U.S. companies expect to spend hundreds of billions of dollars on AI over the next few years, but some investors are worried those plans—and the stock market rally they’ve fueled—depend too much on just a handful of companies.

Chip giant Nvidia (NVDA) revealed in a regulatory filing last month that two direct customers accounted for nearly 40% of total revenue in the second quarter, with “Customer A” representing 23% of sales and “Customer B” 16%.1 At no other point since 2022, when ChatGPT sparked the AI craze, have two individual buyers represented a greater share of Nvidia’s sales.