Start: user router.plugins never reach the route generator (start-plugin-core overwrites them)

Which project does this relate to?

Start

Describe the bug

Generator plugins passed as tanstackStart({ router: { plugins: [...] } }) never reach the route generator — no hook (init, onRouteTreeChanged, afterTransform) is ever invoked, in dev or build, with no error or warning.

Cause: tanStackStartRouter builds its internal generator-plugin list and places it after ...routerConfig in the config it hands to tanstackRouterGenerator, so the user's plugins key is overwritten:

https://github.com/TanStack/router/blob/4eed408f12e0dee55708b1e2b53fefb3441c414d/packages/start-plugin-core/src/vite/start-router-plugin/plugin.ts#L147-L159

tanstackRouterGenerator(() => {
  const routerConfig = getConfig().startConfig.router
  const plugins = [clientTreeGeneratorPlugin, routesManifestPlugin()]
  if (startPluginOpts.prerender?.enabled === true) {
    plugins.push(prerenderRoutesPlugin())
  }
  return {
    ...routerConfig,          // includes the user's routerConfig.plugins…
    target: corePluginOpts.framework,
    routeTreeFileFooter: getRouteTreeFileFooter,
    plugins,                  // …which is clobbered here
  }
}, routerPluginContext)

The same options work correctly with the plain tanstackRouter Vite plugin (no Start), where user config flows through getConfig untouched — so this is Start-specific.

Suggested fix (verified locally via a patched package):

if (routerConfig.plugins) plugins.push(...routerConfig.plugins)

Your Example Website or App

// vite.config.ts — minimal repro
import { defineConfig } from 'vite'
import { tanstackStart } from '@tanstack/react-start/plugin/vite'

export default defineConfig({
  plugins: [
    tanstackStart({
      router: {
        plugins: [
          {
            name: 'probe',
            init: () => console.error('[probe] init'),
            onRouteTreeChanged: () => console.error('[probe] onRouteTreeChanged'),
          },
        ],
      },
    }),
  ],
})

Steps to Reproduce the Bug or Issue

1. Add the probe generator plugin above to any TanStack Start app's vite.config.ts
2. Run vite dev or vite build
3. Neither [probe] line is printed; with the plain tanstackRouter() plugin (non-Start), both print

Expected behavior

User-supplied router.plugins are merged with Start's internal generator plugins and their hooks fire on generator runs — or, if generator plugins are intentionally unsupported under Start, a loud error instead of silent acceptance.

Platform

• @tanstack/react-start: 1.168.27 (start-plugin-core 1.171.19; bug still present on main)
• OS: macOS

Additional context

Footnote once plugins do run: createRouterGeneratorPlugin wraps generator.run() in catch (e) { console.error(e) }, so a generator plugin that throws (e.g. to enforce a project invariant) can log but never fail the build. May be intentional, but it compounds the silence here.

Start in packages/start-plugin-core/src/vite/start-router-plugin/plugin.ts around lines 147-159, then run the minimal Vite configuration from the issue with both vite dev and vite build. Verify that user-supplied router.plugins are preserved alongside Start's internal plugins and that the probe hooks run, or that unsupported plugins produce a loud error.