Alphabet Inc. has become an artificial-intelligence stock, for better or for worse. Though advertising still makes up the lion’s share of the company’s revenue, investors have been focusing more on the part of Alphabet’s business that seems the best proxy for AI momentum.

Cloud revenue in the quarter of $12 billion—a 30% increase from last year—just misses analysts’ estimates of $12.2 billion.

Alphabet comes up short. Overall revenue rose 12% to $96.5 billion in Q4, slightly below analysts' consensus estimate of $96.67 billion. That revenue growth was balanced with sear