Using Vercel AI SDK?
├─ Yes → toUIMessageStreamResponse() (no assistant-stream needed)
└─ No → assistant-stream for custom backends

npm install assistant-stream

import { createAssistantStreamResponse } from "assistant-stream";

export async function POST(req: Request) {
  return createAssistantStreamResponse(async (stream) => {
    stream.appendText("Hello ");
    stream.appendText("world!");

    // Tool call example
    const tool = stream.addToolCallPart({ toolCallId: "1", toolName: "get_weather" });
    tool.argsText.append('{"city":"NYC"}');
    tool.argsText.close();
    tool.setResponse({ result: { temperature: 22 } });

    stream.close();
  });
}

import { useLocalRuntime } from "@assistant-ui/react";

const runtime = useLocalRuntime({
  model: {
    async *run({ messages, abortSignal }) {
      const response = await fetch("/api/chat", {
        method: "POST",
        body: JSON.stringify({ messages }),
        signal: abortSignal,
      });

      const reader = response.body?.getReader();
      const decoder = new TextDecoder();
      let buffer = "";

      while (reader) {
        const { done, value } = await reader.read();
        if (done) break;

        buffer += decoder.decode(value, { stream: true });
        const parts = buffer.split("\n");
        buffer = parts.pop() ?? "";

        for (const chunk of parts.filter(Boolean)) {
          yield { content: [{ type: "text", text: chunk }] };
        }
      }
    },
  },
});

import { AssistantStream, DataStreamDecoder } from "assistant-stream";

const stream = AssistantStream.fromResponse(response, new DataStreamDecoder());
for await (const event of stream) {
  console.log("Event:", JSON.stringify(event, null, 2));
}