createUIMessageStreamResponse

import {
  createUIMessageStream,
  createUIMessageStreamResponse,
  streamText,
  toUIMessageStream,
} from 'ai';

const response = createUIMessageStreamResponse({
  status: 200,
  statusText: 'OK',
  headers: {
    'Custom-Header': 'value',
  },
  stream: createUIMessageStream({
    execute({ writer }) {
      // The outer stream owns the assistant message lifecycle.
      writer.write({ type: 'start' });

      // Write custom data (type must be 'data-<name>')
      writer.write({
        type: 'data-message',
        data: { content: 'Hello' },
      });

      // Write text content using start/delta/end pattern
      writer.write({
        type: 'text-start',
        id: 'greeting-text',
      });
      writer.write({
        type: 'text-delta',
        id: 'greeting-text',
        delta: 'Hello, world!',
      });
      writer.write({
        type: 'text-end',
        id: 'greeting-text',
      });

      // Write source information (flat properties, not nested)
      writer.write({
        type: 'source-url',
        sourceId: 'source-1',
        url: 'https://example.com',
        title: 'Example Source',
      });

      // Merge with LLM stream
      const result = streamText({
        model: "anthropic/claude-sonnet-5.5",
        prompt: 'Say hello',
      });

      writer.merge(
        toUIMessageStream({ stream: result.stream, sendStart: false }),
      );
    },
  }),
});