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OpenAI Functions

The Vercel AI SDK has experimental support for OpenAI functions. Any of the content below is subject to change as OpenAI continues to develop their functions API and we iterate on the Vercel AI SDK. You can see our planned roadmap here.

If you are unfamiliar with OpenAI functions, it's recommended you refer to OpenAI's announcement post. It's important to know that OpenAI does not handle calling the functions, but instead passes the function call to the consumer to handle via a special message and JSON.

Defining Functions

To use functions with the OpenAI API, you need to pass a functions object to the API so the LLM knows it's capabilities. The functions object is an array containing the schema for each function you want to define. Here is an example:

import { OpenAIStream, StreamingTextResponse } from 'ai';
import OpenAI from 'openai';
import type { ChatCompletionCreateParams } from 'openai/resources/chat';
// Create an OpenAI API client
const openai = new OpenAI({
apiKey: process.env.OPENAI_API_KEY || '',
// Function definition:
const functions: ChatCompletionCreateParams.Function[] = [
name: 'get_current_weather',
description: 'Get the current weather',
parameters: {
type: 'object',
properties: {
location: {
type: 'string',
description: 'The city and state, e.g. San Francisco, CA',
format: {
type: 'string',
enum: ['celsius', 'fahrenheit'],
'The temperature unit to use. Infer this from the users location.',
required: ['location', 'format'],
// And use it like this:
export async function POST(req: Request) {
const { messages } = await req.json();
const response = await openai.chat.completions.create({
model: 'gpt-3.5-turbo-0613',
stream: true,
const stream = OpenAIStream(response);
return new StreamingTextResponse(stream);

You can then choose how you want to handle each function call: on the server or on the client.

Handling Function Calls on the Server

On the server, you can pass an experimental_onFunctionCall callback to the OpenAIStream, which will be called when the model calls a function. In order to support recursively calling functions and to construct the message context in the nested OpenAI calls, you can use createFunctionCallMessages to get the "assistant" and "function" messages. You can also return a string which will be sent to the client as the "assistant" message (or returned back to the model as a response to a recursive function call).

const stream = OpenAIStream(response, {
experimental_onFunctionCall: async (
{ name, arguments: args },
) => {
// if you skip the function call and return nothing, the `function_call`
// message will be sent to the client for it to handle
if (name === 'get_current_weather') {
// Call a weather API here
const weatherData = {
temperature: 20,
unit: args.format === 'celsius' ? 'C' : 'F',
// `createFunctionCallMessages` constructs the relevant "assistant" and "function" messages for you
const newMessages = createFunctionCallMessages(weatherData);
return openai.chat.completions.create({
messages: [...messages, ...newMessages],
stream: true,
model: 'gpt-3.5-turbo-0613',
// see "Recursive Function Calls" below

You will then receive a regular "assistant" message on the client containing the output of the function call.

Recursive Function Calls

If you want to support recursive function calls, you need to pass the functions object to the createChatCompletion call in the experimental_onFunctionCall handler. The response from the nested call will be processed by the same logic (and therefore the same handler) as the initial OpenAI call, and the final response will be returned to the client.

Handling Function Calls on the Client

On the client, you can pass an experimental_onFunctionCall handler to the useCompletion and useChat hooks. This callback will be called when the server does not handle a function call and streams it to the client. The handler will be called when the function is invoked. Here's a sample function handler:

const functionCallHandler: FunctionCallHandler = async (
) => {
if (functionCall.name === 'get_current_weather') {
if (functionCall.arguments) {
const parsedFunctionCallArguments = JSON.parse(functionCall.arguments);
// You now have access to the parsed arguments here (assuming the JSON was valid)
// If JSON is invalid, return an appropriate message to the model so that it may retry?
// Generate a fake temperature
const temperature = Math.floor(Math.random() * (100 - 30 + 1) + 30);
// Generate random weather condition
const weather = ['sunny', 'cloudy', 'rainy', 'snowy'][
Math.floor(Math.random() * 4)
const functionResponse: ChatRequest = {
messages: [
id: generateId(),
name: 'get_current_weather',
role: 'function' as const,
content: JSON.stringify({
info: 'This data is randomly generated and came from a fake weather API!',
return functionResponse;

Then just pass the handler to the hook:

const { messages, input, handleInputChange, handleSubmit } = useChat({
experimental_onFunctionCall: functionCallHandler,

Now, when the model calls the get_current_weather function, the OpenAI API will return a specially formatted message with the arguments and the name of the function to call. Your handler will then be invoked on the client to handle the function call and manipulate the chat accordingly.

Rendering messages

The Message type has been updated with an optional function_call value that can either be an CreateChatCompletionRequestFunction object or a string.

A CreateChatCompletionRequestFunction looks like this:

* The name of the function to call.
name?: string;
* The arguments to call the function with, as generated by the model in JSON format. Note that the model does not always generate valid JSON, and may hallucinate parameters not defined by your function schema. Validate the arguments in your code before calling your function.
arguments?: string;

So you can interact with the function call:

message.role === 'function' && message.function_call && (
<p>Function name: {message.function_call.name}</p>
<p>Function arguments: {message.function_call.arguments}</p>

However, while a function is streaming in, it will not be valid JSON that can be parsed to a CreateChatCompletionRequestFunction object, so the function_call value will be a string instead. If you want to stream in the functions raw data, you need to handle this special case:

if (m.function_call) {
const functionCallString =
typeof m.function_call === 'string'
? m.function_call
: JSON.stringify(m.function_call);
return (
{functionCallString.split('\\n').map((line, index) => (
<p key={index}>{line}</p>
} else {
return m.content;

When using the streamData API, the assistant message containing the function_call has no content. Previously, it contained the stringified function_call contents.