Tool Use

Process tools published

Also known as: Tool Usage, Tool Calling, External Tools

Definition

The capability of an LLM to invoke external functions, APIs, or services to perform actions or retrieve information beyond text generation. Tool use extends LLMs from knowledge systems to action systems—they can search the web, run code, query databases, send emails, and interact with any API. The model decides when to use which tool and formulates appropriate inputs.

What this is NOT

  • Not the tools themselves (tool use is the act of invoking tools)
  • Not RAG (RAG is retrieval; tool use includes actions beyond retrieval)
  • Not all structured output (tool use implies execution, not just formatting)

Alternative Interpretations

Different communities use this term differently:

llm-practitioners

A model capability where the LLM outputs structured tool invocations (often JSON) that are executed by the application, with results fed back to the model. Enabled via function calling APIs or tool-use prompts.

Sources: OpenAI Function Calling documentation, Anthropic Tool Use documentation, Google Gemini function calling

cognitive-science

The use of external artifacts to extend cognitive or physical capabilities. Tool use is a marker of intelligence in animals and is fundamental to human cognition (extended mind thesis).

Sources: Clark & Chalmers: The Extended Mind, Tool use in animal cognition research

Examples

  • Model calls a weather API to answer 'What's the weather in Paris?'
  • Model executes Python code to calculate statistics
  • Model searches the web to find recent information
  • Model sends an email on behalf of the user

Counterexamples

Things that might seem like Tool Use but are not:

  • Model generating a response from its training knowledge (no tool invoked)
  • RAG retrieval (retrieval is passive; tool use implies active invocation)
  • Model outputting JSON without execution (that's structured output)

Relations

  • requires function-calling (Function calling is the API mechanism for tool use)
  • requires tool-definition (Tools must be defined for the model)
  • produces tool-result (Tool invocations produce results)
  • overlapsWith agent (Agents use tools to take actions)

See Also

Implementations

Tools and frameworks that implement this concept: