createEffect and onMount don't fire on hydrated routes (Solid Start + TanStack Solid Router)

[mryarbles]

createEffect and onMount don't fire on hydrated routes (Solid Start + TanStack Solid Router)

Summary

For routes rendered via Solid Start (SSR + client hydration), Solid lifecycle hooks (createEffect, onMount) never fire on the client, even though:

• The route component body does execute on the client (signals can be created)
• onCleanup does fire (so a Solid scope is being created and then disposed)
• Plain JS scheduling (setTimeout, queueMicrotask, requestAnimationFrame, Promise.then) all fire normally
• Event handlers attached via JSX work (the hydrated DOM is interactive)

Routes with ssr: false are unaffected — they go through a fresh-render path on the client and lifecycle hooks work as expected.

Versions

• @tanstack/solid-start: ^1.168.13
• @tanstack/solid-router: ^1.170.8
• @tanstack/solid-router-ssr-query: ^1.167.0
• @tanstack/router-plugin: ^1.168.11
• solid-js: ^1.9.13
• vite: ^8.0.0

Minimal reproduction

Any default-SSR route:

// src/routes/index.tsx
import { createFileRoute } from '@tanstack/solid-router'
import { createEffect, createSignal, onCleanup, onMount } from 'solid-js'

export const Route = createFileRoute('/')({ component: Home })

function Home() {
  const tag = `[diag] ${typeof window === 'undefined' ? 'SSR' : 'CLIENT'}`
  console.log(`${tag} 1. body`)

  const [count] = createSignal(0)
  console.log(`${tag} 2. createSignal returned`, count())

  createEffect(() => {
    console.log(`${tag} 3. createEffect`)
  })

  onMount(() => {
    console.log(`${tag} 4. onMount`)
  })

  onCleanup(() => {
    console.log(`${tag} 5. onCleanup`)
  })

  if (typeof window !== 'undefined') {
    queueMicrotask(() => console.log(`${tag} 6. queueMicrotask`))
    setTimeout(() => console.log(`${tag} 7. setTimeout`), 0)
    requestAnimationFrame(() => console.log(`${tag} 8. rAF`))
  }

  return <h1>hello</h1>
}

Observed client console output

[diag] CLIENT 1. body
[diag] CLIENT 2. createSignal returned 0
[diag] CLIENT 6. queueMicrotask
[diag] CLIENT 7. setTimeout
[diag] CLIENT 8. rAF
[diag] CLIENT 5. onCleanup

Missing: 3. createEffect and 4. onMount.

Expected

createEffect and onMount should fire on the client during/after hydration, as they do for non-routed Solid components and for ssr: false routes.

Interpretation

onCleanup firing strongly suggests the route component is called inside a disposable scope (probably as part of route resolution / dry-run for matching) that is torn down before reactive effects can flush. The actual interactive DOM is then hydrated from SSR'd HTML in a separate scope where the function isn't re-invoked, so lifecycle hooks scheduled by the first invocation never run.

If that's the intended design, it should probably be documented prominently (since onMount-for-client-side-init is a near-universal Solid pattern). If it's not intended, the route component's reactive scope needs to be allowed to flush effects before disposal.

Workarounds (currently in use)

For SSR'd routes:

1. Render-body side effect with !isServer guard — works for idempotent operations (writing to documentElement, preloading images, etc.).
2. ssr: false on the route — gives up SSR for the route but lifecycle hooks then work normally.
3. setTimeout(() => ..., 0) from the render body — defers past the disposed scope, fires reliably.

Cross-route observation

In our app, /account/children (which has ssr: false) uses createQuery + createMutation successfully and lifecycle hooks fire. The index route (/) — default ssr: true — exhibits the bug. This is consistent across browser sessions and across pnpm dev restarts.