A broad retreat in artificial-intelligence bellwethers sent U.S. equity benchmarks lower on Tuesday, puncturing the relative calm that had settled over Wall Street during August. Nvidia Corp. slid 3.5% to close below its 20-day moving average for the first time since April, marking its sharpest drop in four months and erasing gains made earlier in the month. Software group Palantir Technologies plunged about 9%, its fifth consecutive decline, after a short-seller report questioned the company’s valuation. Chip designer Arm Holdings lost 5%, while other technology heavyweights such as Microsoft and Meta Platforms also finished in the red, reflecting growing investor unease over whether the surge in AI spending can be sustained. The losses in megacap technology names overshadowed advances in more than 350 constituents of the S&P 500. The tech-heavy Nasdaq 100 fell 1.4%, its second-worst session since April’s 145% tariff on Chinese goods roiled markets, and the broader Nasdaq Composite notched its largest slide since Aug. 1. Treasuries firmed ahead of Federal Reserve Chair Jerome Powell’s speech later this week, as traders weighed the prospect that higher-than-expected AI valuations could make the market more vulnerable to policy surprises.