LUCid: Redefining Relevance For Lifelong Personalization

Work to date has mainly relied on semantic proximity to identify relevant content for lifelong personalization. However, situational relevance is often more important for determining which information is useful for a user's actual task and context. In this paper, we introduce the Proximity Advantage (PA) score, a metric for quantifying semantic proximity bias, and show that existing personalization benchmarks largely conflate semantic and situational proximity, leaving it unclear whether current systems truly capture situational relevance. To support this metric, we introduce LUCid, a diagnostic benchmark of 1,936 user queries paired with long interaction histories, designed to isolate situational relevance from semantic proximity. Our experiments across different stages of the modern personalization pipeline (retrieval, reranking, and generation) reveal significant performance collapse: retrieval recall drops to near zero on the hardest instances, and response alignment remains near 50% even for state-of-the-art models such as Gemini-3-Flash, GPT-5.4, and Claude Haiku, highlighting a fundamental mismatch between the relevance encoded by current systems and what lifelong personalization demands.