streamObject using fastify backend does not seem to work

[Floriferous]

Description

I'm trying to make streamObject work between a NextJS frontend, and a Fastify backend. But I can't get it to work. I can only get the object to appear at once, at the end of the stream.

I tried using createDataStream as well, but when using streamObject I don't have access to result.mergeIntoDataStream nor result.toDataStream.

I'm following this documentation here, but something must be missing:

• https://sdk.vercel.ai/cookbook/api-servers/fastify
• https://sdk.vercel.ai/cookbook/next/stream-object

Here's a reproduction: https://github.com/Floriferous/next-fastify

Code example

No response

AI provider

@ai-sdk/openai: "^1.2.3"

Additional context

No response

[nicoalbanese]

Hey - try updating your backend code to:

fastify.post("/api/v1/stream-object", async function handler(request, reply) {
  try {
    const result = streamObject({
      model: openai("gpt-4o-mini"),
      prompt: "Invent a new holiday and describe its traditions.",
      schema: z.object({
        name: z.string(),
        description: z.string(),
        traditions: z.array(z.string()),
      }),
    });

    reply.header("X-Vercel-AI-Data-Stream", "v1");
    reply.header("Content-Type", "text/event-stream");
    return reply.send(result.textStream);
  } catch (error) {
    console.error("Error:", error);
    return reply.status(500).send({ error: "Internal server error" });
  }
});

I tested this with your reproduction and it works as expected.

[Floriferous]

I tried this out, but the object appears all at once on the frontend, instead of streaming in. Maybe I'm missing something else?

nb: I updated the repro to have generation/streaming side-by-side

[jhoseph88]

I'm encountering the same issue, but with streamText. @Floriferous any chance you found a resolution?

[lgrammel]

@Floriferous @jhoseph88 have you tried setting Content-Encoding to none? https://sdk.vercel.ai/docs/troubleshooting/streaming-not-working-when-proxied

[jhoseph88]

@lgrammel I've confirmed this fixes the issue on my end. Thank you a million times--our customers will be relieved :).