How can I limit the Assistant API to return exactly one message per user input?

Below is an explanation based on the available documentation:

No Built-in Parameter for Message Count

Currently, the Assistant API and related SDK integrations (e.g. when using the Vercel AI SDK with Assistant UI) do not provide a specific parameter to restrict the response to exactly one message per user message. In practice, the assistant may stream multiple message deltas—as seen in the streaming response handling (e.g. using forwardStream)—which could result in what appears to be multiple messages.

Recommended Approach: Custom Business Logic

Since there isn’t an out-of-the-box option to enforce a single message, you would need to manage this on your end. Here are a couple of suggestions:

• Backend Filtering:
  In your route handler (similar to the /app/api/assistant/route.ts example), you can intercept the stream of responses. You could then combine the deltas into one complete message or choose to send only the first complete message to your client.

• UI Filtering:
  On the client side, you might use the UI primitives (like the Message.If components) to conditionally render only one message per user action. For instance, you could maintain state that indicates whether the assistant’s response has already been rendered and ignore any subsequent messages.

Example Concept

While this isn’t a complete implementation, the idea might look like this on the backend:

TSCopied
export async function POST(req: Request) {
  // Parse the request body
  const input = await req.json();
  const threadId = input.threadId ?? (await openai.beta.threads.create({})).id;

  // Add the user's message to the thread
  const createdMessage = await openai.beta.threads.messages.create(threadId, {
    role: "user",
    content: input.message,
  });

  // Process the stream and store the first complete message
  let completeResponse = "";
  const runStream = openai.beta.threads.runs.stream(threadId, { assistant_id: process.env.ASSISTANT_ID! });

  await forwardStream(runStream, (delta) => {
    // Concatenate delta chunks until you determine the message is complete
    completeResponse += delta.content;
    // Optionally, implement logic here to check if the message is fully received,
    // then ignore subsequent deltas.
  });

  return AssistantResponse({ threadId, messageId: createdMessage.id }, async () => {
    // Additional handling if necessary
  });
}

In the example above, once you’ve built the full response, you can ensure that only this single message (or a composed version) is sent back.

Summary

• There isn’t a parameter in the current API to limit messages to one per user's message.
• You need to implement custom logic on either the backend (stream filtering/merging) or the UI (conditional rendering) to meet this requirement.

If you need further details on handling the streaming responses or using the UI primitives, feel free to ask!