[Start] Bare basepath URL (e.g. /preview) redirects to /preview/ on SSR even with trailingSlash: 'never'

Discussed in https://github.com/TanStack/router/discussions/7282

^{Originally posted by Nandan1996 April 28, 2026}

Summary

When basepath is set on the router (e.g. /preview) and a user visits the bare basepath URL (http://localhost:3000/preview), the SSR handler issues a redirect to http://localhost:3000/preview/ (with trailing slash). This happens even though trailingSlash defaults to "never".

Visiting / with no basepath does not redirect to /-with-anything, so the behavior is asymmetric.

Where it happens

In packages/router-core/src/router.ts beforeLoad, on the server, the router rebuilds the location and compares publicHref:

if (isServer ?? this.isServer) {
  const nextLocation = this.buildLocation({
    to: this.latestLocation.pathname,
    search: true, params: true, hash: true, state: true,
    _includeValidateSearch: true,
  })

  if (this.latestLocation.publicHref !== nextLocation.publicHref) {
    const href = this.getParsedLocationHref(nextLocation)
    if (nextLocation.external) throw redirect({ href })
    else throw redirect({ href, _builtLocation: nextLocation })
  }
}

Walkthrough for request GET /preview with basepath: "/preview":

1. Basepath is stripped → internal latestLocation.pathname = /.
2. buildLocation({ to: "/" }) re-joins basepath + / → produces /preview/.
3. Incoming publicHref = /preview, rebuilt publicHref = /preview/ → mismatch → 308 redirect.

Root case (no basepath): both incoming and rebuilt are /, so no redirect. That is why root "just works" but a bare basepath does not.

Expected

With trailingSlash: "never" (the default), GET /preview should be served as the canonical URL — no redirect to /preview/.

Repro

• TanStack Start app
• Set basepath: "/preview" in the router config (and matching dev server base/routerBasepath)
• Visit http://localhost:3000/preview → 308 → http://localhost:3000/preview/

Related

• #4105 — similar shape (visiting / redirects to /basepath/), but the bare-basepath case here looks distinct.

Question

Is this intentional canonicalization (treat bare basepath as non-canonical), or a bug where trailingSlash is not applied to the rebuilt publicHref before the equality check?

Versions: @tanstack/react-router@1.167.5, @tanstack/react-start@1.166.18.

Start in packages/router-core/src/router.ts at the server-side beforeLoad logic that rebuilds the location and compares publicHref. Reproduce GET /preview with basepath "/preview" and trailingSlash set to "never", then verify the canonical bare-basepath URL is served without redirecting to /preview/.