Microsoft, which has invested up to $5 billion in Anthropic and holds a $30 billion Azure compute agreement with the company, filed an amicus curiae brief supporting Anthropic’s lawsuit against the U.S. Department of Defense (DoD). The DoD’s “supply chain risk” designation immediately blocks any defense-related firm from engaging commercially with Anthropic, creating compliance and business risks for Microsoft, whose Copilot and Azure products integrate Anthropic’s Claude. Microsoft argues that while the DoD granted itself a six-month phase-out period, it forced contractors to comply immediately, imposing an unfair burden. The brief challenges the unprecedented use of 10 U.S.C. § 3252—a statute historically reserved for foreign adversaries—against a domestic AI startup. Microsoft warns that such actions, if upheld, present existential risks to any tech provider in the federal supply chain, given the interconnectedness of industry offerings. Microsoft seeks only a temporary restraining order to halt the block, emphasizing principles of due process, orderly transitions, and the risks of using procurement law for policy disputes. The company does not endorse Anthropic’s AI safety positions but wants time for negotiation and continued legality for its own products.