[BUG]: All headers added by tanstack start are malformed when using rsbuild

[sadan4]

Which project does this relate to?

Start

Describe the bug

When using rsbuild, any response headers added by tanstack will be malformed.

actual headers

Headers {
  '0': 'content-type',
  '1': 'text/html; charset=utf-8',
  vary: 'Origin',
  date: 'Tue, 05 May 2026 04:30:34 GMT',
  connection: 'keep-alive',
  'keep-alive': 'timeout=5',
  'transfer-encoding': 'chunked'
}

expected headers

Headers {
  vary: 'Origin',
  'content-type': 'text/html; charset=utf-8',
  date: 'Tue, 05 May 2026 04:30:46 GMT',
  connection: 'keep-alive',
  'keep-alive': 'timeout=5',
  'transfer-encoding': 'chunked'
}

Complete minimal reproducer

bcefc84 (head as of writing)

Steps to Reproduce the Bug

1. cd e2e/react-start/hmr
2. pnpm dev:rsbuild
3. in a separate terminal, run node -e "let r = await fetch('http://localhost:3000'); import {inspect} from 'node:util'; console.log(inspect(await r.headers));"
4. notice that the headers are malformed
5. stop the rsbuild dev server
6. pnpm dev:vite
7. in a separate terminal, run node -e "let r = await fetch('http://localhost:3000'); import {inspect} from 'node:util'; console.log(inspect(await r.headers));"
8. notice that the headers are correct

Expected behavior

I expect the headers not to be malformed

Screenshots or Videos

No response

Platform

• Router / Start Version: bcefc84
• OS: Linux
• Browser: tested with Chrome, Firefox, python requests, and node.js fetch

Additional context

I tested this with curl as well curl -i http://localhost:3000 and it showed the expected headers when every other method i tried (chrome, firefox, etc...) did not.