Safe usage of QueryClient in Next.js with getInitialProps + hydration

[IPPO181]

Hi there

I'm using React Query in a Next.js project where pages rely on getInitialProps (I can't migrate to getServerSideProps yet).

I'm trying to avoid refetching data like authentication status on client-side navigation, so I'm using dehydrate(queryClient) in getInitialProps (on SSR), and relying on hydration + cache on the client. This works well — except that in client-side navigations, getInitialProps runs again and creates a new QueryClient().

To avoid re-fetching, I tried using a singleton QueryClient instance, and it works — but I'm concerned about SSR safety and cross-request data leaks, since I couldn't find any official guidance for this specific case (getInitialProps + SSR + hydration).

Here's the utility I'm using:

import { QueryClient } from '@tanstack/react-query';

let queryClientInstance;

export function getQueryClient() {
  const isServer = typeof window === 'undefined';

  if (isServer) {
    return new QueryClient({
      defaultOptions: {
        queries: {
          refetchOnWindowFocus: false,
          refetchOnMount: false,
          refetchOnReconnect: true,
          retry: false,
          staleTime: Infinity,
        },
      },
    });
  }

  if (!queryClientInstance) {
    queryClientInstance = new QueryClient({
      defaultOptions: {
        queries: {
          refetchOnWindowFocus: false,
          refetchOnMount: false,
          refetchOnReconnect: true,
          retry: false,
          staleTime: Infinity,
        },
      },
    });
  }

  return queryClientInstance;
}

I use it like this:

In _app.js, for QueryClientProvider :

const [queryClient] = React.useState(() => getQueryClient());

In getInitialProps, to create the client for hydration :

  const queryClient = getQueryClient();

  const { isAuthenticated, currentStatus, user } = await fetchSSRAuth(
    false,
    cookie,
    queryClient,
  );

Is this approach considered safe in terms of avoiding SSR data leakage? Or is there a better/officially recommended way to handle QueryClient instantiation across SSR + getInitialProps usage?

Thanks a lot for your work and your time