[Start] SSR build fails when calling function returned by `createServerOnlyFn`

[b-mendoza]

Which project does this relate to?

Start

Describe the bug

After upgrading to @tanstack/start-plugin-core@1.158.2, the SSR build fails with:

[tanstack-start-core::server-fn:ssr] createServerOnlyFn() must be called with a function!

This appears related to changes in #6599.

Your Example Website or App

https://stackblitz.com/~/github.com/b-mendoza/create_server_only_fn-error?file=src/routes/deferred.tsx:L5-L22

Steps to Reproduce the Bug or Issue

1. Create a file with this pattern:

import { createServerOnlyFn } from "@tanstack/react-start";

const myFn = createServerOnlyFn(() => ({ hello: "world" }));
const result = myFn();

2. Run the build script
3. The client build succeeds, but the SSR build crashes with: createServerOnlyFn() must be called with a function!

Expected behavior

myFn() should execute normally during SSR build. Calls to functions returned by createServerOnlyFn should not trigger this error.

Screenshots or Videos

No response

Platform

• Router / Start Version: 1.159.5 (regression introduced in 1.158.2)
• OS: Linux (WSL2)
• Bundler: vite
• Bundler Version: 7.3.1

Additional context

A workaround is to inline the call:

const result = createServerOnlyFn(() => ({ hello: "world" }))();