Global Middleware causes serialisation error when throwing redirects via server function

[mubrik]

Which project does this relate to?

Start

Describe the bug

When there is a global function middleware present in the start instance created in a start.ts file, and a redirect is thrown by any server function a SerovalUnsupportedTypeError occurs.

Your Example Website or App

https://github.com/mubrik/tanstack-start-middleware-issue

Steps to Reproduce the Bug or Issue

start of using the basic install steps:

npx gitpick TanStack/router/tree/main/examples/react/start-basic start-basic
cd start-basic
npm install
npm run dev

create a start.ts file in the src directory, content:

import { createMiddleware, createStart } from '@tanstack/react-start'

export const fnMw = createMiddleware({ type: 'function' })
  .server(({ next }) => {
    console.log("herrr")
    return next({
      context: {
        fromFnMw: true,
      },
    })
  })

export const startInstance = createStart(() => {
  return {
    defaultSsr: true,
    functionMiddleware: [fnMw],
  }
})

update vite config to match:

export default defineConfig({
  server: {
    port: 3000,
  },
  plugins: [
    tsConfigPaths({
      projects: ['./tsconfig.json'],
    }),
    tanstackStart({
      srcDirectory: 'src',
      start: { entry: './start.ts' },
    }),
    viteReact(),
  ],
})

Add a file with the server function throwing redirect:

import { createServerFn } from '@tanstack/react-start';
import { redirect } from '@tanstack/react-router';

export const redirectTest = createServerFn()
    .handler(async () => {
        throw redirect({ to: "/posts" })
    })

// in a component 
function Home() {
  const redir = useServerFn(redirectTest);

  return (
    <div className="p-2">
      <h3>Welcome Home!!!</h3>
      <button onClick={() => redir()}>test redirect</button>
    </div>
  )
}

Expected behavior

I expected the redirect to be propagated to the client router correctly

Screenshots or Videos

Platform

• Router / Start Version: 1.132.38
• OS: macOS
• Browser: Chrome
• Browser Version: 138
• Bundler: vite
• Bundler Version: 7.1.1

Additional context

No response