Kospi plunges 8% and trips a circuit breaker as chips sink

South Korea's main stock index, the Kospi, fell about 8% on October 15, 2026, triggering a circuit breaker, an automatic halt to all trading. SK hynix and Samsung Electronics, the two biggest makers of memory chips, fell around 10%. The drop came two days after the Kospi lost 5.46% on October 13. Worries about US inflation, a selloff in US chip stocks and heavy foreign selling drove it.

What happened

The circuit breaker hit at 12:38 p.m. Seoul time and stopped trading across the market for 20 minutes, The Korea Herald reports. The smaller Kosdaq market hit its own circuit breaker at 12:23 p.m.

Figures moved through the day, so each one below is from a single report:

Measure	Figure	Reported by
Kospi when the halt hit	Down 8.00% at 5,528.05	Bloomingbit
Kospi low of the day	5,506.23, down 8.36%	Bloomingbit
SK hynix	Down 11.35% to 1.26 million won	The Korea Herald
Samsung Electronics	Down 10.13% to 208,500 won	The Korea Herald
Foreign investors' net selling	3.42 trillion won	Bloomingbit
Korean won	1,348.2 per US dollar	Reuters

It was the Kospi's 10th circuit breaker this year and the 16th in its history, Reuters reports. It was the first since July 29, according to The Korea Herald.

SK hynix's market value fell to 929.18 trillion won, below the 1,000 trillion won mark, The Korea Herald says. Bloomingbit notes the Kospi is now about 41% below its record high of 9,385.59, set on June 19.

Why it fell

The reports point to several causes:

• US inflation. US consumer prices rose 3.7% from a year earlier in September, above the 3.6% expected, Bloomingbit reports. That raised fears the US central bank will raise interest rates again.
• US chip stocks. The Philadelphia Semiconductor Index fell 4.3% overnight, Reuters reports. Shares of the AI cloud provider CoreWeave fell 15.46% on concerns about its debt-funded growth, Bloomingbit says.
• No more buybacks. Samsung and SK hynix have finished their share buyback programs, according to J.P. Morgan strategists cited by Finimize. That removed a steady buyer from the market.
• Foreign selling. Foreign investors have been net sellers for seven straight sessions since October 2, Bloomingbit reports. Individual and institutional investors were net buyers on the day.

A rough week for Korean chip stocks

We covered the October 13 selloff, when the Kospi closed down 5.46% and SK hynix fell about 10%. Two days later, the fall went deeper and set off the market's emergency brake.

None of the reports describe a response from South Korea's financial regulators or central bank.

What this means for developers

For teams that buy memory, GPUs or AI cloud capacity:

• Do not read the selloff as a price cut. Falling chip shares reflect what investors expect, not today's contract prices for memory. Budget on current quotes.
• Watch your GPU cloud provider. CoreWeave's 15% fall shows how worried investors are about AI capacity built with borrowed money. Teams that depend on one provider should know its financial footing.
• Interest rates reach AI budgets. Higher rates make it costlier to fund data centers. That can slow how fast new GPU and memory capacity comes online.
• Keep supply options open. Sharp swings in the biggest memory makers are a reminder to keep more than one supplier qualified for large hardware orders.