What is Function Calling?

Function Calling (also called Tool Use) allows language models to request execution of predefined functions with structured arguments. Instead of generating text responses, the model outputs structured function calls that your code can execute.

How It Works

1. Define Functions: Describe available tools with names, descriptions, and parameter schemas
2. User Query: Send user message with function definitions
3. Model Response: Model returns function name + arguments (not the result!)
4. Execute: Your code runs the function
5. Return Result: Send result back to model for final response

Example Flow

User: "What's the weather in Tokyo?"
    ↓
Model: {"function": "get_weather", "args": {"location": "Tokyo"}}
    ↓
Your Code: calls real weather API → "25°C, sunny"
    ↓
Model: "The weather in Tokyo is 25°C and sunny."

Defining Functions

functions = [{
    "name": "get_weather",
    "description": "Get current weather for a location",
    "parameters": {
        "type": "object",
        "properties": {
            "location": {
                "type": "string",
                "description": "City name"
            },
            "unit": {
                "type": "string",
                "enum": ["celsius", "fahrenheit"]
            }
        },
        "required": ["location"]
    }
}]

Common Use Cases

Use Case	Example Functions
Data Retrieval	search_database, get_user_info
Actions	send_email, create_ticket
Calculations	calculate_price, convert_currency
External APIs	get_weather, search_web

Provider Comparison

Provider	Feature Name	Multiple Calls
OpenAI	Function Calling / Tools	Yes (parallel)
Anthropic	Tool Use	Yes
Google	Function Calling	Yes
Open Source	Varies by model	Depends

Best Practices

1. Clear descriptions: Help model understand when to use each function
2. Validate arguments: Never trust model output blindly
3. Handle errors: Return useful error messages to model
4. Limit scope: Only expose necessary functions
5. Test edge cases: Model may call wrong function

Parallel Function Calling

Modern APIs support multiple simultaneous calls:

User: "What's the weather in Tokyo and Paris?"
Model: [
    {"function": "get_weather", "args": {"location": "Tokyo"}},
    {"function": "get_weather", "args": {"location": "Paris"}}
]

Related Concepts

• Agents - Autonomous tool-using systems
• Structured Output - Reliable data generation
• RAG - Knowledge retrieval