Microsoft Files Amicus Brief Supporting Anthropic

On March 10, 2026, Microsoft filed an amicus curiae brief in the U.S. District Court for the District of Columbia supporting Anthropic’s legal challenge to the Pentagon’s supply chain risk designation. The brief advocated for a temporary restraining order to halt the 180-day removal timeline while the case was adjudicated, arguing that the designation created “immediate and irreparable harm” not just to Anthropic but to the entire technology industry’s relationship with the federal government.

Microsoft’s intervention was significant on multiple levels. As the largest investor in OpenAI — the company that had just signed a deal to replace Anthropic on Pentagon classified systems — Microsoft was effectively supporting its investment’s competitor against its investment’s new customer. The filing reflected a calculation that the precedent set by the supply chain designation posed a greater long-term threat to Microsoft’s own government business than any competitive advantage OpenAI might gain from Anthropic’s exclusion.

The brief argued that the supply chain risk framework, if applied to domestic companies for commercial disagreements, would create “a chilling effect on technology companies’ willingness to engage with the Department of Defense.” Microsoft cited its own decades-long history as a defense contractor and argued that every technology company providing services to the Pentagon maintained some form of use-case restrictions or safety policies. If refusing to modify those policies constituted a supply chain risk, then “no technology company can safely contract with the federal government.”

Microsoft also disclosed that it had received informal inquiries from Pentagon officials about its own AI safety policies following the Anthropic designation — a detail that underscored the industry-wide implications. The filing represented a rare moment of solidarity in the fiercely competitive AI industry, driven by the shared recognition that the government’s actions threatened the basic terms on which technology companies engage with defense work.