`createUIMessageStream`

The createUIMessageStream function allows you to create a readable stream for UI messages with advanced features like message merging, error handling, and finish callbacks.

Import

import { createUIMessageStream } from "ai"

Example

const existingMessages: UIMessage[] = [
  /* ... */
];

const stream = createUIMessageStream({
  async execute({ writer }) {
    // Start a text message
    // Note: The id must be consistent across text-start, text-delta, and text-end steps
    // This allows the system to correctly identify they belong to the same text block
    writer.write({
      type: 'text-start',
      id: 'example-text',
    });

    // Write a message chunk
    writer.write({
      type: 'text-delta',
      id: 'example-text',
      delta: 'Hello',
    });

    // End the text message
    writer.write({
      type: 'text-end',
      id: 'example-text',
    });

    // Merge another stream from streamText
    const result = streamText({
      model: "anthropic/claude-sonnet-4.5",
      prompt: 'Write a haiku about AI',
    });

    writer.merge(result.toUIMessageStream());
  },
  onError: error => `Custom error: ${error.message}`,
  originalMessages: existingMessages,
  onFinish: ({ messages, isContinuation, responseMessage }) => {
    console.log('Stream finished with messages:', messages);
  },
});

API Signature

Parameters

execute:

(options: { writer: UIMessageStreamWriter }) => Promise<void> | void

UIMessageStreamWriter

write:

(part: UIMessageChunk) => void

merge:

(stream: ReadableStream<UIMessageChunk>) => void

onError:

(error: unknown) => string

onError:

(error: unknown) => string

originalMessages:

UIMessage[] | undefined

onFinish:

(options: { messages: UIMessage[]; isContinuation: boolean; responseMessage: UIMessage }) => void | undefined

FinishOptions

messages:

UIMessage[]

isContinuation:

boolean

responseMessage:

UIMessage

generateId:

IdGenerator | undefined

Returns

ReadableStream<UIMessageChunk>

A readable stream that emits UI message chunks. The stream automatically handles error propagation, merging of multiple streams, and proper cleanup when all operations are complete.