has partnered with Anthropic on numerous products, warned U.S. warfighters could be “hampered” if companies are forced to immediately alter their existing products and contract configurations with the Pentagon.  

Any immediate implementation, Microsoft said, could have “broad negative ramifications” for the “entire technology sector” and “American business community.” 

Anthropic is asking the court to ultimately reverse the Pentagon’s designation, which has typically been reserved for foreign adversaries and restricts defense contractors from using the company’s products. The AI firm alleges the federal government retaliated against the firm for its “protected viewpoint” on AI safety and the limitations of its own AI models. 

While workers from AI firms OpenAI and Google separately filed an amicus brief Monday in Anthropic’s complaint, Tuesday’s filing makes Microsoft the first standalone company to voice support for such a pause.  

Despite the Pentagon’s position, Anthropic has argued the restrictions cannot prohibit anyone who can do business with the military from doing business with the AI firm. Google, Amazon and Apple said last week that Anthropic’s AI tools will still be available on their platforms for work that does not involve the Pentagon. 

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