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

[sprucepad]

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

repro.mp4

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