base path / trailing slash with loading assets from incorrect path

Describe the bug

Due to base path not being configurable via environment variables(after build)(related: #7242), when using adapters such as adapter-node, and you don't want to build a version of your app for every possible base path.

You can currently get around this by setting
config.kit.paths.base: ""
while also setting export const trailingSlash = 'always'; in a top level +layout.server.ts

This makes it so assets gets loaded from the correct path, https://example.com/somepath/_app/... instead of https://example.com/_app/...

But this also results in #13718, which you can work around by setting either of the following environment variables ORIGIN="https://example.com/somepath" or HOST_HEADER=x-forwarded-host.

This fixes redirects done by sveltekit from non trailing slash to trailing slash when export const trailingSlash = 'always'; is set, but this also effectively undoes the fixes for to the assets paths which means they are returned to being https://example.com/_app/... instead of https://example.com/somepath/_app/...

And the only actual work around for us at the moment is building our app around 30 times to account for all the different base paths instead of once and then just having to set an environment variable

Reproduction

minimal repro using the demo app, with readme explaining setup

Logs

System Info

System:
    OS: Linux 6.17 Garuda Linux
    CPU: (32) x64 AMD Ryzen 9 5950X 16-Core Processor
    Memory: 30.78 GB / 62.70 GB
    Container: Yes
    Shell: 4.2.1 - /usr/bin/fish
  Binaries:
    Node: 24.12.0 - /run/user/1000/fnm_multishells/3392617_1765799124583/bin/node
    npm: 11.6.2 - /run/user/1000/fnm_multishells/3392617_1765799124583/bin/npm
    pnpm: 10.7.0 - /home/$USER/.npm-global/bin/pnpm
    bun: 1.3.4 - /home/$USER/.bun/bin/bun
    Deno: 2.5.6 - /usr/bin/deno
  Browsers:
    Chromium: 143.0.7499.109
    Firefox: 146.0
    Firefox Developer Edition: 146.0
  npmPackages:
    @sveltejs/adapter-node: ^5.4.0 => 5.4.0
    @sveltejs/kit: ^2.49.2 => 2.49.2
    @sveltejs/vite-plugin-svelte: ^6.2.1 => 6.2.1
    svelte: ^5.46.0 => 5.46.0
    vite: ^7.3.0 => 7.3.0

Severity

annoyance

Additional Information

No response

Start by running the linked minimal reproduction and compare asset URLs and redirects with config.kit.paths.base, trailingSlash, ORIGIN, and HOST_HEADER set as described. Trace how adapter-node determines the base path after build; done means assets use the deployed base path while trailing-slash redirects remain correct without rebuilding for each path.