Microsoft has given Anthropic its strongest industry endorsement yet by filing a court brief in a San Francisco federal court that backs the AI company’s legal challenge against the Pentagon’s supply-chain risk designation. The brief called for a temporary restraining order and argued that the designation poses an immediate and serious threat to technology supply chains relied upon by national defense. Amazon, Google, Apple, and OpenAI have also endorsed Anthropic through a joint filing, making this the most broadly backed legal intervention in the AI industry’s history.
Anthropic’s legal fight began after the Pentagon labeled it a supply-chain risk following the breakdown of a $200 million contract in which the company refused to allow its Claude AI to be used for mass surveillance or autonomous lethal weapons. Defense Secretary Pete Hegseth formalized the designation, triggering the cancellation of Anthropic’s government contracts. Anthropic filed two simultaneous lawsuits in California and Washington DC, arguing the designation was unconstitutional.
Microsoft’s endorsement carries special weight given its direct use of Anthropic’s technology in federal military systems and its partnership in the Pentagon’s $9 billion cloud computing contract. Additional federal agreements spanning defense, intelligence, and civilian agencies further deepen Microsoft’s stake in the outcome of this case. Microsoft publicly argued that responsible AI governance and robust national security were goals the government and technology sector needed to pursue together.
Anthropic’s court filings argued that the supply-chain risk designation was an unconstitutional act of ideological retaliation for the company’s public advocacy of responsible AI development. The company disclosed that it does not believe Claude is currently safe or reliable enough for lethal autonomous operations, which it said was the genuine basis for its contract demands. The Pentagon’s technology chief publicly ruled out any renegotiation.
Congressional Democrats have separately demanded answers from the Pentagon about whether AI was used in a strike in Iran that reportedly killed over 175 civilians at a school. Their formal letters ask about AI targeting systems and human oversight. Together, Microsoft’s strongest endorsement, the industry coalition, and congressional scrutiny are intensifying the legal fight and elevating Anthropic’s case to a matter of national importance.