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Tool Calling

Last updated January 21, 2026

The OpenResponses API supports tool calling to give models access to external functions. Define tools in your request with a name, description, and JSON schema for parameters. When the model determines it needs a tool to answer the user's question, it returns a function_call output with the tool name and arguments for you to execute.

tool-calls.ts
const apiKey = process.env.AI_GATEWAY_API_KEY;
 
const response = await fetch('https://ai-gateway.vercel.sh/v1/responses', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
    Authorization: `Bearer ${apiKey}`,
  },
  body: JSON.stringify({
    model: 'openai/gpt-5.4',
    input: [
      {
        type: 'message',
        role: 'user',
        content: 'What is the weather like in New York?',
      },
    ],
    tools: [
      {
        type: 'function',
        function: {
          name: 'get_weather',
          description: 'Get the current weather in a location',
          parameters: {
            type: 'object',
            properties: {
              location: {
                type: 'string',
                description: 'The city and state, e.g. San Francisco, CA',
              },
            },
            required: ['location'],
          },
        },
      },
    ],
    tool_choice: 'auto',
  }),
});
tool-calls.py
import os
from openai import OpenAI
 
client = OpenAI(
    api_key=os.getenv('AI_GATEWAY_API_KEY'),
    base_url='https://ai-gateway.vercel.sh/v1',
)
 
response = client.responses.create(
    model='openai/gpt-5.4',
    input=[
        {
            'type': 'message',
            'role': 'user',
            'content': 'What is the weather like in New York?',
        },
    ],
    tools=[
        {
            'type': 'function',
            'function': {
                'name': 'get_weather',
                'description': 'Get the current weather in a location',
                'parameters': {
                    'type': 'object',
                    'properties': {
                        'location': {
                            'type': 'string',
                            'description': 'The city and state, e.g. San Francisco, CA',
                        },
                    },
                    'required': ['location'],
                },
            },
        },
    ],
    tool_choice='auto',
)

Tool call response

When the model decides to call a tool, the response includes a function_call output:

{
  "output": [
    {
      "type": "function_call",
      "name": "get_weather",
      "arguments": "{\"location\": \"New York, NY\"}",
      "call_id": "call_abc123"
    }
  ]
}

Tool choice options

  • auto - The model decides whether to call a tool
  • required - The model must call at least one tool
  • none - The model cannot call any tools
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