Function Calling & Built-in Tools
Understand how models invoke external functions, build tool schemas from scratch, and learn when built-in tools beat custom ones.
Function Call Playground
Type a natural language prompt and watch how the model decides which tool to call, constructs arguments, receives results, and formulates a final response.
Tool Schema Designer
Build a tool schema interactively. Define the name, description, and parameters — the JSON schema generates in real-time on the right.
Built-in vs Custom Tools
Most platforms ship built-in tools for common tasks. Here is when to use them versus building your own.
Key Insight: Descriptions Drive Selection
Tool descriptions are as important as your model prompts. The model chooses which tool to call based on the description, not the function name. A tool named fetch_data with the description "Retrieves current weather conditions for any city worldwide" will be selected for weather queries even though the name says nothing about weather. Write descriptions like you are explaining the tool to a smart colleague who has never seen it before.