A handful of giant tech companies are propping up the whole US stock market, and it all rests on one bet — AI — which makes this a fragile setup for investors.

• The top 10 stocks are almost all making the same AI bet, so the market is far less diversified than it looks.
• Nvidia sells the chips, Micron supplies the memory, and Microsoft, Meta, Google, and Amazon buy them — money circling between the same few companies.
• That circular spending flatters everyone's earnings and keeps the growth story believable.
• Big tech is spending three quarters of a trillion dollars a year on AI, roughly the entire revenue of the AI chip trade.
• The weak spot: that spending is still bringing in only a few billion in profit, and CEOs will eventually ask whether it is worth it.

Outlook: If even one of these giants cuts back on AI spending, the earnings holding up the market could unravel fast.