AXIS: Explainable Time Series Anomaly Detection with Large Language Models

• URL: http://arxiv.org/abs/2509.24378v1
• Date: Mon, 29 Sep 2025 07:24:22 GMT
• Title: AXIS: Explainable Time Series Anomaly Detection with Large Language Models
• Authors: Tian Lan, Hao Duong Le, Jinbo Li, Wenjun He, Meng Wang, Chenghao Liu, Chen Zhang,
• Abstract summary: AXIS is a framework that conditions a frozen Large Language Models (LLMs) for nuanced time-series understanding.<n>LLMs operate on discrete tokens and struggle to directly process long, continuous signals.<n>We introduce a new benchmark featuring multi-format questions and rationales that supervise contextual grounding and pattern-level semantics.
• Score: 33.68487894996624
• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: Time-series anomaly detection (TSAD) increasingly demands explanations that articulate not only if an anomaly occurred, but also what pattern it exhibits and why it is anomalous. Leveraging the impressive explanatory capabilities of Large Language Models (LLMs), recent works have attempted to treat time series as text for explainable TSAD. However, this approach faces a fundamental challenge: LLMs operate on discrete tokens and struggle to directly process long, continuous signals. Consequently, naive time-to-text serialization suffers from a lack of contextual grounding and representation alignment between the two modalities. To address this gap, we introduce AXIS, a framework that conditions a frozen LLM for nuanced time-series understanding. Instead of direct serialization, AXIS enriches the LLM's input with three complementary hints derived from the series: (i) a symbolic numeric hint for numerical grounding, (ii) a context-integrated, step-aligned hint distilled from a pretrained time-series encoder to capture fine-grained dynamics, and (iii) a task-prior hint that encodes global anomaly characteristics. Furthermore, to facilitate robust evaluation of explainability, we introduce a new benchmark featuring multi-format questions and rationales that supervise contextual grounding and pattern-level semantics. Extensive experiments, including both LLM-based and human evaluations, demonstrate that AXIS yields explanations of significantly higher quality and achieves competitive detection accuracy compared to general-purpose LLMs, specialized time-series LLMs, and time-series Vision Language Models.

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