Forcing markdown for streamText

[hous]

I'm using streamText with gpt-5-nano. I'd expect to be able to force markdown formatting, but I can't figure out how to do that...

        const result = streamText({
            model: 'openai/gpt-5-nano',
            prompt: `${prompt}`,
            format: 'markdown',
        });

I'd love to use Streamdown but my issue is getting actual markdown content from the OpenAI model using the Vercel AI SDK - anyone know how to force that?

[michaelgrewal]

I'm also very disappointed with GPT-5 models never responding in markdown. It used to be so easy for GPT4o to return in markdown, I'd just ask in the system prompt. But doesn't work with GPT5.

How do we get markdown with GPT5?

[Nishchit14]

Highly needed