---
title: "Function Calling"
original_url: "https://tds.s-anand.net/#/function-calling?id=function-calling-with-openai"
downloaded_at: "2025-06-08T23:26:04.973121"
---

[Function Calling with OpenAI](#/function-calling?id=function-calling-with-openai)
----------------------------------------------------------------------------------

[Function Calling](https://platform.openai.com/docs/guides/function-calling) allows Large Language Models to convert natural language into structured function calls. This is perfect for building chatbots and AI assistants that need to interact with your backend systems.

OpenAI supports [Function Calling](https://platform.openai.com/docs/guides/function-calling) – a way for LLMs to suggest what functions to call and how.

[![OpenAI Function Calling - Full Beginner Tutorial](https://i.ytimg.com/vi_webp/aqdWSYWC_LI/sddefault.webp)](https://youtu.be/aqdWSYWC_LI)

Here’s a minimal example using Python and OpenAI’s function calling that identifies the weather in a given location.

```
# /// script
# requires-python = ">=3.11"
# dependencies = [
#   "httpx",
# ]
# ///

import httpx
import os
from typing import Dict, Any


def query_gpt(user_input: str, tools: list[Dict[str, Any]]) -> Dict[str, Any]:
    response = httpx.post(
        "https://api.openai.com/v1/chat/completions",
        headers={
            "Authorization": f"Bearer {os.getenv('OPENAI_API_KEY')}",
            "Content-Type": "application/json",
        },
        json={
            "model": "gpt-4o-mini",
            "messages": [{"role": "user", "content": user_input}],
            "tools": tools,
            "tool_choice": "auto",
        },
    )
    return response.json()["choices"][0]["message"]


WEATHER_TOOL = {
    "type": "function",
    "function": {
        "name": "get_weather",
        "description": "Get the current weather for a location",
        "parameters": {
            "type": "object",
            "properties": {
                "location": {"type": "string", "description": "City name or coordinates"}
            },
            "required": ["location"],
            "additionalProperties": False,
        },
        "strict": True,
    },
}

if __name__ == "__main__":
    response = query_gpt("What is the weather in San Francisco?", [WEATHER_TOOL])
    print([tool_call["function"] for tool_call in response["tool_calls"]])Copy to clipboardErrorCopied
```

### [How to define functions](#/function-calling?id=how-to-define-functions)

The function definition is a [JSON schema](https://json-schema.org/) with a few OpenAI specific properties.
See the [Supported schemas](https://platform.openai.com/docs/guides/structured-outputs#supported-schemas).

Here’s an example of a function definition for scheduling a meeting:

```
MEETING_TOOL = {
    "type": "function",
    "function": {
        "name": "schedule_meeting",
        "description": "Schedule a meeting room for a specific date and time",
        "parameters": {
            "type": "object",
            "properties": {
                "date": {
                    "type": "string",
                    "description": "Meeting date in YYYY-MM-DD format"
                },
                "time": {
                    "type": "string",
                    "description": "Meeting time in HH:MM format"
                },
                "meeting_room": {
                    "type": "string",
                    "description": "Name of the meeting room"
                }
            },
            "required": ["date", "time", "meeting_room"],
            "additionalProperties": False
        },
        "strict": True
    }
}Copy to clipboardErrorCopied
```

### [How to define multiple functions](#/function-calling?id=how-to-define-multiple-functions)

You can define multiple functions by passing a list of function definitions to the `tools` parameter.

Here’s an example of a list of function definitions for handling employee expenses and calculating performance bonuses:

```
tools = [
    {
        "type": "function",
        "function": {
            "name": "get_expense_balance",
            "description": "Get expense balance for an employee",
            "parameters": {
                "type": "object",
                "properties": {
                    "employee_id": {
                        "type": "integer",
                        "description": "Employee ID number"
                    }
                },
                "required": ["employee_id"],
                "additionalProperties": False
            },
            "strict": True
        }
    },
    {
        "type": "function",
        "function": {
            "name": "calculate_performance_bonus",
            "description": "Calculate yearly performance bonus for an employee",
            "parameters": {
                "type": "object",
                "properties": {
                    "employee_id": {
                        "type": "integer",
                        "description": "Employee ID number"
                    },
                    "current_year": {
                        "type": "integer",
                        "description": "Year to calculate bonus for"
                    }
                },
                "required": ["employee_id", "current_year"],
                "additionalProperties": False
            },
            "strict": True
        }
    }
]Copy to clipboardErrorCopied
```

Best Practices:

1. **Use Strict Mode**
   * Always set `strict: True` to ensure valid function calls
   * Define all required parameters
   * Set `additionalProperties: False`
2. **Use tool choice**
   * Set `tool_choice: "required"` to ensure that the model will always call one or more tools
   * The default is `tool_choice: "auto"` which means the model will choose a tool only if appropriate
3. **Clear Descriptions**
   * Write detailed function and parameter descriptions
   * Include expected formats and units
   * Mention any constraints or limitations
4. **Error Handling**
   * Validate function inputs before execution
   * Return clear error messages
   * Handle missing or invalid parameters

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