Sector Bear Market: A decline of 20% or more from a recent peak in a sector index—the technical threshold separating a correction (10%+) from a bear market—signaling that selling has moved beyond profit-taking into a fundamental repricing of the group’s prospects. The Philadelphia Semiconductor Index (SOX) crossed that line Friday, closing more than 20% below its late-June record after falling 1.8% on the session and over 4% Thursday, capping the worst week for chip stocks since the April 2025 tariff meltdown. The mechanics of this bear market are instructive because it arrived while the broader indexes remain within a few percent of all-time highs—the S&P 500 is down only modestly from its records even as its highest-flying sector lost a fifth of its value in three weeks. That divergence defines a rotation-driven bear market rather than a systemic one: capital isn’t leaving equities, it’s leaving a crowded trade. Three forces converged to produce it. First, valuation exhaustion—the SOX roughly doubled during the second quarter, leaving no margin for disappointment. Second, the spending-justification question: hyperscalers’ capital expenditure keeps climbing (and increasingly requires equity raises, secondaries, and bond issuance) while AI revenue conversion lags, a tension that IBM’s 25% collapse and Netflix’s guidance miss sharpened even though neither is a chipmaker. Third, competitive shock: Moonshot’s Kimi K3—a 2.8 trillion-parameter open-weight model claiming parity with U.S. frontier systems—compressed the assumed moat that justified premium multiples across the American AI stack, echoing the DeepSeek scare but at larger claimed scale. For portfolio purposes, sector bear markets resolve along two paths. The benign path: valuations reset, strong hands accumulate, and the sector reclaims leadership once earnings validate demand (the pattern after April 2025, which preceded the SOX’s best stretch in years). The malign path: the de-rating spreads as index-level concentration transmits sector weakness broadly—three stocks still drive an outsized share of S&P earnings revisions, so a sustained chip bear market mathematically caps index upside. Friday afternoon’s dip-buying, Apple’s brief ascent past Nvidia as the world’s most valuable company, and the rotation into insurers, staples, and managed care suggest the market is currently pricing the benign version: a leadership handoff, not a top. The tell in coming weeks is whether memory pricing and hyperscaler capex guidance hold firm—if they do, this bear market is a valuation event; if they crack, it becomes an earnings event, and those are the ones that spread.