ML Security PapersInference-Only Prompt Projection for Safe Text-to-Image Generation with TV Guarantees
Minhyuk Lee, Hyekyung Yoon, Myungjoo Kang · Seoul National University
Inference-time prompt projection defense rewrites unsafe T2I prompts into safe ones using LLM+VLM without retraining the generator
Text-to-Image (T2I) diffusion models enable high-quality open-ended synthesis, but their real-world deployment demands safeguards that suppress unsafe generations without degrading benign prompt-image alignment. We formalize this tension through a total variation (TV) lens: once the reference conditional distribution is fixed, any nontrivial reduction in unsafe generations necessarily incurs TV deviation from the reference, yielding a principled Safety-Prompt Alignment Trade-off (SPAT). Guided by this view, we propose an inference-only prompt projection framework that selectively intervenes on high-risk prompts via a surrogate objective with verification, mapping them into a tolerance-controlled safe set while leaving benign prompts effectively unchanged, without retraining or fine-tuning the generator. Across four datasets and three diffusion backbones, our approach achieves 16.7-60.0% relative reductions in inappropriate percentage (IP) versus strong model-level alignment baselines, while preserving benign prompt-image alignment on COCO near the unaligned reference.