Repeated assistant messages in useChat

Issue

When using useChat with streamText on the server, the assistant's messages appear duplicated in the UI - showing both the previous message and the new message, or showing the same message multiple times. This can occur when using tool calls or complex message flows.

// Server-side code that may experience assistant message duplication on the client
export async function POST(req: Request) {
  const { messages } = await req.json();

  const result = streamText({
    model: openai('gpt-4o-mini'),
    messages: convertToModelMessages(messages),
    tools: {
      weather: {
        description: 'Get the weather for a location',
        parameters: z.object({
          location: z.string(),
        }),
        execute: async ({ location }) => {
          return { temperature: 72, condition: 'sunny' };
        },
      },
    },
  });

  return result.toUIMessageStreamResponse();
}

Background

The duplication occurs because toUIMessageStreamResponse generates new message IDs for each new message.

Solution

Pass the original messages array to toUIMessageStreamResponse using the originalMessages option. By passing originalMessages, the method can reuse existing message IDs instead of generating new ones, ensuring the client properly updates existing messages rather than creating duplicates.

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

  const result = streamText({
    model: openai('gpt-4o-mini'),
    messages: convertToModelMessages(messages),
    tools: {
      weather: {
        description: 'Get the weather for a location',
        parameters: z.object({
          location: z.string(),
        }),
        execute: async ({ location }) => {
          return { temperature: 72, condition: 'sunny' };
        },
      },
    },
  });

  return result.toUIMessageStreamResponse({
    originalMessages: messages, // Pass the original messages here
    generateMessageId: generateId,
    onFinish: ({ messages }) => {
      saveChat({ id, messages });
    },
  });
}