Cerebras Systems stock fell nearly 20 percent this week to its lowest level since the chipmaker’s initial public offering in May, after a report that Nvidia will power a key feature of OpenAI’s newest model rather than Cerebras hardware.

The shares closed at $166.43 on Friday. The company, which ended its first day of trading with a market value of $95 billion, is now worth just over $39 billion, and the stock has lost more than half its value since the pop that followed its debut on the Nasdaq.

The damage started Wednesday, when the research firm SemiAnalysis posted on X that OpenAI will run the “Ultrafast” mode for its GPT-6.1 Sol model on Nvidia graphics processing units instead of Cerebras chips.

A second pressure came from inside the company. Up to 19.4 million shares held by directors, officers, non-executive employees and non-employee holders became available for sale on Wednesday, equal to 8 percent of shares outstanding, according to the Cerebras prospectus. Before that, as many as 14.6 million shares have unlocked every two weeks since Aug. 19.

The stock clawed back almost 3 percent in extended trading on Friday after Sam Altman, the chief executive of OpenAI, posted on X in response to what he called “speculation about our partnership with Cerebras.” “Cerebras is a close partner, and we have a deep engagement pushing on the frontiers of speed,” Mr. Altman wrote.

Cerebras went public on May 14, and its first-day close fell just short of the group of companies, including Meta, Alibaba and now SpaceX, that ended their initial trading days valued above $100 billion. Investors had embraced the prospect of an alternative to Nvidia in artificial intelligence.

Cerebras makes dinner plate-sized ASICs, custom chips built for inference workloads, and leases them from its own data centers as a cloud service. In January, it struck a deal worth more than $10 billion to supply OpenAI with 750 megawatts of computing power through 2028. According to McKinsey, inference is set to pass training as the dominant A.I. data center workload by the end of the decade, so losing OpenAI inference traffic to Nvidia lands hard.

The selling has not been limited to public investors. Andrew Feldman, the chief executive, and Sean Lie, the chief technology officer, both made billionaires by the offering, sold more than $240 million of Class A shares between Aug. 20 and Sept. 25 under trading plans adopted shortly after the I.P.O. Other executives have sold shares worth millions of dollars.