Reported upstream as withastro/astro#17034.

getRelativeLocaleUrl(), getAbsoluteLocaleUrl() and getAbsoluteLocaleUrlList() from astro:i18n return URLs with a trailing slash under trailingSlash: 'never' whenever the locale-prefixed path is empty ('') or root ('/') — and, when prependWith is used, whenever the input path carries a trailing slash.

// astro.config.mjs: trailingSlash: 'never', i18n: { defaultLocale: 'en', locales: ['en', 'pl'] }
getRelativeLocaleUrl('pl');                                  // '/pl'                  ✅
getRelativeLocaleUrl('pl', '');                              // '/pl/'                 ❌ expected '/pl'
getRelativeLocaleUrl('pl', '/');                             // '/pl/'                 ❌ expected '/pl'
getRelativeLocaleUrl('pl', 'docs/setup');                    // '/pl/docs/setup'       ✅ (nested paths are fine)
getRelativeLocaleUrl('pl', 'docs/setup/', { prependWith: 'blog' });
                                                             // '/blog/pl/docs/setup/' ❌ expected '/blog/pl/docs/setup'
getAbsoluteLocaleUrl('pl', '');                              // 'https://example.com/pl/' ❌
getAbsoluteLocaleUrlList('');                                // ['https://example.com', 'https://example.com/pl/'] ❌
//                          inconsistent within a single call ─────────────────^

The last call returns one URL per locale — the shape an hreflang block needs — and is inconsistent within a single call: the default locale gets no slash, every other locale gets one.

The affected URLs are the most-linked pages of a localized site — every language switcher and every hreflang block includes each locale’s home page. Each affected navigation costs an extra 301 round trip (an extra billed invocation on serverless/edge), and hreflang/canonical alternates built from getAbsoluteLocaleUrlList() point crawlers at redirecting URLs — contradicting what @astrojs/sitemap, which honors trailingSlash: 'never', emits for the same pages.

npm install
npm test        # astro build + starts the built server + assertions over HTTP

npm test builds the site (output: 'server', @astrojs/node standalone), starts dist/server/entry.mjs, and asserts in two sections:

1. URL generation — the seven calls above; five fail.
2. Server enforcement of the same config — these pass, demonstrating the contradiction: the slash-less URLs the helpers should return are real pages (/pl, /pl/docs/setup, /blog/pl/docs/setup → 200), while the URLs they do return are redirected away (/pl/, /blog/pl/docs/setup/ → 301; 308 for non-GET — see TrailingSlashHandler in astro/src/core/routing/trailing-slash-handler.ts).

Every URL exercised by the tests exists as a page:

src/pages/index.astro               → /
src/pages/pl/index.astro            → /pl
src/pages/docs/setup.astro          → /docs/setup
src/pages/pl/docs/setup.astro       → /pl/docs/setup
src/pages/blog/pl/docs/setup.astro  → /blog/pl/docs/setup   (the prependWith: 'blog' mount)

The process exits non-zero because of the five failing generation cases.

Alternatively npm run dev and open http://localhost:4321/ — the rendered list shows the same outputs, and clicking the “Polski” language link navigates to /pl/, which dev answers with 404 and the notice “Your site is configured with trailingSlash set to never. Do you want to go to /pl instead?”

So the URLs produced by astro:i18n are URLs that Astro itself refuses to serve under the very same configuration.