serverFn.url used as a form action throws "forgot to return a response from your server route handler"

Which project does this relate to?

Start

Describe the bug

When calling a server function directly, it works fine, but when calling a server function through .url (e.g. in a form action), it gives an HTTPError (full paths censored):

Error: It looks like you forgot to return a response from your server route handler. If you want to defer to the app router, make sure to have a component set in this route.
    at throwRouteHandlerError (.../tanstack-http-error-repro/node_modules/.pnpm/@tanstack+start-server-core@1.169.16_crossws@0.4.8_srvx@0.11.20_/node_modules/@tanstack/start-server-core/src/createStartHandler.ts:187:9)
    ... 5 lines matching cause stack trace ...
    at async file://.../tanstack-http-error-repro/node_modules/.pnpm/@tanstack+start-plugin-core@1.171.19_@tanstack+react-router@1.170.17_react-dom@19.2.7_r_d72b1ab9da4814157965ad4aab276fbb/node_modules/@tanstack/start-plugin-core/dist/esm/vite/dev-server-plugin/plugin.js:79:36 {
  cause: Error: It looks like you forgot to return a response from your server route handler. If you want to defer to the app router, make sure to have a component set in this route.
      at throwRouteHandlerError (.../tanstack-http-error-repro/node_modules/.pnpm/@tanstack+start-server-core@1.169.16_crossws@0.4.8_srvx@0.11.20_/node_modules/@tanstack/start-server-core/src/createStartHandler.ts:187:9)
      at getFinalResponse (.../tanstack-http-error-repro/node_modules/.pnpm/@tanstack+start-server-core@1.169.16_crossws@0.4.8_srvx@0.11.20_/node_modules/@tanstack/start-server-core/src/createStartHandler.ts:257:7)
      at executeMiddleware (.../tanstack-http-error-repro/node_modules/.pnpm/@tanstack+start-server-core@1.169.16_crossws@0.4.8_srvx@0.11.20_/node_modules/@tanstack/start-server-core/src/createStartHandler.ts:325:33)
      at process.processTicksAndRejections (node:internal/process/task_queues:104:5)
      at async startRequestResolver (.../tanstack-http-error-repro/node_modules/.pnpm/@tanstack+start-server-core@1.169.16_crossws@0.4.8_srvx@0.11.20_/node_modules/@tanstack/start-server-core/src/createStartHandler.ts:524:50)
      at async Object.fetch (.../tanstack-http-error-repro/node_modules/.pnpm/@tanstack+react-start@1.168.27_crossws@0.4.8_srvx@0.11.20__esbuild@0.28.1_react-dom@19._13289cfff4aadcbbf2e856a5b2ef2e8e/node_modules/@tanstack/react-start/dist/plugin/default-entry/server.ts:16:14)
      at async file://.../tanstack-http-error-repro/node_modules/.pnpm/@tanstack+start-plugin-core@1.171.19_@tanstack+react-router@1.170.17_react-dom@19.2.7_r_d72b1ab9da4814157965ad4aab276fbb/node_modules/@tanstack/start-plugin-core/dist/esm/vite/dev-server-plugin/plugin.js:79:36,
  status: 500,
  statusText: undefined,
  headers: undefined,
  data: undefined,
  body: undefined,
  unhandled: true
}

Complete minimal reproducer

https://github.com/sprucepad/tanstack-http-error-repro

Steps to Reproduce the Bug

1. Run pnpm install && pnpm run dev
2. Go to localhost:3000, localhost:3001 and localhost:3002
3. Click the "Through button" buttons in each, see the logs
4. Click the "Through form" buttons in each, see the errors and logs

Expected behavior

I expected testFn.url to work for forms, as that's what's listed in the docs. Instead, I get an HTTP error.

Screenshots or Videos

https://github.com/user-attachments/assets/f697c066-b3a6-4f68-a92a-a21e9e6a982d

Platform

• Router / Start Version: Start 1.168.27, Router 1.170.17 (from the template)
• OS:Linux
• Browser: Zen (Firefox)
• Browser Version: 1.21.4b (Firefox 152.0.3)
• Bundler: Vite
• Bundler Version: 8.0.0

Additional context

No response

Start with the linked minimal reproducer and run pnpm install && pnpm run dev, then compare the button and form flows across the three localhost ports. Trace the .url request through createStartHandler.ts, which appears in the stack trace. Done means testFn.url works as a form action without the HTTP error while direct calls continue to work.