Cerebras Systems tumbled nearly 20 percent this week to a post-IPO low of $166.43, cutting its market value to just over $39 billion from the $95 billion notched on its May debut. The slide accelerated after SemiAnalysis reported that OpenAI will run the "Ultrafast" tier of its new GPT-6.1 Sol model on Nvidia GPUs rather than Cerebras silicon, undercutting the core investment thesis that the startup could carve out a meaningful slice of inference demand.
The OpenAI pivot
SemiAnalysis posted the finding on X Wednesday, stating that Nvidia hardware will power the speed-optimized mode of OpenAI's latest model. That disclosure matters because inference, not training, is projected by McKinsey to become the dominant data-center workload before 2030. Cerebras had secured a contract worth more than $10 billion in January to deliver 750 megawatts of compute to OpenAI through 2028, a deal predicated on the startup's dinner-plate ASICs handling exactly this kind of low-latency serving. Losing the flagship "Ultrafast" tier to the incumbent raises questions about whether that contract volume will materialize as modeled.
Altman pushes back
OpenAI chief executive Sam Altman responded on X Friday, calling Cerebras a close partner with deep engagement on speed frontiers. The stock recovered roughly 3 percent in after-hours trading on the comment, but the session's gain barely dented the week's decline. The market's skepticism is readable: a partnership affirmation without a workload commitment is not a revenue guarantee.
The unlock overhang
Pressure also came from the expiration of post-IPO lockups. The prospectus shows up to 19.4 million shares, 8 percent of total outstanding, held by directors, officers, non-executive employees and other non-employee holders became eligible for sale Wednesday. That tranche follows biweekly unlocks of up to 14.6 million shares that have been rolling since August 19. Supply has been steady; demand has not.
Insider selling at scale
Chief executive Andrew Feldman and chief technology officer Sean Lie, both billionaires on paper after the listing, sold more than $240 million of Class A stock between August 20 and September 25 under Rule 10b5-1 plans adopted shortly after the offering. Other executives have liquidated holdings valued in the millions. The volume of planned selling by the founding team, concurrent with the first major competitive setback, gives investors a clear signal about how insiders are weighting the near-term risk-reward.