fix(router): apply trailingSlash config before basepath redirect comparison on SSR

.changeset/fix-ssr-bare-basepath-redirect.md

@@ -0,0 +1,7 @@
+---
+'@tanstack/router-core': patch
+---
+
+fix(router-core): apply the `trailingSlash` option to the rewritten basepath `publicHref` before the SSR redirect comparison.
+
+Fixes [#7291](https://github.com/TanStack/router/issues/7291): requesting a bare basepath URL (for example `/preview` with `basepath: '/preview'`) on the server always answered with a `308` redirect to `/preview/`, even with `trailingSlash: 'never'` (the default). `rewriteBasepath.output` joins the basepath and the internal pathname, which yields a trailing slash for the root route, and `buildLocation` never reconciled that with `trailingSlash` before comparing it to the incoming request URL. The rewritten pathname is now trimmed under `'never'`, given a trailing slash under `'always'` and left as is under `'preserve'`. This also applies to custom `rewrite.output` implementations: under `'never'` a trailing slash they emit is trimmed from `publicHref`.

packages/router-core/src/router.ts

@@ -2139,7 +2139,19 @@ export class RouterCore<
         } else {
           // A same-origin rewrite can produce a pathname like "//evil.example".
           // Normalize it to "/evil.example" so the link stays on this origin.
-          publicHref = normalizeProtocolRelative(getUrlPath(rewrittenUrl))
+          const trailingSlashOpt = this.options.trailingSlash ?? 'never'
+          let rewrittenPathname = rewrittenUrl.pathname
+          if (trailingSlashOpt === 'never') {
+            rewrittenPathname = trimPathRight(rewrittenPathname)
+          } else if (
+            trailingSlashOpt === 'always' &&
+            !rewrittenPathname.endsWith('/')
+          ) {
+            rewrittenPathname += '/'
+          }
+          publicHref = normalizeProtocolRelative(
+            rewrittenPathname + rewrittenUrl.search + rewrittenUrl.hash,
+          )
         }
       } else {
         // Fast path: no rewrite, skip URL construction entirely

packages/router-core/tests/rewrite.test.ts

@@ -237,6 +237,30 @@ describe('router rewrites', () => {
     },
   )
 
+  test.each([
+    { trailingSlash: 'never' as const, root: '/app', nested: '/app/posts' },
+    { trailingSlash: 'always' as const, root: '/app/', nested: '/app/posts/' },
+    { trailingSlash: 'preserve' as const, root: '/app/', nested: '/app/posts' },
+  ])(
+    'applies trailingSlash=$trailingSlash to the rewritten basepath publicHref',
+    ({ trailingSlash, root, nested }) => {
+      const router = createTestRouter({
+        routeTree: new BaseRootRoute({}),
+        history: createMemoryHistory({ initialEntries: ['/app'] }),
+        basepath: '/app',
+        trailingSlash,
+      })
+
+      // The bare basepath is where the SSR redirect check used to see
+      // '/app' !== '/app/' and answer with a spurious 308.
+      expect(router.buildLocation({ to: '/' }).publicHref).toBe(root)
+      expect(router.buildLocation({ to: '/posts' }).publicHref).toBe(nested)
+      expect(
+        router.buildLocation({ to: '/posts', search: { page: 2 } }).publicHref,
+      ).toBe(`${nested}?page=2`)
+    },
+  )
+
   test('rebuilds rewrites and stored locations when basepath or custom rewrite changes', () => {
     const router = createTestRouter({
       routeTree: new BaseRootRoute({}),