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About this paper

Yushi Hu, Reyhane Askari-Hemmat, Melissa Hall, Emily Dinan, Luke Zettlemoyer, Marjan Ghazvininejad

Multimodal RewardBench 2 (MMRB2) introduces the first comprehensive benchmark for evaluating reward models on multimodal understanding and interleaved text and image generation. It reveals that frontier MLLM judges like Gemini 3 Pro achieve the highest human agreement (up to 80%) but still struggle with nuanced distinctions and exhibit biases in reasoning tasks, while demonstrating a strong correlation between benchmark performance and downstream generative AI task success.

Key insights

  • The benchmark successfully highlights current limitations of MLLM-as-a-judge, revealing that even frontier models like Gemini 3 Pro, while leading, do not fully match human judgment.
  • MLLM judges struggle with fine-grained differentiation between similar outputs from the same model and exhibit a bias towards image-containing responses in multimodal reasoning tasks.
  • Performance on MMRB2 shows a strong correlation (Pearson's r > 0.8) with success in various downstream generative AI tasks, affirming its practical value for model selection and optimization.

Summary by alphaXiv.

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