Samsung Electronics booked the biggest quarterly operating profit in its history on October 29, yet the AI spending behind the result looks shakier than it did a month ago.

The South Korean chipmaker (KRX: 005930) made an operating profit of KRW107.4 trillion ($80.2bn) in July-September, 782.8% more than a year earlier and the first time it has cleared KRW100 trillion in a single quarter. Heavy buying of AI hardware by the hyperscalers carried the figure past the KRW106 trillion consensus compiled by FnGuide.

Samsung now earns more in a quarter than Microsoft, Alphabet or Meta, the US groups whose data-centre budgets fill its order book. Memory makers have been the surest winners of the AI build-out so far, but their profits depend on those budgets holding, and investors spent October asking whether they will.

The rest of the income statement was less convincing. Revenue rose 126.9% to KRW195.3 trillion, short of the KRW200.3 trillion analysts had pencilled in, and net profit of KRW75.1 trillion fell well below the KRW87.4 trillion they expected.

SK hynix (KRX: 000660), the main supplier of high-bandwidth memory (HBM) to Nvidia, set the tone a day earlier. Its operating profit climbed 557.2% year on year to KRW83.33 trillion, at a margin above 80%, and it announced a fresh shareholder return plan worth KRW60 trillion on top of a KRW40 trillion programme it had just completed.

"Virtually all of our DRAM and HBM production capacity for the next three years through 2029 has already been sold out," the company told analysts on its results call.

Seoul took that as a licence to buy. The Kospi rose 6.6% to 6,012.44 on October 28, its steepest one-day gain in three months, as SK hynix shares added 10%, Saxo Bank said in a market note.

The rally came after a punishing month. The index lost 15.7% over the first three weeks of October, and on October 15 an 8% slide tripped a circuit breaker in Seoul as investors questioned the debt piling up behind US data-centre construction. SK hynix shelved a US listing of its Solidigm subsidiary on October 23, citing market conditions.

US results that evening clouded the picture again. Alphabet shares dropped about 10% in after-hours trading on disappointing revenue and negative free cash flow of $9.07bn for the third quarter, according to Saxo. Microsoft gained 3% as its Azure cloud sales grew 48%, but its capital spending of $39.4bn came in well below estimates, which is cold comfort to anyone selling it chips. Nasdaq 100 futures were down 0.9% in early Asian trade on October 29, and the Kospi opened flat at 6,017.22.

Swings of this size in both directions within a fortnight say more about investors' nerves than about the chipmakers' order books. Samsung's preliminary guidance put its operating margin at a record 55.1%, KB Securities expects its HBM prices to more than double in 2027, and SK hynix sees memory shortages lasting until 2030. The doubt lies with the buyers. While Alphabet, Microsoft and their peers keep their capex plans intact, Korea's memory makers will keep posting records. The first clear sign of a cutback would land hardest in Seoul, where the market's mood now rises and falls with two companies.