Microsoft backs Anthropic in its legal fight against the Pentagon

• Microsoft is showing its support for Anthropic.
• In a legal filing, Microsoft urged a federal court to temporarily block Anthropic's blacklisting.
• On Monday, Anthropic filed to stop the Pentagon from labeling the company as a supply chain risk.

Microsoft is squarely in Anthropic's corner.

In a legal filing on Tuesday, Microsoft warned that the Pentagon's effective blacklisting of Anthropic has "negative ramifications for the entire technology sector and American business community."

"This is not the time to put at risk the very AI ecosystem that the Administration has helped to champion," a lawyer for the tech giant wrote in a proposed amicus brief to the California federal court overseeing Anthropic's case.

On Monday, Anthropic sued Hegseth, the Pentagon, the Executive Office of the Presidency, and a host of federal agencies in a bid to both temporarily block Hegseth's order and to permanently prevent its implementation.

Microsoft, as it stated in the filing, has a vested interest in whether Defense Secretary Pete Hegseth's decision to label Anthropic and its products as a supply chain risk remains in place. In November, Microsoft pledged to invest up to $5 billion in Anthropic, as part of an expanded partnership between Anthropic, Microsoft, and Nvidia.

"Immediate implementation of the Determination imposes substantial and wide-ranging costs and risks on Microsoft and other U.S. government contractors that partner with Anthropic and rely on Anthropic's technology as a foundational layer of their own products and services, which they provide to the U.S. military," Microsoft's lawyers wrote in the filing.

In the brief, Microsoft echoed Anthropic CEO Dario Amodei's concerns about how AI could be abused if the Pentagon did not take into account proper safeguards.

"Microsoft's position is that AI should be focused on lawful and appropriately guarded use cases. For example, AI should not be used to conduct domestic mass surveillance or put the country in a position where autonomous machines could independently start a war," the company's lawyers wrote in the filing.

Anthropic has received support for its fight from Silicon Valley. OpenAI CEO Sam Altman had urged the Pentagon not to follow through with the supply chain designation, even as OpenAI faced intense criticism for a rushed deal with the Defense Department.

Dozens of OpenAI and Google employees, in their personal capacities, including Google's chief scientist Jeff Dean, backed a separate amicus brief on Monday opposing the Pentagon's action against Anthropic.

Last week, the Pentagon formally informed Anthropic that it was taking the unprecedented step of requiring defense contractors to cease doing business with Anthropic. Related to the action, President Donald Trump has also ordered all federal agencies to phase out their usage of Anthropic's models within the next six months.

Microsoft noted that Hegseth's formal determination does not have the same six-month window for contractors that the federal government gave itself to transition away from Anthropic's products.

"By setting a six-month period for DoW itself to transition services away from Anthropic, DoW recognized that, even if the Determination stands, an orderly transition takes time," the filing stated. "But the Determination does not provide the same transition period for contractors that may use Anthropic products or services to perform under DoW contracts."

Hegseth and top Pentagon officials have rejected Amodei's concerns about how AI could be deployed in fully autonomous weapons or potentially abused to spy on Americans.

A spokesperson for Microsoft said all sides "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 in a statement to Business Insider. 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."