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): apply trailingSlash config to the rewritten basepath URL before the SSR redirect comparison.
+
+Fixes [#7291](https://github.com/TanStack/router/issues/7291): visiting a bare basepath URL (e.g. `/preview`) on the server always triggered a spurious `308` redirect to `/preview/` even when `trailingSlash` was `'never'` (the default), because the rewritten public href was synthesised from `URL` parsing — which always normalises to a trailing slash — and never reconciled with the user's `trailingSlash` config before being compared to the incoming request URL.

packages/router-core/src/router.ts

@@ -2020,8 +2020,24 @@ export class RouterCore<
           publicHref = rewrittenUrl.href
           external = true
         } else {
+          // Apply trailingSlash config to the rewritten pathname so that the
+          // publicHref is canonical before it is compared to the incoming URL
+          // in the SSR redirect check. Without this, visiting the bare basepath
+          // (e.g. /preview) always produces publicHref = /preview/ from the
+          // rewrite output and triggers a spurious 308 redirect even when
+          // trailingSlash is 'never'. See: https://github.com/TanStack/router/issues/7291
+          const trailingSlashOpt = this.options.trailingSlash ?? 'never'
+          let rewrittenPathname = rewrittenUrl.pathname
+          if (trailingSlashOpt === 'never') {
+            rewrittenPathname = trimPathRight(rewrittenPathname)
+          } else if (
+            trailingSlashOpt === 'always' &&
+            !rewrittenPathname.endsWith('/')
+          ) {
+            rewrittenPathname += '/'
+          }
           publicHref =
-            rewrittenUrl.pathname + rewrittenUrl.search + rewrittenUrl.hash
+            rewrittenPathname + rewrittenUrl.search + rewrittenUrl.hash
         }
       } else {
         // Fast path: no rewrite, skip URL construction entirely