Anthropic is drawing criticism for being the only major frontier AI lab not to sign an open letter advocating for open-weight AI, as Washington considers restrictions on some Chinese models. On Friday, over two dozen companies including Nvidia, Meta, and Microsoft signed "Open Weights and American AI Leadership," endorsing open-weights. Over the weekend, OpenAI, Google, and SpaceX also signed. As of Monday, the only major frontier AI lab not to back the defense of open models is Anthropic. "The entire tech industry (save for Anthropic) has come out in favor of open source AI," said David Sacks. "They won't stop until they kneecap open source," he wrote. "Proud that @OpenAI signed the letter. Ant is silent on it," wrote Peter Steinberger. Bill Gurley criticized Anthropic, writing, "Excited to see Anthropic acknowledge the real problem with open is it competes with their corporate economic strategy." Kai-Fu Lee wrote, "Who DIDN'T sign the Open Weight letter is far more interesting than who did." Anthropic did not immediately respond to a request for comment. Anthropic CEO Dario Amodei has previously warned that developers lose control of frontier AI models once their weights are released. The debate has been supercharged by the launch of Kimi K3, an open-weight model from Chinese lab Moonshot. Last week, a top White House official accused Moonshot of using Anthropic