Global stock markets fell sharply today as renewed concerns about an AI valuation bubble triggered broad risk-off flows. Tech and semiconductor names led declines across the U.S., Europe and Asia; the Nasdaq and key Asian indices posted notable losses. Portfolio managers cited concentrated exposure to AI winners and thin liquidity as amplifiers, prompting rotations into Treasuries and defensive sectors. Analysts warned the move may be a valuation re-rating rather than a macro shock, but stressed that corporate earnings and policy signals will determine whether the sell-off proves temporary or deeper.