In Next.js SSR/app-router, do not use useSuspenseQuery unless your server prefetch + hydration setup guarantees the data is already in the cache such that the component will not suspend.

Why: with standard hydration, if the query isn’t actually present on the client yet, useSuspenseQuery may suspend and you’ll get undesirable loading/empty HTML or server/client state mismatch. If you can’t guarantee “no suspend,” prefer useQuery (so the component renders with isLoading/pending state instead of relying on Suspense streaming).

Apply this rule:

• If you use the “dehydrate pending queries” + streaming-friendly pattern (v5.40+), useSuspenseQuery can be appropriate because pending promises can be hydrated and consumed.
• Otherwise (common case with the standard HydrationBoundary + awaited prefetch), use useQuery in the client component to avoid Suspense-related mismatches.

Also keep SSR hydration details correct:

• For non-JSON serialization, deserialize on the client via hydrate.transformPromise.
• Ensure prefetchQuery({ queryFn }) uses a function queryFn: () => getPosts().then(serialize).

Example (safe default for app-router with awaited prefetch):

// app/posts/page.tsx (Server)
import { dehydrate, HydrationBoundary, QueryClient } from '@tanstack/react-query'
import Posts from './posts'
import { getPosts } from './api'

export default async function PostsPage() {
  const queryClient = new QueryClient()
  await queryClient.prefetchQuery({
    queryKey: ['posts'],
    queryFn: getPosts,
  })

  return (
    <HydrationBoundary state={dehydrate(queryClient)}>
      <Posts />
    </HydrationBoundary>
  )
}

// app/posts/posts.tsx (Client)
'use client'
import { useQuery } from '@tanstack/react-query'
import { getPosts } from './api'

export default function Posts() {
  const { data } = useQuery({ queryKey: ['posts'], queryFn: getPosts })
  return <div>{data?.length}</div>
}