When Neither Side Confirms: Destructive Tool Approval Across MCP Servers and Hosts

Junwei Yao, Zike Yang, Zhijun Wang, Yongxi Zhou, Wenbo Ye

LLM agents call tools that delete data or move money through the Model Context Protocol (MCP). The specification asks for a human who can deny tool calls but splits the mechanism between the server, which can annotate a tool or ask the user through the host (elicitation), and the host application, which decides whether to prompt. We measure both sides from source code. In a 1,000-repository sample of the official MCP registry, an estimated 90% of destructive tools (95% CI 82-95) have no server-side confirmation, and most servers that elicit proceed, or fall back to a flag the model can set, when the host cannot relay the question. Of 41 popular open-source hosts and agent SDKs, 32 have no approval step by default, including 16 of 25 applications. Pairing sampled destructive tools flagged by a rule-based extractor with these applications, 60.7% of pairs have no enforced human approval by default. Through four agent SDKs, four of five low-cost models executed an injected deletion in 479 of 960 runs. If hosts prompted for every tool not annotated read-only or non-destructive, the unapproved share of these pairs would fall to 5.4%; universal elicitation support would leave 56.7%.…