Prerendered +server routes don't use the specified HTTP response headers

[BMorearty]

Describe the bug

Prerendering a +server.ts file causes its HTTP response headers to be ignored.

E.g., I want to prerender an atom.xml file for a blog site. I'd like to override the Cache-Control and Content-Type headers. When I add export const prerender = true and run pnpm preview, this page renders with content-type: text/html and no cache-control header at all:

import type { RequestHandler } from './$types';

export const prerender = true;

export const GET: RequestHandler = async ({ url }) => {
  const body =
    '<?xml version="1.0" encoding="utf-8" ?>\n<feed xmlns="http://www.w3.org/2005/Atom">\n</feed>';
  const headers = {
    'Cache-Control': `max-age=0, must-revalidate`,
    'Content-Type': 'application/atom+xml',
  };
  return new Response(body, {
    status: 200,
    headers,
  });
};

Workaround is to only prerender HTML pages since their content type is text/html, and only those that don't need header overrides such as Cache-Control.

Reproduction

https://github.com/BMorearty/sveltekit-prerender-headers

Clone this repo and build it for production (bug does not reproduce in dev mode):
$ pnpm install && pnpm build && pnpm preview

Then in your browser, open the Network debug panel.

Then go to http://localhost:4173/atom.xml. Note in the Network panel that the Content-Type
is text/html when i should be application/atom+xml, and there is no Cache-Control
header. Both headers are specified in /src/routes/atom.xml/+server.ts but are being ignored.

Also note that atom.xml renders as an HTML page, not as XML. That's due to the incorrect
Content-Type.

Logs

No response

System Info

System:
    OS: macOS 13.2.1
    CPU: (8) arm64 Apple M1
    Memory: 226.09 MB / 16.00 GB
    Shell: 5.8.1 - /bin/zsh
  Binaries:
    Node: 18.12.1 - ~/.nvm/versions/node/v18.12.1/bin/node
    Yarn: 1.22.19 - ~/.nvm/versions/node/v18.12.1/bin/yarn
    npm: 8.19.2 - ~/.nvm/versions/node/v18.12.1/bin/npm
  Browsers:
    Chrome: 110.0.5481.177
    Firefox: 110.0.1
    Safari: 16.3
  npmPackages:
    @sveltejs/adapter-auto: ^2.0.0 => 2.0.0
    @sveltejs/kit: ^1.11.0 => 1.11.0
    svelte: ^3.56.0 => 3.56.0
    vite: ^4.1.4 => 4.1.4

Severity

serious, but I can work around it

Additional Information

No response

[dummdidumm]

This is a general prerender limitation. Prerendering means the result is on disk as a static asset, which means whatever serves those assets is in charge of defining those headers - there's no SvelteKit-aware runtime involved that could add these.
The content-type at least should be fine on most deployment platforms because they know what content-type to give a .xml file.

[benmccann]

there might be some trickery around CSP nonces to figure out if we did this: #9235

though this is a feature that could never be supported by adapter-static and would be limited to things like adapter-vercel/adapter-netlify where we can set headers for prerendered pages

for adapter-static, we could try to detect if the user sets headers as suggested in #8428 (comment) and warn (for user-enabled prerendering) or opt-out of prerendering (for automatically-enabled prerendering) in that case