The tech giant's shares were down 8% in extended trading. Alphabet has gained about 9% so far this year.
CEO Sundar Pichai said the company expected to spend about $75 billion in capital expenditures for 2025.
Wall Street had been expecting 2025 capital expenditures of about $58 billion, according to LSEG data. That would mark a modest increase over the $52.5 billion spending in 2024.
Alphabet has been spending heavily on an infrastructure buildout to support AI research and integration into products such as search and cloud services.
But investors are growing cautious about costs and the possibility that search users would opt to use rival chatbots such as ChatGPT. The emergence of China's DeepSeek, which offers cut-rate AI, also has raised new questions about capital spending by Google and U.S. rivals.
Digital advertising revenue, which represents about three-quarters of Alphabet's overall revenue, rose 10.6% to $72.46 billion in the fourth quarter. That beat the third quarter's 10.4% growth and it topped analysts' estimates of $71.84 billion, according to LSEG.
Google's Cloud business posted a 30% rise in revenue to $11.96 billion in the fourth quarter, slowing down from the 35% increase in the September quarter. Analysts were expecting a rise of 32.3% to $12.16 billion, according to data compiled by LSEG.
The soft cloud numbers come even as Google has built out AI features within its cloud computing platform. Larger cloud rival Microsoft also reported weaker-than-expected growth in its Azure cloud platform last week.
Shares of Amazon, the largest cloud provider, which will publish quarterly results on Thursday, fell 1.5% in after-hours trade.
Google's revenue rose 12% to $96.47 billion in the fourth quarter, compared with the average analyst estimate of $96.56 billion, according to data compiled by LSEG.
The company reported a profit of $2.15 per share, beating estimates of $2.13 per share.
