Nvidia and Korea's giants are pouring huge money into AI, a boom that's lifting markets but raising real doubts about whether the spending pays off.

• Nvidia, OpenAI, Anthropic and Korea's Samsung and SK held an AI summit, with Samsung and SK sealing $950 billion in deals to supply chips and memory.
• Korea's plan is simple: if America wants AI in everything, Korea will sell the chips and profit.
• The bond market is nervous because companies are borrowing heavily on a promise AI will pay off later, and much of it is still cash burn.
• Record S&P 500 profit margins are being credited to AI, which Wall Street uses to say keep buying stocks.
• Nvidia says it can't get enough memory, land, power, or workers, and expects the data center buildout to double each year for a decade.

Outlook: AI spending keeps climbing, but growing worry over costs and debt could trigger more selling in tech stocks.