Beyond Static Toolsets: Self-Evolving LLM Tool Agents via Continual Documentation Adaptation

Large language models (LLMs) increasingly act as tool-using agents, and existing methods for evaluating and optimizing tool usage by LLMs typically assume a static tool environment with fixed APIs and documentation. In practice, toolsets evolve as tools are added, changed, or deprecated, introducing instability for agents that must retain prior competence while adapting to new capabilities. We formalize this challenge as the stability-adaptation dilemma. To address it, we propose Con-tDa, a continual documentation adaptation framework that provides a generalizable solution to this problem, enabling LLM agents to self-evolve by updating tool documentation. ContDa combines relation-guided exploration, which leverages functionally related existing tools as anchors to probe and identify new tool capabilities, with relation-aware adjustment that organizes overlapping tools and explicitly encodes usage preferences and fallback options among them. We then introduce complementary metrics that disentangle performance from stability and adaptation. Experiments across three evolution patterns on dynamic extensions of StableToolBench and RestBench show that ContDa consistently improves average performance by enhancing the discovery of new capabilities while incurring only limited loss of previously solved tasks, demonstrating documentation adaptation as an effective and lightweight mechanism for robust tool use in evolving environments. Our code is available at https://github.com/Bingo-W/ContDa.