From Reasoning to Pixels: Benchmarking the Alignment Gap in Unified Multimodal Models

Unified multimodal models (UMMs) aim to integrate multimodal understanding and generation within a unified architecture, yet it remains unclear to what extent their representations are truly aligned across modalities. To investigate this question, we use reasoning-guided image generation as a diagnostic task, where models produce textual reasoning first and then generate images. We introduce UReason, a benchmark for evaluating cross-modal alignment in this paradigm, consisting of 2,000 manually curated instances spanning five reasoning-intensive tasks: Code, Arithmetic, Spatial, Attribute and Text. To enable controlled analysis, we develop an evaluation framework that compares direct generation, reasoning-guided generation and de-contextualized generation, which conditions only on the refined prompt extracted from reasoning. Across eight widely used UMMs, while we find that reasoning-guided generation yields improvements over direct generation, somewhat surprisingly, de-contextualized generation consistently outperforms reasoning-guided generation by a large margin. Our results suggest that the intended visual semantics in textual reasoning are not reliably reflected in the generated images. This finding indicates that, despite unified design and training, current UMMs still do not robustly align representations across modalities. Overall, UReason serves as a practical litmus test for cross-modal alignment and provides a challenging benchmark for developing next-generation, more tightly aligned UMMs.