Microsoft Backs Anthropic in Legal Fight Against Pentagon Blacklisting

Microsoft has thrown its weight behind Anthropic's legal challenge against the Pentagon, filing an amicus brief that argues the AI company's designation as a "supply-chain risk" should be temporarily blocked.

In a filing to a federal court in San Francisco, Microsoft—itself one of the Pentagon's most deeply embedded tech contractors—said a temporary restraining order was necessary to prevent serious disruption to suppliers whose products rely on Anthropic's technology. Google, Amazon, Apple, and OpenAI have also signed on to a brief in support of Anthropic.

The dispute stems from collapsed contract negotiations last month over a $200 million deal to deploy Anthropic's AI on classified military systems. Talks fell apart after Anthropic insisted its technology should not be used for mass surveillance of US citizens or to power autonomous lethal weapons. Defense Secretary Pete Hegseth responded by designating Anthropic a supply-chain risk—a designation previously applied only to companies with ties to foreign adversaries like China, and never before used against a US company.

"The Department of War needs reliable access to the country's best technology," Microsoft said in a statement to The Guardian. "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."

On Thursday, the Pentagon's chief technology officer told CNBC there's "no chance" the agency would renegotiate with Anthropic after the designation. Anthropic has launched two lawsuits—one in federal court in California and one in the DC circuit—challenging the Pentagon's decision.