Tool-Using Agent

Actor agents published

Also known as: Tool Agent, Function-Calling Agent

Definition

An agent that extends its capabilities by invoking external tools—APIs, functions, code interpreters, or other services. The agent decides when to use which tool based on the task, formulates appropriate inputs, calls the tool, and incorporates the result into its reasoning. Tool use is what lets LLM agents affect the world beyond generating text.

What this is NOT

  • Not the same as having access to tools (must actually use them based on decisions)
  • Not a pipeline that always calls certain tools (tool selection must be dynamic)
  • Not an API wrapper (the agent reasons about when/how to use the tool)

Alternative Interpretations

Different communities use this term differently:

llm-practitioners

An LLM-based agent that uses function calling or tool-use APIs to invoke external services. The model outputs structured tool calls, the runtime executes them, and results are fed back to the model.

Sources: OpenAI Function Calling documentation, Anthropic Tool Use documentation, LangChain Tools documentation

cognitive-science

An agent that uses external artifacts (physical or digital tools) to extend its cognitive or physical capabilities beyond what it can do unaided.

Sources: Extended mind thesis (Clark & Chalmers), Tool use in animal cognition research

Examples

  • An agent that searches Google, reads web pages, and synthesizes answers
  • A coding agent that runs code in a sandbox and iterates on errors
  • A data analyst agent that queries databases and generates visualizations
  • An email agent that reads, composes, and sends emails via API

Counterexamples

Things that might seem like Tool-Using Agent but are not:

  • A chatbot that only generates text responses
  • A RAG system where retrieval is hardcoded, not decided by the model
  • A pipeline that always executes the same tools in sequence

Relations

  • specializes agent (Tool-using agents are a type of agent)
  • requires tool-use (By definition, uses tools)
  • overlapsWith code-executing-agent (Code execution is a specific type of tool use)
  • overlapsWith function-calling (Function calling is the API mechanism for tool use)

See Also

Implementations

Tools and frameworks that implement this concept: