Which project does this relate to?

Start

Describe the bug

When running TanStack Start in a pnpm monorepo (Rush in this case), the client fails to hydrate with a 404 error for the virtual client entry module:

GET http://localhost:3000/@id/virtual:tanstack-start-client-entry net::ERR_ABORTED 404 (Not Found)
Uncaught (in promise) TypeError: Failed to fetch dynamically imported module: http://localhost:3000/@id/virtual:tanstack-start-client-entry

The SSR renders correctly, but client-side hydration completely fails.

Root Cause Analysis

The tanstackStart() plugin calculates the default client entry path relative to its own location:

const defaultEntryDir = path.resolve(currentDir, "..", "..", "plugin", "default-entry")

In a pnpm monorepo, the package is symlinked to the pnpm store:

node_modules/@tanstack/react-start -> ../../../../common/temp/node_modules/.pnpm/@tanstack+react-start@1.135.2_.../

The plugin sets up a Vite alias from virtual:tanstack-start-client-entry to this symlinked path. However, when the browser requests /@id/virtual:tanstack-start-client-entry, Vite's module server doesn't intercept the request - it falls through to TanStack Start's catch-all router middleware, which returns a 404 HTML page.

Workaround

Creating a local src/client.tsx file bypasses the default entry resolution:

import { StrictMode, startTransition } from 'react'
import { hydrateRoot } from 'react-dom/client'
import { StartClient } from '@tanstack/react-start/client'

startTransition(() => {
  hydrateRoot(
    document,
    <StrictMode>
      <StartClient />
    </StrictMode>,
  )
})

This works because the plugin then uses the local file instead of resolving to the pnpm store path.

Your Example Website or App

This occurs in a Rush monorepo with pnpm. A similar issue was reported by an NX monorepo user: https://www.answeroverflow.com/m/1446212939764990075

Steps to Reproduce the Bug or Issue

1. Create a TanStack Start app inside a pnpm monorepo (Rush, Turborepo, or plain pnpm workspaces)
2. Run vite dev
3. Open browser - SSR works but console shows the 404 error
4. Any client-side interactivity fails

Expected behavior

The virtual client entry module should be served correctly by Vite regardless of whether the package is symlinked.

Screenshots or Videos

No response

Platform

• Start Version: 1.135.2
• Vite Version: 7.2.2
• OS: macOS
• Package Manager: pnpm (Rush monorepo)

Additional context

The official start-workos example does not include a src/client.tsx, confirming this file should not be required. The issue appears to be in how Vite 7's Environment API handles aliases pointing to symlinked paths in the pnpm store.