Start: createServerFn in beforeLoad returns undefined during dev SPA navigation despite valid serverFn response

[TomoFromEarth]

Which project does this relate to?

Start

Describe the bug

A createServerFn called from root route beforeLoad returns undefined during client-side navigation in Vite dev.

The same server function works during SSR, and a direct fetch to the generated server function URL returns 200 with a serialized result.

Repro output:

[serverFn beforeLoad repro] {
  environment: 'server',
  auth: { userId: 'user_mock_repro', token: 'token_mock_repro' }
}

[serverFn beforeLoad repro] {
  environment: 'browser',
  auth: 'undefined'
}

[serverFn beforeLoad repro] raw serverFn fetch {
  status: 200,
  ok: true,
  serialized: 'true',
  hasSerializedResult: true,
  hasUserId: true,
  hasToken: true
}

Complete minimal reproducer

https://github.com/TomoFromEarth/tanstack-start-clerk-beforeload-repro

Steps to Reproduce the Bug

1. Clone the repo:

git clone https://github.com/TomoFromEarth/tanstack-start-clerk-beforeload-repro.git
cd tanstack-start-clerk-beforeload-repro

2. Install and run:

pnpm install
pnpm dev

3. Open http://localhost:3000.

4. Click between Home and Repro.

5. Observe the browser console/dev server logs. During browser-side navigation, fetchAuth() returns undefined, then the raw fetch to the same server function URL returns 200 with a serialized result.

Expected behavior

Expected: await fetchAuth() returns the server function result during client navigation.

Actual: await fetchAuth() returns undefined, while the underlying endpoint returns the expected serialized payload.

Screenshots or Videos

No response

Platform

• Router / Start Version: @tanstack/react-router 1.170.16 / @tanstack/react-start 1.168.26
• OS: macOS 26.3.1
• Browser: Chrome
• Browser Version: 149.0.7827.197 (Official Build) (arm64)
• Bundler: Vite
• Bundler Version: 7.3.2

Additional context

A hard refresh/direct load of the route works because beforeLoad runs through the SSR path and receives the server function result.

[schiller-manuel]

i wasnt able to reproduce this using your project.