C3.ai stock has fallen 21% in 2026 as revenue declined 35% and losses more than doubled, leaving investors to bet on whether founder Thomas Siebel can reverse the decline after returning as CEO.

What happened: C3.ai generated $250.3 million（約400億円） in revenue during fiscal 2026 (ended April 30), a 35% decline from the prior year, and posted a $470.4 million（約750億円） loss, 63% higher than fiscal 2025. After founder Thomas Siebel stepped down in September to deal with health issues, the company's sales fell sharply. Siebel returned to the CEO role on May 8 and has cut roughly 35% of the workforce. Management now forecasts revenue between $210 million（約340億円） and $240 million（約380億円） for fiscal 2027, which would decline year-over-year even at the high end.

Why it matters: C3.ai helps enterprises adopt AI by offering 40 ready-made applications that can be customized across industries—Shell uses them for equipment monitoring, retailers for inventory optimization, and banks for anti-money laundering. The company's severe revenue drop and mounting losses raise questions about whether its business model can survive without the customer relationships that Siebel built. With $575.4 million（約920億円） in cash on hand at fiscal year-end, C3.ai cannot afford another large loss, making the next 12 months critical.

What to watch: The stock trades at a price-to-sales ratio of 6.1 (below its five-year average of 10.5), but the forward P/S ratio is 6.9 because revenue is forecast to shrink further in fiscal 2027. Investors will have to endure at least one more year of declining sales before knowing whether Siebel can turn the company around.