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/upload-pages-artifact@v4.0.0
        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

[Nic-Polumeyv]

The error means the prerenderer requested a path that doesn't start with your configured base. When that happens, the full message includes (linked from /locutionis/...), and that referrer is the fastest way to find the offending link.

The reason it only shows up in CI is most likely the NODE_ENV ternary. vite build sets NODE_ENV=production itself, but only if it isn't already set, and your config is evaluated after that happens. I verified this on kit 2.x, a plain vite build sees production at config evaluation time while NODE_ENV=staging vite build sees staging. So if your local pnpm run build passes while CI fails, something in your local environment (shell profile, direnv, .env tooling) is setting NODE_ENV to something other than production, which turns base off locally and hides every unprefixed link. Tying base to NODE_ENV makes the build behave differently depending on who runs it.

Two more things worth knowing. actions/configure-pages with static_site_generator: sveltekit already sets kit.paths.base (and kit.prerender.origin) in your svelte.config.js during the workflow, so you can drop the ternary entirely, or switch to a dedicated variable like BASE_PATH that you only set in the workflow. A dedicated variable also gives you a one-liner to reproduce the CI behavior locally, BASE_PATH=/locutionis pnpm build.

On the workaround, I'd remove it. Returning early from handleHttpError hides genuinely broken links, and anyone clicking one on the deployed site gets a 404. I had a look at your current code and all internal links now go through resolve() (including FeaturedFigureCard, which resolves its href prop internally), so the original / offender may be gone already. Build once with base forced on, and if the error still appears, the (linked from ...) part tells you exactly which page to fix.