To add on this thread, to anyone struggling with this in 2025 and with the lack of information from google and thanks to some info spared on internet I found that you can just set your headers as follow (Here I'm using express)

it is important that status is 200. Now this is streaming for me on a prompt ui for IA using vercel ui sdk.

Not need of SSE or GRCP

  res.status(200);
  res.setHeader('transfer-encoding', 'chunked');
  res.setHeader('Content-Type', 'plain/text');
  res.setHeader('cache-control', 'no-cache');
  res.setHeader('x-accel-buffering', 'no');