LM Arena's anonymity assumption does not hold: a preference-learning approach identifies which model wrote a response
AspireAI community · arXiv
Strict anonymity of model responses is a key condition for the reliability of voting-based leaderboards such as LM Arena, and prior attempts to break that assumption with statistical features like TF-IDF or bag-of-words have lacked the discriminative power to separate stylistically similar or within-family models. INTERPOL takes a model-driven route, learning to distinguish target models from others using interpolated preference data, with hard negative samples synthesized through model interpolation and an adaptive curriculum learning strategy. The paper reports that this significantly outperforms existing baselines on identification accuracy, and quantifies the real-world consequence through ranking manipulation simulations on Arena battle data. The work was accepted at EMNLP 2026 with a highlight award, which signals reviewer confidence but not deployment readiness. The honest limitation is that Arena has not published an incident, so this is a demonstrated attack surface rather than evidence of a realised breach, and the manipulation results come from simulation rather than a live leaderboard.
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