Send Custom Body from useChat

By default, useChat sends all messages as well as information from the request to the server. However, it is often desirable to control the body content that is sent to the server, e.g. to:

only send the last message
send additional data along with the message
change the structure of the request body

The prepareSendMessagesRequest option allows you to customize the body content that is sent to the server. The function receives the message list, the request data, and the request body from the append call. It should return the body content that will be sent to the server.

Example

This example shows how to only send the text of the last message to the server. This can be useful if you want to reduce the amount of data sent to the server.

Client

app/page.tsx

'use client';

import { useChat } from '@ai-sdk/react';
import { DefaultChatTransport } from 'ai';
import { useState } from 'react';

export default function Chat() {
  const [input, setInput] = useState('');
  const { messages, sendMessage } = useChat({
    transport: new DefaultChatTransport({
      prepareSendMessagesRequest: ({ id, messages }) => {
        return {
          body: {
            id,
            message: messages[messages.length - 1],
          },
        };
      },
    }),
  });

  return (
    <div>
    {messages.map((message, index) => (
      <div key={index}>
        {message.role === 'user' ? 'User: ' : 'AI: '}
        {message.parts.map((part) => {
          switch (part.type) {
            case "text":
              return <div key={`${message.id}-text`}>{part.text}</div>;
          }
        })}
      </div>
    ))}

      <form onSubmit={(e) => {
        e.preventDefault();
        sendMessage({text: input});
        setInput('');
      }}>
        <input value={input} onChange={(e) => setInput(e.currentTarget.value)} />
      </form>
    </div>
  );
}

Server

We need to adjust the server to receive the custom request format with the chat ID and last message. The rest of the message history can be loaded from storage.

app/api/chat/route.ts

import { openai } from '@ai-sdk/openai';
import { convertToModelMessages, streamText } from 'ai';

// Allow streaming responses up to 30 seconds
export const maxDuration = 30;

export async function POST(req: Request) {
  const { id, message } = await req.json();

  // Load existing messages and add the new one
  const messages = await loadMessages(id);
  messages.push(message);

  // Call the language model
  const result = streamText({
    model: openai('gpt-4.1'),
    messages: convertToModelMessages(messages),
  });

  // Respond with the stream
  return result.toUIMessageStreamResponse({
    originalMessages: messages,
    onFinish: ({ messages: newMessages }) => {
      saveMessages(id, newMessages);
    },
  });
}