@dandoen I was playing with your example this morning and noted something:

The stream route is statically rendered

If you force the route to be dynamic:

export async function GET(request: NextRequest) {
  headers().has("whatever");
  // ...
});

Then the route is dynamic:

And streaming works as expected.

Still next dev streams without having to force the route to be dynamic.

Oh wow, good spot. Confirmed. Thx @vicb

We should keep this one opened while we figure out why it doesn't work for pre-rendered pages

This one can be closed, this is expected.
If the route is ISR/SSG, the result are retrieved from the incremental cache and sent right away (it's the same behavior in next/start )
next dev is specific and doesn't use the incremental cache, that's why it "worked" there