Function-calling has enabled large language models (LLMs) to act as tool-using agents, but injecting thousands of tool schemas into the prompt is costly and error-prone. We introduce MCP-Zero, a proactive agent framework that lets the LLM itself decide when and which external tools to retrieve, thereby assembling a task-specific toolchain from scratch. The framework is built upon three components: (1) Proactive Tool Request, where the model emits a structured block that explicitly specifies the desired server and task; (2) Hierarchical Vector Routing, a coarse-to-fine retrieval algorithm that first selects candidate servers and then ranks tools within each server based on the semantic similarity; (3) Iterative Proactive Invocation, enabling multi-round, cross-domain toolchain construction with minimal context overhead, and allowing the model to iteratively revise its request when the returned tools are insufficient. To evaluate our approach we also compile MCP-tools, a retrieval dataset comprising 308 MCP servers and 2,797 tools extracted from the official Model-Context-Protocol repository and normalized into a unified JSON schema. Experiments show that MCP-Zero (i) effectively addresses the context overhead problem of existing methods and accurately selects the correct tool from a pool of nearly 3,000 candidates (248.1k tokens); (ii) reduces token consumption by 98\% on the APIbank while maintaining high accuracy; and (iii) supports multi-turn tool invocation with consistent accuracy across rounds.
View on arXiv@article{fei2025_2506.01056,
title={ MCP-Zero: Active Tool Discovery for Autonomous LLM Agents },
author={ Xiang Fei and Xiawu Zheng and Hao Feng },
journal={arXiv preprint arXiv:2506.01056},
year={ 2025 }
}