Shares of Cerebras Systems Inc. (CBRS) fell about 7% to $181.40 on Wednesday, September 30, after semiconductor research firm SemiAnalysis reported that OpenAI's newest flagship model not running on Cerebras chips.

In a post published Wednesday, SemiAnalysis said OpenAI's GPT-6.1 Sol Ultrafast tier is instead being served on Nvidia hardware, potentially including Blackwell-class chips such as the GB300. Neither Cerebras nor OpenAI responded immediately to requests for comment on the report. Without confirmation from either company, some uncertainty remains about the exact setup.

The report, if accurate, lands in one of the most competitive corners of the AI hardware market. Speed-optimized inference tiers such as OpenAI's Ultrafast offering sell raw throughput as part of the product, which makes the serving hardware behind them a competitive differentiator rather than a back-end detail. Nvidia's accelerators remain the standard infrastructure for large-scale AI serving, while Cerebras has staked its business on Wafer-Scale Engine chips that lean on very large amounts of on-chip memory to push serving speeds higher.

Why the Report Matters for Cerebras

OpenAI is Cerebras' largest customer by revenue backlog, a concentration that leaves the chipmaker unusually exposed to developments involving a single client. Losing ground with that customer therefore carries outsized weight for the stock.

The timing also matters. In August, Cerebras announced it would power the Ultrafast tier for GPT-5.6 Sol at speeds of up to 750 tokens per second. That announcement was framed at the time as a signature win for the company's Wafer-Scale Engine chips.

The new GPT-6.1 Sol Ultrafast tier, unveiled at OpenAI's DevDay on September 29, reportedly runs at only around 300 tokens per second — a steep drop from the 750 tokens per second advertised for the prior model. The slower figure has raised questions about whether the larger model is testing the limits of Cerebras' on-chip memory design. It is the kind of technical detail that normally stays in the background, but not this week.

A Stock Already Under Pressure

Wednesday's drop did not happen in a vacuum. Cerebras has faced a rocky run since its IPO priced at $350 back in May. The company posted a GAAP net loss of $450.5 million in its most recent quarter, and insiders have sold roughly $266 million worth of stock over the past three months.

Shares now trade much closer to their 52-week low of $160.81 than their 52-week high of $386.34, a gap that says a lot about how sentiment has shifted since the IPO.

The broader market gave Cerebras no cover on Wednesday either. The S&P 500 was higher on the day, and the Nasdaq climbed 0.9%, gains that made Cerebras one of the clearer underperformers in the chip sector for the session. Nvidia, meanwhile, moved in the opposite direction, rising about 2%.

There is also a version of this story in which nothing has actually changed over the long term: Cerebras could still end up powering later versions of the Ultrafast mode once capacity or optimization issues get sorted out. For now, though, the market is trading on the headline rather than the hypothetical — and the headline is not a good one for Cerebras.

How the situation resolves should show up in a few observable places: whether either company confirms or disputes the serving arrangement, what Cerebras' next quarterly report says about its OpenAI relationship, and whether Ultrafast throughput on the new model climbs as optimization work progresses — a data point that would help distinguish a structural loss from a temporary capacity decision.

As of Wednesday morning, Cerebras stock sat around $181.40, down sharply from its 52-week high of $386.34 and not far above its 52-week low of $160.81. No further comment has been issued by either company on the reported hardware switch.

This article originally appeared on CoinCentral.