Astro Info

Astro                    v6.0.6
Vite                     v7.3.1
Node                     v22.21.1
System                   macOS (arm64)
Package Manager          npm
Output                   server
Adapter                  @astrojs/vercel (v10.0.1)
Integrations             none

If this issue only occurs in one browser, which browser is a problem?

No response

Describe the Bug

When using domain-based i18n routing with trailingSlash: "never", requests to the root path (/) of a non-default locale's domain return 404.

The bug is in computePathnameFromDomain() in dist/core/app/base.js (lines 183-190). When a request arrives at the mapped domain, the method:

1. Correctly resolves the locale from the domain and prepends it to the path: / → /en
2. Then checks if (url.pathname.endsWith("/")) and appends a trailing slash, producing /en/

Since the root path / inherently ends with /, this condition is always true for root-level requests. The resulting /en/ fails to match the route pattern ^\/en$ generated for trailingSlash: "never", causing a 404.

// base.js lines 183-190
if (locale) {
    pathname = prependForwardSlash(
        joinPaths(normalizeTheLocale(locale), this.removeBase(url.pathname))
    );
    if (url.pathname.endsWith("/")) {
        pathname = appendForwardSlash(pathname);
    }
}

The trailing slash append logic does not consult this.manifest.trailingSlash, so it conflicts with the route patterns when trailing slashes are disabled.

Config to reproduce:

export default defineConfig({
  site: "https://example.fi",
  output: "server",
  trailingSlash: "never",
  adapter: vercel(),
  i18n: {
    locales: ["fi", "en"],
    defaultLocale: "fi",
    domains: {
      en: "https://example.com",
    },
  },
});

The default locale's domain works fine because computePathnameFromDomain() returns undefined for it, bypassing the buggy code path entirely.

Workaround: remove trailingSlash: "never" (use the default "ignore").

What's the expected result?

Requests to the root path of a domain-mapped locale (e.g., https://example.com/) should return 200 and render the correct page, regardless of the trailingSlash setting. The computePathnameFromDomain() method should respect the configured trailingSlash value when deciding whether to append a trailing slash to the locale-prefixed pathname.

Link to Minimal Reproducible Example

https://stackblitz.com/edit/github-khcpagmm?file=README.md

Participation

• I am willing to submit a pull request for this issue.