What Remains OOD? OODConceptBench for Open-World Concept Recognition in Vision-Language Models
In the era of large-scale pretrained models, zero-shot generalization has increasingly blurred the boundary between known and unknown concepts. However, current evaluations do not directly reveal which open-world concepts different vision-language models can recognize. To study this problem, we introduce OODConceptBench, a hierarchical benchmark for open-world concept recognition on image-concept pairs. The full benchmark set (L1) contains 30,752 images, and the harder subset (L2) contains 14,285 images. The benchmark covers 560 concept phrases across seven open-world categories. We evaluate 37 vision-language models across release year, model family, and variant. We obtain three main findings. First, target-concept recognition is lower on L2 than on L1 across all 37 evaluated models. Second, later-released models do not necessarily achieve stronger open-world concept recognition. Third, an increase in overall recognition does not imply consistent improvement across all benchmark categories. Together, these results show that OOD remains a challenge even in the era of large-scale vision-language pretraining.