Definition
Function calling (tool use) exposes named functions with JSON schemas. The model emits a structured call; your runtime executes it and returns results for the model to continue.
It turns chat models into systems that can fetch live data and take actions.
In simple terms
Instead of guessing the weather, the assistant fills out a form that triggers the weather API and then reads the result aloud.
Where you see it
- Booking assistants calling calendar APIs.
- Data agents running SQL queries.
- RAG pipelines invoked as retrieval tools.
How it works
1.Declare tools
Name, description, and parameter schema.
2.Model selects a call
Structured arguments in the response.
3.Execute and continue
Feed tool output back into the conversation.
Why it matters
- Function calling is the bridge from text generation to useful agents.
Often confused
The model executes code by itself.
Your application runs the function; the model only proposes the call.