redirect() ignores Headers(), making it impossible to set multiple headers with the same name

[frenzzy]

Which project does this relate to?

Start

Describe the bug

It is not possible to set multiple headers using the redirect() method with the Headers() interface, because all headers are ignored in this case:

import { redirect } from '@tanstack/react-router';
import { createServerFn } from '@tanstack/react-start';

export const action = createServerFn({ method: 'GET' }).handler(() => {
  const headers = new Headers()
  headers.append('x-example', 'a')
  headers.append('x-example', 'b')
  throw redirect({ to: '/', headers }) // no response headers
  // throw redirect({ to: '/', headers: { 'x-example': 'c' } }) // works
})

Your Example Website or App

https://tanstack.com/start/latest/docs/framework/react/examples/start-basic?path=examples%2Freact%2Fstart-basic%2Fsrc%2Futils%2Fposts.tsx

Steps to Reproduce the Bug or Issue

1. Create a server function with redirect()
2. Pass a Headers() object to redirect()
3. Make a request and check the response headers

Expected behavior

Headers should be present in the response

Screenshots or Videos

No response

Platform

• OS: any
• Browser: any
• Version: latest on Apr 24, 2025 (1.117.0)

Additional context

No response