South Korea's KOSPI Fell 7.6% to a Six-Month Low. TSMC's Record Quarter Couldn't Stop It.

Trading on South Korea's stock exchange hit its circuit breaker Thursday afternoon, the tenth such halt on the KOSPI this year and the first since July. By the closing bell, the index had shed 7.6% of its value, settling at 5,554.22, a level it hadn't closed below since April.

Samsung Electronics closed down 9.5%. SK Hynix fell 11.1%. Those two stocks alone account for more than half the weight of Korea's benchmark index, so when both fall together on the same day, the entire market falls with them.

The timing is what makes this one sting. Roughly six hours earlier, Taiwan Semiconductor Manufacturing Company had reported a profit of about $23.6 billion for the third quarter, up 68.7% from a year ago and its sixth consecutive record. Revenue came in at $46.7 billion, a figure Pulse24 had already flagged as likely once TSMC's monthly sales disclosures pointed to a record quarter more than a week earlier. None of it stopped the bleeding. Asian chip stocks sold off anyway, dragging down the index that holds two of the world's largest memory makers.

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What Changed

Two separate pressures collided on Thursday. One is specific to the AI trade itself: investors have spent the past several weeks questioning whether the hundreds of billions of dollars committed to AI infrastructure will generate the returns their backers are promising, a doubt Pulse24 has tracked since OpenAI's own revenue fell short of what investors had been told to expect. China's CXMT added to that unease this week by announcing a new memory architecture that threatens the pricing power Samsung and SK Hynix have enjoyed during the AI memory boom.

The other pressure is macro, and it arrived from Washington. September's consumer price index rose 3.7% from a year earlier, beating every forecast, a reading Pulse24 covered the day it landed. The 10-year Treasury yield has pushed back toward 5.3% since, a level last seen in 2007, and futures traders have raised their odds of another Fed rate hike in the months ahead. Wall Street absorbed that combination badly on Wednesday, with the S&P 500 down 1.1% and the Nasdaq down 1.9% by the close. Asia inherited the hangover Thursday morning.

Why It Matters

Samsung's stock has now fallen twice in three sessions on two different storylines. Earlier this week it dropped 7.4% on the CXMT threat alone. Thursday's 9.5% decline came from a broader macro shock layered on top of that same competitive worry, and a company-specific threat compounding a market-wide rate scare is a harder combination for any stock to shake off than either problem on its own.

It also complicates the simplest story markets have told themselves about AI spending this year: that strong fundamentals at companies like TSMC would eventually override sentiment. TSMC's own numbers argue AI demand is not slowing. High performance computing chips made up 68% of its revenue last quarter, up from 57% a year earlier, and its most advanced 2nm process jumped from 3% to 8% of wafer revenue in a single quarter. If a company posting those figures cannot stop a selloff, financing costs and sentiment are driving this market more than earnings are right now, at least for this stretch.

Rising yields explain part of why. A large share of AI infrastructure spending is funded with debt, and that debt gets more expensive every time the 10-year yield climbs. The same inflation data lifting Treasury yields is also feeding expectations for more Fed hikes, which raises the discount rate investors apply to future AI profits before those profits have even arrived. Valuations that looked defensible when rates sat lower look considerably less so now, a tension Pulse24 flagged when US stocks hit their second-priciest level in 145 years even as the Fed kept raising rates.

What to Watch Next

Three things will say more about whether Thursday was a one day scare or the start of something longer. South Korea's circuit breaker could trip again in the sessions ahead if the selling continues, which would signal this isn't over. TSMC's upcoming guidance commentary is worth tracking too, since management is expected to address 2026 capital spending plans and whether CoWoS packaging capacity, the bottleneck that has constrained AI chip supply all year, is finally catching up with demand. The 10-year Treasury yield matters most of all; if it keeps climbing past 5.3%, the valuation math on every AI-adjacent stock gets harder, regardless of how good any single company's earnings look.

The Pulse24 Take

Markets have a habit of treating two separate questions as one: is the technology real, and is the price right. TSMC's quarter answered the first question about as clearly as a single earnings report can. AI demand is not cooling. The second question, whether current valuations and the debt funding them can survive a world where the 10-year yield sits near 5.3%, remains unresolved, and Thursday's selloff in Seoul is a reminder that good earnings alone won't settle it. Investors who treat those two questions as the same one are the ones most likely to be caught off guard by days like this.