U.S. stocks delivered a mixed September performance despite the month’s historical weakness, with artificial-intelligence enthusiasm supporting technology shares while surging Treasury yields pressured broader markets. The S&P 500 fell 0.4% and remained 1.9% below its August record, compared with its 0.6% average September decline since 1950. The Nasdaq Composite rose 1.8% after reaching a new peak on Sept. 22, led by Meta, AMD and other semiconductor stocks, while the Dow Jones Industrial Average dropped about 4.3% and the Russell 2000 declined 5.4%. Interactive Brokers chief strategist Steve Sosnick described the market as "AI or bust" but cautioned that underlying conditions remained weak. The 2-year and 10-year Treasury yields each climbed roughly half a percentage point, while the 30-year yield rose nearly 40 basis points from August’s end, marking the largest combined move since September 2022. Tighter financial conditions have also affected the initial public offering (IPO) market: Oura postponed its offering, while Holtec Nuclear and Bamboo Insurance also cited uncertain conditions, and Anthropic reportedly delayed its planned debut from October to November. UBS chief economist Arend Kapteyn said AI investment and the wealth effect from rising AI stocks account for 80% or more of U.S. economic growth. UBS raised its estimate for AI-driven capital spending next year by $200 billion to $1.4 trillion, while non-AI component investment is running at zero. Kapteyn warned that an AI reversal could weaken both business investment and consumer spending, although he said each dollar of AI capital spending this quarter appears to generate about $1.50 of investment in the following quarter, with only about one-third directed to technology.