React Router transaction name has a double slash when lazyRouteManifest is used with a basename ending in / #25758

Is there an existing issue for this?

• I have checked for existing issues https://github.com/getsentry/sentry-javascript/issues
• I have reviewed the documentation https://docs.sentry.io/
• I am using the latest SDK release https://github.com/getsentry/sentry-javascript/releases

How do you use Sentry?

Sentry Saas (sentry.io)

Which SDK are you using?

@sentry/react

SDK Version

11.9.0 (reproduced on develop at 7d1f14f; the same line is in 10.77.1)

Framework Version

react-router 7.18.3 and 6.30.4

Link to Sentry event

No response

Reproduction Example/SDK Setup

Sentry.init({
  dsn: '...',
  integrations: [
    Sentry.reactRouterV7BrowserTracingIntegration({
      useEffect: React.useEffect,
      useLocation,
      useNavigationType,
      createRoutesFromChildren,
      matchRoutes,
      enableAsyncRouteHandlers: true,
      lazyRouteManifest: ['/users/:id'],
    }),
  ],
  tracesSampleRate: 1,
});

const router = Sentry.wrapCreateBrowserRouterV7(createBrowserRouter)(
  [
    { path: '/', element: <Home /> },
    { path: '/users/:id', element: <User /> },
  ],
  { basename: '/app/' },
);

Steps to Reproduce

1. Set up the integration with enableAsyncRouteHandlers: true and a lazyRouteManifest.
2. Create the router with a basename that ends in /, e.g. /app/ or / (for example import.meta.env.BASE_URL in Vite).
3. Open /app/users/123 and look at the pageload transaction name. Navigations behave the same.

Expected Result

/app/users/:id (and /users/:id for basename /) with source route, the same as with basename /app.

Actual Result

/app//users/:id for basename /app/, and //users/:id for basename /. Source is route, and this is the name that gets sent.

The manifest branch of resolveRouteNameAndSource prepends the basename as passed in:

// packages/react/src/reactrouter-compat-utils/utils.ts:372
return [(stripBasename ? '' : basename) + manifestMatch, 'route'];

On develop the router-match branch trims the trailing slash first (utils.ts:341 and :349, since #25639). The manifest branch has concatenated the raw basename since lazyRouteManifest was added. React Router accepts a basename with a trailing slash, and matchRouteManifest already handles it when stripping (route-manifest.test.ts, "handles basename with trailing slash"). Only the concatenation misses the trim.

The manifest is checked before route matching, so this happens for any route the manifest matches, lazy or not. I reproduced it through wrapCreateBrowserRouterV7, wrapCreateMemoryRouterV6, and the @sentry/react/react-router entry. It does not happen with stripBasename: true or with a basename without a trailing slash.

Additional Context

A one-line fix (trimSlash(basename) on that line) makes my repro tests pass, and the rest of the @sentry/react tests still pass with it. If this looks right to you, I'd be happy to open a PR with those tests.