Grounding-based open-vocabulary detectors provide a widely used framework for text-conditioned localization, with a phrase-grounding formulation that aligns object queries to individual text tokens and recovers class scores through token-to-class aggregation. We observe that this token-level scoring design becomes a bottleneck for fine-grained open-vocabulary object detection, where the target category is defined by a complete compositional description rather than by isolated tokens. Based on this observation, we propose SL-Ground, a sentence-level grounding reformulation for grounding-based open-vocabulary detection. Instead of scoring queries against fragmented token evidence, SL-Ground encodes each category description as a complete sentence-level representation and directly scores object queries against these representations. To evaluate fine-grained recognition across label granularity, we construct a multi-granularity Stanford Cars benchmark spanning vehicle type, brand, and car model. At the finest car-model granularity, SL-Ground raises AP over the token-level variant from 14.5 to 70.6 under matched training candidate sampling and to 86.0 with full-vocabulary training, while better preserving open-vocabulary capability and avoiding repeated prompt chunking. Beyond Stanford Cars, SL-Ground also improves FG-OVD performance, achieving the best average AP among compared methods. These results show that sentence-level grounding provides a more suitable scoring unit for fine-grained OVOD and offers a practical path toward fine-grained grounding-based open-vocabulary detectors.

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