High-quality representations are a core requirement for effective recommendation. In this work, we study the problem of LLM-based descriptor generation, i.e., generating keyphrase-like natural language item representations with minimal constraints on downstream applications. However, open-ended generation provides little control over the generation space, leading to high cardinality, low-performance descriptors that render downstream modeling challenging. To this end, we propose AgenticTagger, a framework that queries LLMs to represent items with text descriptors drawn from an automatically mined vocabulary.

AgenticTagger features two core stages: (1) a vocabulary-building stage in which a set of hierarchical, low-cardinality, and high-quality descriptors is identified, and (2) a vocabulary-assignment stage in which LLMs assign in-vocabulary descriptors to items. To ensure the vocabulary effectively describes the item corpus of interest, we design a multi-agent reflection mechanism in which an architect LLM iteratively refines the vocabulary guided by parallelized feedback from annotator LLMs that validate the vocabulary against item data. Experiments on public and private datasets show AgenticTagger brings consistent improvements across diverse recommendation scenarios, including generative retrieval, ranking, and critique-based recommendation.