Cerebras Systems stock has had a rough time since its much-hyped initial public offering earlier this year. Fears that it could lose some of its OpenAI business to Nvidia aren't helping.

Cerebras shares dived 7% to $181.40 Wednesday. Late Tuesday, semiconductor research firm SemiAnalysis said in a social media post that OpenAI is using Nvidia chips rather than Cerebras hardware to power "Ultrafast" mode for its latest GPT6.1 Sol artificial-intelligence model.

Cerebras and OpenAI didn't immediately respond to requests for comment. But if confirmed, the news raises significant concerns for Cerebras. OpenAI is its biggest customer by revenue backlog, and the two companies specifically announced Cerebras would be powering Ultrafast mode when it was launched in August.

Barron's previously wrote that Cerebras stock could be risky because of its high valuation and the company's dependence on OpenAI. The stock opened trading at $350 following its IPO in May. But initial enthusiasm about the potential for the company's unusually large AI chips-which excel at running models at high speeds-has faded as earnings failed to impress investors.

Even if it is confirmed that OpenAI isn't currently using Cerebras for its latest model, it still could in the future. There could be capacity or optimization issues which have to be resolved. But for a stock already struggling to impress, it's another headwind.