> ## Documentation Index
> Fetch the complete documentation index at: https://openrouter.ai/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Tool Calling

> Function calling and tool integration with the Responses API

The Responses API supports comprehensive tool calling capabilities, allowing models to call functions, execute tools in parallel, and handle complex multi-step workflows.

## Basic Tool Definition

Define tools using the OpenAI function calling format:

<CodeGroup>
  ```typescript title="TypeScript" expandable lines theme={null}
  const weatherTool = {
    type: 'function' as const,
    name: 'get_weather',
    description: 'Get the current weather in a location',
    strict: null,
    parameters: {
      type: 'object',
      properties: {
        location: {
          type: 'string',
          description: 'The city and state, e.g. San Francisco, CA',
        },
        unit: {
          type: 'string',
          enum: ['celsius', 'fahrenheit'],
        },
      },
      required: ['location'],
    },
  };

  const response = await fetch('https://openrouter.ai/api/v1/responses', {
    method: 'POST',
    headers: {
      'Authorization': 'Bearer YOUR_OPENROUTER_API_KEY',
      'Content-Type': 'application/json',
    },
    body: JSON.stringify({
      model: 'openai/o4-mini',
      input: [
        {
          type: 'message',
          role: 'user',
          content: [
            {
              type: 'input_text',
              text: 'What is the weather in San Francisco?',
            },
          ],
        },
      ],
      tools: [weatherTool],
      tool_choice: 'auto',
      max_output_tokens: 9000,
    }),
  });

  const result = await response.json();
  console.log(result);
  ```

  ```python title="Python" expandable lines theme={null}
  import requests

  weather_tool = {
      'type': 'function',
      'name': 'get_weather',
      'description': 'Get the current weather in a location',
      'strict': None,
      'parameters': {
          'type': 'object',
          'properties': {
              'location': {
                  'type': 'string',
                  'description': 'The city and state, e.g. San Francisco, CA',
              },
              'unit': {
                  'type': 'string',
                  'enum': ['celsius', 'fahrenheit'],
              },
          },
          'required': ['location'],
      },
  }

  response = requests.post(
      'https://openrouter.ai/api/v1/responses',
      headers={
          'Authorization': 'Bearer YOUR_OPENROUTER_API_KEY',
          'Content-Type': 'application/json',
      },
      json={
          'model': 'openai/o4-mini',
          'input': [
              {
                  'type': 'message',
                  'role': 'user',
                  'content': [
                      {
                          'type': 'input_text',
                          'text': 'What is the weather in San Francisco?',
                      },
                  ],
              },
          ],
          'tools': [weather_tool],
          'tool_choice': 'auto',
          'max_output_tokens': 9000,
      }
  )

  result = response.json()
  print(result)
  ```

  ```bash title="cURL" expandable lines theme={null}
  curl -X POST https://openrouter.ai/api/v1/responses \
    -H "Authorization: Bearer YOUR_OPENROUTER_API_KEY" \
    -H "Content-Type: application/json" \
    -d '{
      "model": "openai/o4-mini",
      "input": [
        {
          "type": "message",
          "role": "user",
          "content": [
            {
              "type": "input_text",
              "text": "What is the weather in San Francisco?"
            }
          ]
        }
      ],
      "tools": [
        {
          "type": "function",
          "name": "get_weather",
          "description": "Get the current weather in a location",
          "strict": null,
          "parameters": {
            "type": "object",
            "properties": {
              "location": {
                "type": "string",
                "description": "The city and state, e.g. San Francisco, CA"
              },
              "unit": {
                "type": "string",
                "enum": ["celsius", "fahrenheit"]
              }
            },
            "required": ["location"]
          }
        }
      ],
      "tool_choice": "auto",
      "max_output_tokens": 9000
    }'
  ```
</CodeGroup>

## Tool Choice Options

Control when and how tools are called:

| Tool Choice                             | Description                         |
| --------------------------------------- | ----------------------------------- |
| `auto`                                  | Model decides whether to call tools |
| `none`                                  | Model will not call any tools       |
| `{type: 'function', name: 'tool_name'}` | Force specific tool call            |

### Force Specific Tool

<CodeGroup>
  ```typescript title="TypeScript" expandable lines theme={null}
  const response = await fetch('https://openrouter.ai/api/v1/responses', {
    method: 'POST',
    headers: {
      'Authorization': 'Bearer YOUR_OPENROUTER_API_KEY',
      'Content-Type': 'application/json',
    },
    body: JSON.stringify({
      model: 'openai/o4-mini',
      input: [
        {
          type: 'message',
          role: 'user',
          content: [
            {
              type: 'input_text',
              text: 'Hello, how are you?',
            },
          ],
        },
      ],
      tools: [weatherTool],
      tool_choice: { type: 'function', name: 'get_weather' },
      max_output_tokens: 9000,
    }),
  });
  ```

  ```python title="Python" expandable lines theme={null}
  import requests

  response = requests.post(
      'https://openrouter.ai/api/v1/responses',
      headers={
          'Authorization': 'Bearer YOUR_OPENROUTER_API_KEY',
          'Content-Type': 'application/json',
      },
      json={
          'model': 'openai/o4-mini',
          'input': [
              {
                  'type': 'message',
                  'role': 'user',
                  'content': [
                      {
                          'type': 'input_text',
                          'text': 'Hello, how are you?',
                      },
                  ],
              },
          ],
          'tools': [weather_tool],
          'tool_choice': {'type': 'function', 'name': 'get_weather'},
          'max_output_tokens': 9000,
      }
  )
  ```
</CodeGroup>

### Disable Tool Calling

<CodeGroup>
  ```typescript title="TypeScript" expandable lines theme={null}
  const response = await fetch('https://openrouter.ai/api/v1/responses', {
    method: 'POST',
    headers: {
      'Authorization': 'Bearer YOUR_OPENROUTER_API_KEY',
      'Content-Type': 'application/json',
    },
    body: JSON.stringify({
      model: 'openai/o4-mini',
      input: [
        {
          type: 'message',
          role: 'user',
          content: [
            {
              type: 'input_text',
              text: 'What is the weather in Paris?',
            },
          ],
        },
      ],
      tools: [weatherTool],
      tool_choice: 'none',
      max_output_tokens: 9000,
    }),
  });
  ```

  ```python title="Python" expandable lines theme={null}
  import requests

  response = requests.post(
      'https://openrouter.ai/api/v1/responses',
      headers={
          'Authorization': 'Bearer YOUR_OPENROUTER_API_KEY',
          'Content-Type': 'application/json',
      },
      json={
          'model': 'openai/o4-mini',
          'input': [
              {
                  'type': 'message',
                  'role': 'user',
                  'content': [
                      {
                          'type': 'input_text',
                          'text': 'What is the weather in Paris?',
                      },
                  ],
              },
          ],
          'tools': [weather_tool],
          'tool_choice': 'none',
          'max_output_tokens': 9000,
      }
  )
  ```
</CodeGroup>

## Multiple Tools

Define multiple tools for complex workflows:

<CodeGroup>
  ```typescript title="TypeScript" expandable lines theme={null}
  const calculatorTool = {
    type: 'function' as const,
    name: 'calculate',
    description: 'Perform mathematical calculations',
    strict: null,
    parameters: {
      type: 'object',
      properties: {
        expression: {
          type: 'string',
          description: 'The mathematical expression to evaluate',
        },
      },
      required: ['expression'],
    },
  };

  const response = await fetch('https://openrouter.ai/api/v1/responses', {
    method: 'POST',
    headers: {
      'Authorization': 'Bearer YOUR_OPENROUTER_API_KEY',
      'Content-Type': 'application/json',
    },
    body: JSON.stringify({
      model: 'openai/o4-mini',
      input: [
        {
          type: 'message',
          role: 'user',
          content: [
            {
              type: 'input_text',
              text: 'What is 25 * 4?',
            },
          ],
        },
      ],
      tools: [weatherTool, calculatorTool],
      tool_choice: 'auto',
      max_output_tokens: 9000,
    }),
  });
  ```

  ```python title="Python" expandable lines theme={null}
  calculator_tool = {
      'type': 'function',
      'name': 'calculate',
      'description': 'Perform mathematical calculations',
      'strict': None,
      'parameters': {
          'type': 'object',
          'properties': {
              'expression': {
                  'type': 'string',
                  'description': 'The mathematical expression to evaluate',
              },
          },
          'required': ['expression'],
      },
  }

  response = requests.post(
      'https://openrouter.ai/api/v1/responses',
      headers={
          'Authorization': 'Bearer YOUR_OPENROUTER_API_KEY',
          'Content-Type': 'application/json',
      },
      json={
          'model': 'openai/o4-mini',
          'input': [
              {
                  'type': 'message',
                  'role': 'user',
                  'content': [
                      {
                          'type': 'input_text',
                          'text': 'What is 25 * 4?',
                      },
                  ],
              },
          ],
          'tools': [weather_tool, calculator_tool],
          'tool_choice': 'auto',
          'max_output_tokens': 9000,
      }
  )
  ```
</CodeGroup>

## Parallel Tool Calls

The API supports parallel execution of multiple tools:

<CodeGroup>
  ```typescript title="TypeScript" expandable lines theme={null}
  const response = await fetch('https://openrouter.ai/api/v1/responses', {
    method: 'POST',
    headers: {
      'Authorization': 'Bearer YOUR_OPENROUTER_API_KEY',
      'Content-Type': 'application/json',
    },
    body: JSON.stringify({
      model: 'openai/o4-mini',
      input: [
        {
          type: 'message',
          role: 'user',
          content: [
            {
              type: 'input_text',
              text: 'Calculate 10*5 and also tell me the weather in Miami',
            },
          ],
        },
      ],
      tools: [weatherTool, calculatorTool],
      tool_choice: 'auto',
      max_output_tokens: 9000,
    }),
  });

  const result = await response.json();
  console.log(result);
  ```

  ```python title="Python" expandable lines theme={null}
  import requests

  response = requests.post(
      'https://openrouter.ai/api/v1/responses',
      headers={
          'Authorization': 'Bearer YOUR_OPENROUTER_API_KEY',
          'Content-Type': 'application/json',
      },
      json={
          'model': 'openai/o4-mini',
          'input': [
              {
                  'type': 'message',
                  'role': 'user',
                  'content': [
                      {
                          'type': 'input_text',
                          'text': 'Calculate 10*5 and also tell me the weather in Miami',
                      },
                  ],
              },
          ],
          'tools': [weather_tool, calculator_tool],
          'tool_choice': 'auto',
          'max_output_tokens': 9000,
      }
  )

  result = response.json()
  print(result)
  ```
</CodeGroup>

## Tool Call Response

When tools are called, the response includes function call information:

```json expandable lines theme={null}
{
  "id": "resp_1234567890",
  "object": "response",
  "created_at": 1234567890,
  "model": "openai/o4-mini",
  "output": [
    {
      "type": "function_call",
      "id": "fc_abc123",
      "call_id": "call_xyz789",
      "name": "get_weather",
      "arguments": "{\"location\":\"San Francisco, CA\"}"
    }
  ],
  "usage": {
    "input_tokens": 45,
    "output_tokens": 25,
    "total_tokens": 70
  },
  "status": "completed"
}
```

## Tool Responses in Conversation

Include tool responses in follow-up requests:

<CodeGroup>
  ```typescript title="TypeScript" expandable lines theme={null}
  const response = await fetch('https://openrouter.ai/api/v1/responses', {
    method: 'POST',
    headers: {
      'Authorization': 'Bearer YOUR_OPENROUTER_API_KEY',
      'Content-Type': 'application/json',
    },
    body: JSON.stringify({
      model: 'openai/o4-mini',
      input: [
        {
          type: 'message',
          role: 'user',
          content: [
            {
              type: 'input_text',
              text: 'What is the weather in Boston?',
            },
          ],
        },
        {
          type: 'function_call',
          id: 'fc_1',
          call_id: 'call_123',
          name: 'get_weather',
          arguments: JSON.stringify({ location: 'Boston, MA' }),
        },
        {
          type: 'function_call_output',
          id: 'fc_output_1',
          call_id: 'call_123',
          output: JSON.stringify({ temperature: '72°F', condition: 'Sunny' }),
        },
        {
          type: 'message',
          role: 'assistant',
          id: 'msg_abc123',
          status: 'completed',
          content: [
            {
              type: 'output_text',
              text: 'The weather in Boston is currently 72°F and sunny. This looks like perfect weather for a picnic!',
              annotations: []
            }
          ]
        },
        {
          type: 'message',
          role: 'user',
          content: [
            {
              type: 'input_text',
              text: 'Is that good weather for a picnic?',
            },
          ],
        },
      ],
      max_output_tokens: 9000,
    }),
  });
  ```

  ```python title="Python" expandable lines theme={null}
  import requests

  response = requests.post(
      'https://openrouter.ai/api/v1/responses',
      headers={
          'Authorization': 'Bearer YOUR_OPENROUTER_API_KEY',
          'Content-Type': 'application/json',
      },
      json={
          'model': 'openai/o4-mini',
          'input': [
              {
                  'type': 'message',
                  'role': 'user',
                  'content': [
                      {
                          'type': 'input_text',
                          'text': 'What is the weather in Boston?',
                      },
                  ],
              },
              {
                  'type': 'function_call',
                  'id': 'fc_1',
                  'call_id': 'call_123',
                  'name': 'get_weather',
                  'arguments': '{"location": "Boston, MA"}',
              },
              {
                  'type': 'function_call_output',
                  'id': 'fc_output_1',
                  'call_id': 'call_123',
                  'output': '{"temperature": "72°F", "condition": "Sunny"}',
              },
              {
                  'type': 'message',
                  'role': 'assistant',
                  'id': 'msg_abc123',
                  'status': 'completed',
                  'content': [
                      {
                          'type': 'output_text',
                          'text': 'The weather in Boston is currently 72°F and sunny. This looks like perfect weather for a picnic!',
                          'annotations': []
                      }
                  ]
              },
              {
                  'type': 'message',
                  'role': 'user',
                  'content': [
                      {
                          'type': 'input_text',
                          'text': 'Is that good weather for a picnic?',
                      },
                  ],
              },
          ],
          'max_output_tokens': 9000,
      }
  )
  ```
</CodeGroup>

<Info>
  **Optional Field**

  The `id` field is optional for `function_call_output` objects. Only `type`, `call_id`, and `output` are required — `call_id` is what pairs the output with its originating `function_call`. The examples above include `id` for completeness, but you can safely omit it.
</Info>

### Multimodal tool outputs

`function_call_output.output` accepts either a string or an array of input content parts (`input_text`, `input_image`, `input_file`) — the same shape as user-message content. Use the array form to return images or files from a tool; the non-text parts are only forwarded to supported multimodal models.

```json lines theme={null}
{
  "type": "function_call_output",
  "call_id": "call_123",
  "output": [
    {
      "type": "input_text",
      "text": "{\"results\":[{\"title\":\"Golden Gate Bridge\"}]}"
    },
    {
      "type": "input_image",
      "image_url": "https://example.com/image.jpg"
    }
  ]
}
```

## Streaming Tool Calls

Monitor tool calls in real-time with streaming:

<CodeGroup>
  ```typescript title="TypeScript" expandable lines theme={null}
  const response = await fetch('https://openrouter.ai/api/v1/responses', {
    method: 'POST',
    headers: {
      'Authorization': 'Bearer YOUR_OPENROUTER_API_KEY',
      'Content-Type': 'application/json',
    },
    body: JSON.stringify({
      model: 'openai/o4-mini',
      input: [
        {
          type: 'message',
          role: 'user',
          content: [
            {
              type: 'input_text',
              text: 'What is the weather like in Tokyo, Japan? Please check the weather.',
            },
          ],
        },
      ],
      tools: [weatherTool],
      tool_choice: 'auto',
      stream: true,
      max_output_tokens: 9000,
    }),
  });

  const reader = response.body?.getReader();
  const decoder = new TextDecoder();

  while (true) {
    const { done, value } = await reader.read();
    if (done) break;

    const chunk = decoder.decode(value);
    const lines = chunk.split('\n');

    for (const line of lines) {
      if (line.startsWith('data: ')) {
        const data = line.slice(6);
        if (data === '[DONE]') return;

        try {
          const parsed = JSON.parse(data);
          if (parsed.type === 'response.output_item.added' &&
              parsed.item?.type === 'function_call') {
            console.log('Function call:', parsed.item.name);
          }
          if (parsed.type === 'response.function_call_arguments.done') {
            console.log('Arguments:', parsed.arguments);
          }
        } catch (e) {
          // Skip invalid JSON
        }
      }
    }
  }
  ```

  ```python title="Python" expandable lines theme={null}
  import requests
  import json

  response = requests.post(
      'https://openrouter.ai/api/v1/responses',
      headers={
          'Authorization': 'Bearer YOUR_OPENROUTER_API_KEY',
          'Content-Type': 'application/json',
      },
      json={
          'model': 'openai/o4-mini',
          'input': [
              {
                  'type': 'message',
                  'role': 'user',
                  'content': [
                      {
                          'type': 'input_text',
                          'text': 'What is the weather like in Tokyo, Japan? Please check the weather.',
                      },
                  ],
              },
          ],
          'tools': [weather_tool],
          'tool_choice': 'auto',
          'stream': True,
          'max_output_tokens': 9000,
      },
      stream=True
  )

  for line in response.iter_lines():
      if line:
          line_str = line.decode('utf-8')
          if line_str.startswith('data: '):
              data = line_str[6:]
              if data == '[DONE]':
                  break
              try:
                  parsed = json.loads(data)
                  if (parsed.get('type') == 'response.output_item.added' and
                      parsed.get('item', {}).get('type') == 'function_call'):
                      print(f"Function call: {parsed['item']['name']}")
                  if parsed.get('type') == 'response.function_call_arguments.done':
                      print(f"Arguments: {parsed.get('arguments', '')}")
              except json.JSONDecodeError:
                  continue
  ```
</CodeGroup>

## Tool Validation

Ensure tool calls have proper structure:

```json lines theme={null}
{
  "type": "function_call",
  "id": "fc_abc123",
  "call_id": "call_xyz789",
  "name": "get_weather",
  "arguments": "{\"location\":\"Seattle, WA\"}"
}
```

Required fields:

* `type`: Always "function\_call"
* `id`: Unique identifier for the function call object
* `name`: Function name matching tool definition
* `arguments`: Valid JSON string with function parameters
* `call_id`: Unique identifier for the call

## Best Practices

1. **Clear descriptions**: Provide detailed function descriptions and parameter explanations
2. **Proper schemas**: Use valid JSON Schema for parameters
3. **Error handling**: Handle cases where tools might not be called
4. **Parallel execution**: Design tools to work independently when possible
5. **Conversation flow**: Include tool responses in follow-up requests for context

## Next Steps

* Learn about [Web Search](./web-search) integration
* Explore [Reasoning](./reasoning) with tools
* Review [Basic Usage](./basic-usage) fundamentals