Vision-Language Models (VLMs) have shown strong performance in Spatio-Temporal Video Grounding (STVG), yet they are still evaluated mostly in a zero-shot manner on general-purpose benchmarks of everyday scenes. This creates a critical disconnect from real-world applications in specialized domains, where models inevitably encounter rare visual or textual concepts. Since exhaustive pre-training across infinite data distributions is infeasible, the ability to adapt to novel domains with limited data is essential. To bridge this gap, we introduce AnyGroundBench, a domain-adaptation benchmark designed to shift the STVG evaluation paradigm from static zero-shot testing to rigorous domain adaptation. Targeting five specialized domains (animal, industry, sports, surgery, and public security), AnyGroundBench pairs newly captured, expert-annotated videos with established datasets, unifying them through dense, high-fidelity spatio-temporal annotations. Crucially, the benchmark provides dedicated limited training subsets, enabling systematic evaluation of domain adaptability under limited training data. We benchmark 23 state-of-the-art VLMs in the zero-shot setting and further evaluate five adaptation strategies, spanning training-free and fine-tuning-based approaches, on representative models, assessing their zero-shot generalization and adaptation capacity. Our results show that current VLMs remain far from practical performance in the zero-shot setting, that training-free adaptation produces highly variable effects depending on the model and domain, and that fine-tuning-based adaptation, though more effective, still falls short of real-world requirements, with gains varying markedly across domains. These findings expose fundamental limitations in current VLMs' spatio-temporal reasoning, pointing to concrete directions for future research.

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