HorizonBench: Long-Horizon Personalization with Evolving Preferences

Shuyue Stella Li; Bhargavi Paranjape; Kerem Oktar; Zhongyao Ma; Gelin Zhou; Lin Guan; Na Zhang; Sem Park; Lin Chen; Diyi Yang; Yulia Tsvetkov; Asli Celikyilmaz

arXiv · 2604.17283

HorizonBench: Long-Horizon Personalization with Evolving Preferences

Shuyue Stella Li·Bhargavi Paranjape·Kerem Oktar·Zhongyao Ma·Gelin Zhou·Lin Guan·Na Zhang·Sem Park·Lin Chen·Diyi Yang·Yulia Tsvetkov·Asli Celikyilmaz

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Abstract

User preferences evolve across months of interaction, and tracking them requires inferring when a stated preference has been changed by a subsequent life event. We define this problem as long-horizon personalization and observe that progress on it is limited by data availability and measurement, with no existing resource providing both naturalistic long-horizon interactions and the ground-truth provenance needed to diagnose why models fail. We introduce a data generator that produces conversations from a structured mental state graph, yielding ground-truth provenance for every preference change across 6-month timelines, and from it construct HorizonBench, a benchmark of 4,245 items from 360 simulated users with 6-month conversation histories averaging ~4,300 turns and ~163K tokens. HorizonBench provides a testbed for long-context modeling, memory-augmented architectures, theory-of-mind reasoning, and user modeling. Across 25 frontier models, the best model reaches 52.8% and most score at or below the 20% chance baseline. When these models err on evolved preferences, over a third of the time they select the user's originally stated value without tracking the updated user state. This belief-update failure persists across context lengths and expression explicitness levels, identifying state-tracking capability as the primary bottleneck for long-horizon personalization.

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Shuyue Stella Li, Bhargavi Paranjape, Kerem Oktar, Zhongyao Ma, Gelin Zhou, Lin Guan, Na Zhang, Sem Park, Lin Chen, Diyi Yang, Yulia Tsvetkov, Asli Celikyilmaz. 2026-04-19. HorizonBench: Long-Horizon Personalization with Evolving Preferences. https://arxiv.org/abs/2604.17283

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