Implementation:Openai Openai python Pydantic Function Tool

Knowledge Sources	openai-python OpenAI Function Calling
Domains	NLP, Data_Extraction
Last Updated	2026-02-15 00:00 GMT

Overview

Concrete tool for converting Pydantic models into API-compatible tool definitions and JSON schemas provided by the OpenAI Python SDK.

Description

The pydantic_function_tool() function converts a Pydantic model class into a ChatCompletionToolParam dictionary suitable for the tools parameter. Internally it uses to_strict_json_schema() to convert the model's fields into a strict JSON schema. The function name defaults to the class name, and the description defaults to the class docstring.

Usage

Use when defining tools for function calling or when you want to convert a Pydantic model to a tool definition without manually writing JSON schemas.

Code Reference

Source Location

Repository: openai-python
File: src/openai/lib/_tools.py
Lines: L40-66
File: src/openai/lib/_pydantic.py
Lines: L1-155 (to_strict_json_schema)

Signature

def pydantic_function_tool(
    model: type[pydantic.BaseModel],
    *,
    name: str | None = None,
    description: str | None = None,
) -> ChatCompletionToolParam:
    """
    Converts a Pydantic model into a function tool definition.

    Args:
        model: Pydantic model class defining the tool's parameters.
        name: Override tool name (defaults to model class name).
        description: Override description (defaults to model docstring).

    Returns:
        ChatCompletionToolParam dict with type="function" and strict JSON schema.
    """

Import

from openai.lib import pydantic_function_tool

I/O Contract

Inputs

Name	Type	Required	Description
model	type[pydantic.BaseModel]	Yes	Pydantic model defining tool parameters
name	str or None	No	Custom tool name (default: class name)
description	str or None	No	Custom description (default: class docstring)

Outputs

Name	Type	Description
tool_param	ChatCompletionToolParam	Dict with type="function", function name, description, and strict JSON schema

Usage Examples

Basic Tool Definition

from pydantic import BaseModel
from openai.lib import pydantic_function_tool

class GetWeather(BaseModel):
    """Get the current weather for a location."""
    city: str
    unit: str = "celsius"

tool = pydantic_function_tool(GetWeather)
# Result:
# {
#     "type": "function",
#     "function": {
#         "name": "GetWeather",
#         "description": "Get the current weather for a location.",
#         "parameters": {"type": "object", "properties": {...}, "required": [...]}
#     }
# }

With Custom Name

tool = pydantic_function_tool(
    GetWeather,
    name="weather_lookup",
    description="Look up weather conditions",
)

Used with Chat Completions

from openai import OpenAI
from openai.lib import pydantic_function_tool
from pydantic import BaseModel

class SearchWeb(BaseModel):
    """Search the web for information."""
    query: str
    max_results: int = 5

client = OpenAI()
response = client.chat.completions.create(
    model="gpt-4o",
    messages=[{"role": "user", "content": "Find info about Python 3.12"}],
    tools=[pydantic_function_tool(SearchWeb)],
)

Related Pages

Page Connections

Double-click a node to navigate. Hold to expand connections.

Principle

Implementation

Heuristic

Environment