Anthropic has issued a formal warning to investors about unauthorized trading of its shares. The AI company updated its website this week, naming eight specific platforms it says are not permitted to facilitate buy or sell transactions in Anthropic stock. The list includes Open Doors Partners, Unicorns Exchange, Pachamama Capital, Lionheart Ventures, Hiive, Forge Global, Sydecar, and Upmarket. The company's blog post explicitly states that "any sale or transfer of Anthropic stock, or any interest in Anthropic stock, offered by these firms is void and will not be recognized on our books and records." Forge Global pushed back on its inclusion, telling TechCrunch the company was added in error. "We are working with Anthropic to remove Forge's name from this alert," the platform said. "Forge does not facilitate transactions in any private company's shares without the explicit approval of the company." The warning comes as demand for AI company shares has surged through secondary markets and alternative investment vehicles. Anthropic, which is reportedly seeking fresh funding at a $900 billion valuation, has become particularly difficult to source. Some secondary market brokers recently told TechCrunch that Anthropic is among the "hardest" stocks to obtain. Over the past year, platforms including crypto exchange OKX have launched investment products offering exposure to AI companies through pre-IPO perpetual futures contracts and special purpose vehicles, which allow investors to buy shares in entities holding stakes in private companies. Anthropic emphasized that both its preferred and common stock carry transfer restrictions, meaning transactions conducted through unauthorized channels carry significant legal and financial risk for investors.