Grounded Post-Training with Hard Examples for Reducing Hallucination in Multimodal Large Language Models

Hallucination remains a major challenge in vision-language models (VLMs), particularly when linguistically plausible responses are unsupported by visual evidence. We study whether multimodal hallucination can be reduced by concentrating post-training supervision on hard grounding boundaries, where preferred and rejected responses are semantically close but differ in their support from observable visual evidence. Under a frozen visual encoder and cross-modal alignment pathway, we first use supervised fine-tuning (SFT) to establish broad decoder-side multimodal behavior and then construct hard grounding preference pairs for Direct Preference Optimization (DPO). These pairs target evidence utilization, calibration, and grounding consistency across fine-grained recognition, spatial reasoning, OCR, ambiguous or insufficient evidence, and false-premise queries. The resulting DPO model consistently improves over its SFT initialization on DocVQA, TextVQA, MMBench, and VQAv2. Across approximately 839 additional multimodal examples, it further achieves 6.2--8.2 percentage-point higher pairwise win rates under three independent LLM judges. We additionally release HardVQA-DPO, a curated and growing resource with more than 3K hard-grounding SFT examples and an initial set of DPO preference pairs.https://huggingface.co/datasets/vlmgrounding/hardvqa-sft-dpo These results show that decoder-side adaptation alone can improve a meaningful subset of grounding failures under a fixed multimodal representation, while preserving broader multimodal capabilities.