Use function calling with Gemini using OpenAI SDK

This code sample demonstrates how to do function calling in Gemini using Chat Completions API in the OpenAI SDK.

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Code sample

Python

Before trying this sample, follow the Python setup instructions in the Vertex AI quickstart using client libraries. For more information, see the Vertex AI Python API reference documentation.

To authenticate to Vertex AI, set up Application Default Credentials. For more information, see Set up authentication for a local development environment. 

import vertexai
import openai

from google.auth import default, transport

# TODO(developer): Update & uncomment below line
# PROJECT_ID = "your-project-id"
location = "us-central1"

vertexai.init(project=PROJECT_ID, location=location)

# Programmatically get an access token
credentials, _ = default(scopes=["https://www.googleapis.com/auth/cloud-platform"])
auth_request = transport.requests.Request()
credentials.refresh(auth_request)

# OpenAI Client
client = openai.OpenAI(
    base_url=f"https://{location}-aiplatform.googleapis.com/v1beta1/projects/{PROJECT_ID}/locations/{location}/endpoints/openapi",
    api_key=credentials.token,
)

tools = [
    {
        "type": "function",
        "function": {
            "name": "get_current_weather",
            "description": "Get the current weather in a given location",
            "parameters": {
                "type": "object",
                "properties": {
                    "location": {
                        "type": "string",
                        "description": "The city and state, e.g. San Francisco, CA or a zip code e.g. 95616",
                    },
                },
                "required": ["location"],
            },
        },
    }
]

messages = []
messages.append(
    {
        "role": "system",
        "content": "Don't make assumptions about what values to plug into functions. Ask for clarification if a user request is ambiguous.",
    }
)
messages.append({"role": "user", "content": "What is the weather in Boston, MA?"})

response = client.chat.completions.create(
    model="google/gemini-2.0-flash-001",
    messages=messages,
    tools=tools,
    tool_choice="auto",
)

print("Function:", response.choices[0].message.tool_calls[0].id)
print("Arguments:", response.choices[0].message.tool_calls[0].function.arguments)
# Example response:
# Function: get_current_weather
# Arguments: {"location":"Boston"}

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