Tool Calls
Last updated March 7, 2026
The Chat Completions API supports function calling, allowing models to call tools and functions. This follows the same specification as the OpenAI Function Calling API.
tool-calls.ts
import OpenAI from 'openai';
const apiKey = process.env.AI_GATEWAY_API_KEY || process.env.VERCEL_OIDC_TOKEN;
const openai = new OpenAI({
apiKey,
baseURL: 'https://ai-gateway.vercel.sh/v1',
});
const tools: OpenAI.Chat.Completions.ChatCompletionTool[] = [
{
type: 'function',
function: {
name: 'get_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',
},
unit: {
type: 'string',
enum: ['celsius', 'fahrenheit'],
description: 'The unit for temperature',
},
},
required: ['location'],
},
},
},
];
const completion = await openai.chat.completions.create({
model: 'anthropic/claude-opus-4.6',
messages: [
{
role: 'user',
content: 'What is the weather like in San Francisco?',
},
],
tools: tools,
tool_choice: 'auto',
stream: false,
});
console.log('Assistant:', completion.choices[0].message.content);
console.log('Tool calls:', completion.choices[0].message.tool_calls);tool-calls.py
import os
from openai import OpenAI
api_key = os.getenv('AI_GATEWAY_API_KEY') or os.getenv('VERCEL_OIDC_TOKEN')
client = OpenAI(
api_key=api_key,
base_url='https://ai-gateway.vercel.sh/v1'
)
tools = [
{
'type': 'function',
'function': {
'name': 'get_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'
},
'unit': {
'type': 'string',
'enum': ['celsius', 'fahrenheit'],
'description': 'The unit for temperature'
}
},
'required': ['location']
}
}
}
]
completion = client.chat.completions.create(
model='anthropic/claude-opus-4.6',
messages=[
{
'role': 'user',
'content': 'What is the weather like in San Francisco?'
}
],
tools=tools,
tool_choice='auto',
stream=False,
)
print('Assistant:', completion.choices[0].message.content)
print('Tool calls:', completion.choices[0].message.tool_calls)Controlling tool selection: By default, tool_choice is set to 'auto', allowing the model to decide when to use tools. You can also:
- Set to
'none'to disable tool calls - Force a specific tool with:
tool_choice: { type: 'function', function: { name: 'your_function_name' } }
When the model makes tool calls, the response includes tool call information:
{
"id": "chatcmpl-123",
"object": "chat.completion",
"created": 1677652288,
"model": "anthropic/claude-opus-4.6",
"choices": [
{
"index": 0,
"message": {
"role": "assistant",
"content": null,
"tool_calls": [
{
"id": "call_123",
"type": "function",
"function": {
"name": "get_weather",
"arguments": "{\"location\": \"San Francisco, CA\", \"unit\": \"celsius\"}"
}
}
]
},
"finish_reason": "tool_calls"
}
],
"usage": {
"prompt_tokens": 82,
"completion_tokens": 18,
"total_tokens": 100
}
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