What Is Tool Use in AI Agents

Tool use is the capability that transforms a language model from a text generator into an agent. When an AI system can call external tools, it can read databases, send emails, modify files, execute code, and interact with any system that exposes an API.

In the context of AI agents, a tool is a function with a defined interface. The function has a name, a description that helps the model understand when to use it, and an input schema that specifies what parameters it accepts. The model generates a structured tool call, the runtime executes it, and the result is fed back to the model for the next step.

The Model Context Protocol (MCP) standardizes this pattern. An MCP server exposes tools with JSON Schema definitions. An MCP client connects to the server and makes those tools available to the agent. The agent sees the tool descriptions, decides which to call, constructs the input, and receives the output.

Tool use introduces safety challenges that do not exist in pure text generation:

Irreversible actions. A model that generates bad text can be corrected. A model that deletes a database row cannot undo that action. Tool use means the agent can affect the real world.

Privilege escalation. If an agent has access to an admin tool, it might use it even when the current task does not require admin privileges. Without enforcement, the agent operates with the full set of available capabilities.

Parameter manipulation. The agent constructs tool parameters from context, including potentially untrusted user input. An indirect prompt injection could manipulate these parameters to target unintended resources.

Chained exploitation. Individual tool calls might be safe, but a sequence of calls can achieve harmful outcomes. Reading a credential file and then calling an API with those credentials is a multi-step attack.

Securing tool use requires policy enforcement at the tool invocation layer. Every tool call should be evaluated against a policy that specifies which tools are allowed, with what parameters, and under what conditions. This is the core function of Authensor's policy engine.