assistant-ui Streaming

Always consult assistant-ui.com/llms.txt for latest API.

The assistant-stream package handles streaming from AI backends.

References

• ./references/data-stream.md — AI SDK data stream format
• ./references/assistant-transport.md — Native assistant-ui format
• ./references/encoders.md — Encoders and decoders

When to Use

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

Installation

npm install assistant-stream

Custom Streaming Response

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();
  });
}

With useLocalRuntime

useLocalRuntime expects ChatModelRunResult chunks. Yield content parts for streaming:

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 }] };
        }
      }
    },
  },
});

Debugging Streams

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));
}

Stream Event Types

• part-start with part.type = "text" | "reasoning" | "tool-call" | "source" | "file"
• text-delta with streamed text
• result with tool results
• step-start, step-finish, message-finish
• error strings

Common Gotchas

Stream not updating UI

• Check Content-Type is text/event-stream
• Check for CORS errors

Tool calls not rendering

• addToolCallPart needs both toolCallId and toolName
• Register tool UI with makeAssistantToolUI

Partial text not showing

• Use text-delta events for streaming