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