An open letter urging the US to support rather than restrict open-weight AI models has received public support from all major tech companies except Anthropic. CEO Dario Amodei explained why in a Monday blog post, stating he "agrees with much" of the letter but disagrees with its assertion that open models help defenders more than attackers. The letter, initially backed by 25 companies including Microsoft and Nvidia, has grown to 77 signatories, including OpenAI, Google, and SpaceX. Amodei wrote that Anthropic has "never advocated for a ban on open-weights models," noting they "expand access to the AI economy" and "strengthen competition." However, he argued, "It seems at least as likely to me that the opposite will be true" regarding whether open models aid defenders. He cited the risk of a malicious group using AI to create a bioweapon based on a pandemic-level virus, calling defense "a multi-year operational task." Amodei’s larger message is that a strict "protectionist ban" on US businesses adopting open-weight AI would fail to address national security concerns, including the risk of China developing a more powerful AI for "permanent military superiority." He added that open-weights models "do potentially present a higher risk than closed models," but "banning the use of these models by US businesses does nothing to address this risk, because bad actors are unlikely to be legitimate US businesses."