CI only error with SSG: Error: 404 / does not begin with base

[pBouillon]

Hi!

I'm attempting to deploy a website built with SvelteKit to GitHub Pages.
The whole site is SSG only and I have no issue with pnpm dev, pnpm build or pnpm serve locally.

However, it seems that that the GitHub Actions CI keeps on throwing the same error: Error: 404 / does not begin with 'base':

With this job:

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout
        uses: actions/checkout@v4

      - name: Install pnpm
        uses: pnpm/action-setup@v4
        with:
          version: 9
          run_install: false

      - name: Setup Node.js
        uses: actions/setup-node@v4
        with:
          node-version: 22
          cache: 'pnpm'

      - name: Setup Pages
        uses: actions/configure-pages@v5
        with:
          static_site_generator: sveltekit

      - name: Install dependencies
        run: pnpm install --frozen-lockfile

      - name: Build
        env:
          NODE_ENV: production
        run: pnpm run build

      - name: Upload artifact
        uses: actions/[email protected]
        with:
          path: build

What is surprising is that I cannot reproduce the issue, even when using the same environment variables, and it makes me wondering if that's something internal to SvelteKit that is throwing this.

I am clueless as what I'm supposed to do, as all my paths are starting with this prefix (I also noticed the docs recommending to use resolve(...) but this fails too).

I attempted:

• Downgrading the pnpm version in my build script
• Use {base} and resolve(...) without success
• Write a custom +error.svelte as I was wondering if the default error page was the issue (it wasn't)
• Ask several AI agents, that were unable to find it either
• Browse Setting paths.base in svelte.config.js causes adapter-static to fail (404) #4528 but that didn't worked either

My svelte.config.js is the following:

const config = {
  preprocess: [vitePreprocess(), mdsvex()],
  kit: {
    adapter: adapter(),
    paths: {
      base: process.env.NODE_ENV === 'production' ? '/locutionis/' : '/'
    }
  },
  extensions: ['.svelte', '.svx'],
  compilerOptions: { runes: true }
};

I'm a completely out of idea, does anyone has a new one I havn't tried yet?
If that's of any help, the repo is the following: https://github.com/pBouillon/locutionis

---

EDIT: I've silenced the error with:

     kit: {
    adapter: adapter(),
    paths: {
      base: process.env.NODE_ENV === 'production' ? '/locutionis/' : '/'
    },
    prerender: {
      handleHttpError: ({ path, referrer, message }) => {
        const isBasePathError = path === '/' && message.includes('does not begin with `base`');

        if (isBasePathError) {
          return;
        }

        throw new Error(message);
      }
    }
  },

but it doesn't seems like the good way of addressing the issue