Streaming custom data in express.

[nicstrong]

In AI SDK 5.0 i can't find a way to stream custom data from express.
The examples show how to do this by calling createUIMessageStream<T> using the writer in execute. I couldn't find a way to copy the stream returned from this call to the express Response object. For exress the docs show using pipeUIMessageStreamToResponse<T> to pipe the message stream to the Response but this API doesn't expose a writer. In v4 this could be done with pipeDataStreamToResponse which had an execute callback.

Thanks,
Nic

[lgrammel]

Do these examples work for you? https://ai-sdk.dev/cookbook/api-servers/express

[nicstrong]

That link uses pipeDataStreamToResponse which did work for me in v4. But after porting to v5 i now use pipeUIMessageStreamToResponse (as per this guide).
pipeUIMessageStreamToResponse doesn't expose a execute callback that gives access to a writer to write custom data.

[nicstrong]

Is there any guidance on this? Seems like a major piece of function missed from v4 -> v5 for express.

[lgrammel]

please check out https://ai-sdk.dev/cookbook/api-servers/express#sending-custom-data

[BORNTOBEBURN]

Hey! 👋

In AI SDK 5.0, the streaming API changed a bit. The old pipeDataStreamToResponse with its execute callback is gone, but you can still stream custom data from Express by separating three steps:

Create the UI Message Stream

Write your custom data

Pipe the stream to Express

Here’s a minimal example:

import express from "express";
import {
createUIMessageStream,
pipeUIMessageStreamToResponse,
createWriter,
} from "ai-sdk";

const app = express();
app.use(express.json());

app.post("/ai", async (req, res) => {
// 1️⃣ Create a writer & UI stream
const writer = createWriter();
const uiStream = createUIMessageStream(writer);

// 2️⃣ Write your custom data
writer.write({ type: "text", content: "Hello, world!" });
writer.write({ type: "image", url: "https://example.com/img.png" });
writer.close(); // important!

// 3️⃣ Pipe to Express response
await pipeUIMessageStreamToResponse(uiStream, res);

// Alternatively, if you need raw access:
// uiStream.getReadable().pipe(res);
});

app.listen(3000, () => console.log("Server listening on 3000"));

Key points:

createUIMessageStream gives you both a writer and a Readable stream.

pipeUIMessageStreamToResponse sets headers and closes the response automatically.

If you need to transform the stream (e.g. to JSON), you can convert it using the SDK’s bridge:

const dataStream = aiBridge.convertUIMessageStreamToDataStream(uiStream);
dataStream.pipe(res);

This way you can keep the same streaming behavior as before without the old execute API.