TanStack Start: Undefined middleware on HMR

[sdorra]

Which project does this relate to?

Start

Describe the bug

When I use the same middleware in a loader and inside a component (useServerFn), I get the following error on HMR:

8:24:07 AM [vite] (client) hmr update /src/styles.css?direct, /src/routes/private/index.tsx, /src/routes/private/index.tsx?tsr-split=component
8:24:07 AM [vite] (ssr) hmr update /@id/virtual:cloudflare/worker-entry, /src/routes/private/index.tsx?tss-serverfn-split, /src/components/UserId.tsx?tss-serverfn-split
[vite] TypeError: Cannot use 'in' operator to search for 'Symbol(TSS_SERVER_FUNCTION_FACTORY)' in undefined
    at /Users/sdorra/Desktop/hmr-issue-sec/node_modules/.pnpm/@tanstack+start-client-core@1.168.2/node_modules/@tanstack/start-client-core/src/createServerFn.ts:99:13
    at Array.map (<anonymous>)
    at fun.middleware (/Users/sdorra/Desktop/hmr-issue-sec/node_modules/.pnpm/@tanstack+start-client-core@1.168.2/node_modules/@tanstack/start-client-core/src/createServerFn.ts:98:18)
    at /Users/sdorra/Desktop/hmr-issue-sec/src/routes/private/index.tsx:7:4
    at Object.runInlinedModule (workers/runner-worker/index.js:215:4)
    at ModuleRunner.directRequest (workers/runner-worker/vite/module-runner:1204:59)
    at ModuleRunner.cachedRequest (workers/runner-worker/vite/module-runner:1111:73)
    at ModuleRunner.import (workers/runner-worker/vite/module-runner:1061:10)
    at HMRClient.fetchUpdate (workers/runner-worker/vite/module-runner:476:21)
    at async Promise.all (index 1)

Sometimes the development server becomes unusable and requires a restart. The error occurs only when I change SSR-related code.

Complete minimal reproducer

https://github.com/sdorra/tanstack-start-hmr-issue

Steps to Reproduce the Bug

1. Start the application pnpm run dev
2. Visit the application (http://localhost:3000)
3. Change the handler of UserId component a few times (src/components/UserId.tsx)
4. Check the server logs for the error

Expected behavior

Hmr works, no error is thrown and the dev server keeps alive.

Screenshots or Videos

No response

Platform

• Router / Start Version: @tanstack/react-start@1.167.65
• OS: macOS
• Browser: Chrome
• Bundler: vite
• Bundler Version: 8.0.11

Additional context

No response