Preserving middleware-set status drops middleware-set headers #7647

Which project does this relate to?

Start

Describe the bug

I have a request middleware that classifies a document request, sets setResponseStatus(410) and setResponseHeader("cache-control", "no-store"), then continues into the router so the normal React shell/error UI renders.

To preserve the middleware status, I wrap defaultStreamHandler and rebuild the response with getResponseStatus(). That preserves the status, but middleware-set headers disappear from the final document response. Route headers still work.

More details are in the repro repo.

Complete minimal reproducer

https://github.com/alex35mil/tanstack-start-middleware-response-headers

Steps to Reproduce the Bug

pnpm install
pnpm dev
curl -i http://127.0.0.1:3000/

Expected behavior

The final document response preserves both the middleware-classified status and headers:

HTTP/1.1 418
x-route-header: present
x-middleware-header: present

Screenshots or Videos

No response

Platform

• Router / Start Version:

    "@tanstack/react-router": "1.170.15",
    "@tanstack/react-start": "1.168.25",
    "@vitejs/plugin-react": "6.0.2",

• OS: macOS
• Browser: Arc
• Browser Version: 1.146.0
• Bundler: vite
• Bundler Version: 8.0.16

Additional context

Additional context is in the repro repo.