When Anonymity Breaks: Identifying Models Behind Text-to-Image Leaderboards

Text-to-image (T2I) models are increasingly popular, producing a large share of AI-generated images online. To compare their quality, voting-based leaderboards have become the standard, relying on anonymized model outputs for fairness. In this work, we show that such anonymity can be easily broken. We find that generations from each T2I model form distinctive clusters in the image embedding space, enabling accurate deanonymization without prompt control or training data. Using 22 models and 280 prompts (150K images), our centroid-based method achieves high accuracy and reveals systematic model-specific signatures. We further introduce a prompt-level distinguishability metric and conduct large-scale analyses showing how certain prompts amplify these signatures. Our findings expose fundamental security risks in T2I leaderboards and motivate stronger anonymization defenses.