Server-Sent Events

You have a normal HTTP server and you want the chat reply to stream. Use Server-Sent Events (SSE). It is the default transport. It works in all browsers, it passes through most proxies, and it is easy to debug.

1. Return SSE from the server

Wrap the chat() stream with toServerSentEventsResponse():

import {
  chat,
  chatParamsFromRequest,
  toServerSentEventsResponse,
} from "@tanstack/ai";
import { openaiText } from "@tanstack/ai-openai";

export async function POST(request: Request) {
  const { messages, threadId, runId } = await chatParamsFromRequest(request);
  const stream = chat({
    adapter: openaiText("gpt-5.6"),
    messages,
    threadId,
    runId,
  });
  return toServerSentEventsResponse(stream);
}

2. Connect from the client

Pass fetchServerSentEvents() to useChat:

import { useChat, fetchServerSentEvents } from "@tanstack/ai-react";

const { messages, sendMessage } = useChat({
  connection: fetchServerSentEvents("/api/chat"),
});

To add headers, a dynamic URL, or extra body data, see Request Options.

Resume a dropped stream

A connection can drop in the middle of a reply. If the server sends SSE id: values, fetchServerSentEvents can continue the reply:

Reconnect. If the connection drops after an id arrived, the adapter reconnects with Last-Event-ID. It removes the replayed chunks that it already has.
joinRun(runId). It sends a read-only GET with offset=-1 and the run id. The server replays an in-flight or finished run from the start.

The ids appear only when the server passes a durability adapter to toServerSentEventsResponse. The ids are opaque tokens that the durability adapter owns. The chat client does not create, parse, or store them. Without ids, the adapter does one plain fetch.

Add a GET handler next to POST, so that joinRun works in a second tab or after a reload:

POST starts new runs. It also gets the automatic reconnects, because the adapter sends the same body again with Last-Event-ID.
GET replays a known run from the start.

import {
  chat,
  chatParamsFromRequest,
  memoryStream,
  resumeServerSentEventsResponse,
  toServerSentEventsResponse,
} from "@tanstack/ai";
import { openaiText } from "@tanstack/ai-openai";

export async function POST(request: Request) {
  const { messages, threadId, runId } = await chatParamsFromRequest(request);
  const stream = chat({ adapter: openaiText("gpt-5.6"), messages, threadId, runId });
  return toServerSentEventsResponse(stream, {
    durability: { adapter: memoryStream(request) },
  });
}

// joinRun hits GET ?offset=-1&runId=... (replay only, no messages sent).
export async function GET(request: Request) {
  return resumeServerSentEventsResponse({ adapter: memoryStream(request) });
}

The GET handler does not call a provider. On a replay, resumeFrom() on the durability adapter is not null (it comes from ?offset). So the handler replays the log. If the request has no resume offset, resumeServerSentEventsResponse returns a 400.

Other adapters resume the same way: