LLMTabBench: Evaluating LLMs on Binary Tabular Classification From Zero to Few Shots

Supervised classification for tabular data remains a core machine learning task, yet its reliance on large labeled datasets limits applicability in data-scarce domains. For such few-shot scenarios, specialized methods like TabPFN - a state-of-the-art Prior-Data Fitted Network - have set a high standard by leveraging large-scale synthetic pretraining, though they still require a context of labeled examples to function. In contrast, Large Language Models (LLMs) could offer a more flexible alternative via zero- and few-shot in-context learning directly from task descriptions, but their performance on tabular data remains inconsistent and poorly understood. We introduce LLMTabBench, a benchmark designed to systematically evaluate LLMs for tabular classification under data-scarce conditions. LLMTabBench explicitly probes (i) how LLM prior knowledge interacts with in-context information (task descriptions and few-shot examples), and (ii) how model performance scales with increasing data complexity, using both real-world and controlled synthetic datasets. Our findings include: (1) LLMs are highly competitive in zero-shot settings and can outperform alternative models, even when those models have access to few-shot examples; (2) incorporating additional few-shot examples can conflict with LLM prior knowledge, limiting or even degrading performance; and (3) there is a data complexity threshold beyond which LLMs' performance declines and few-shot examples become less effective. Together, these findings reveal fundamental constraints of in-context learning for tabular data and provide practical guidance for deploying LLMs in low-data regimes.