This React 18 error occurs when a Suspense boundary receives a state update before completing server-side hydration, causing the boundary to switch to client rendering. It typically happens with SSR applications when context providers, external stores, or async updates fire during the hydration phase.
This error is specific to React 18's Concurrent Features and occurs during server-side rendering (SSR) with Suspense boundaries. When React hydrates server-rendered content, it expects the client-side render to match exactly what was rendered on the server. If a Suspense boundary receives any updates (like state changes, context updates, or external store changes) before it finishes hydrating, React cannot guarantee this match. React then falls back to client-side rendering for that Suspense boundary, which defeats the performance benefits of SSR. The error message suggests wrapping the update in startTransition, but this alone may not solve the underlying timing issue. The root problem is typically an update happening too early in the component lifecycle—before React has had a chance to reconcile the server HTML with the client component tree. This commonly occurs with state management libraries, authentication providers, theme providers, or any code that updates state immediately on mount.
Enable React DevTools Profiler and add debug logging to narrow down the source:
// Add to suspect components
useEffect(() => {
console.log('Component mounted:', ComponentName);
}, []);Look for components that:
- Wrap your Suspense boundary (providers, contexts)
- Use external state management
- Make API calls on mount
- Access browser APIs (localStorage, window, etc.)
Check the browser console for the full error stack trace to identify the specific boundary and component.
Use a hydration-safe pattern to prevent updates during SSR:
import { useEffect, useState } from 'react';
function useHydrated() {
const [hydrated, setHydrated] = useState(false);
useEffect(() => {
setHydrated(true);
}, []);
return hydrated;
}
// In your component
function MyComponent() {
const hydrated = useHydrated();
// Only run client-side code after hydration
useEffect(() => {
if (!hydrated) return;
// Safe to update state here
dispatch(someAction());
}, [hydrated]);
}This ensures updates only happen after the component has hydrated on the client.
If using a context provider (theme, auth, etc.), implement getServerSnapshot:
import { useSyncExternalStore } from 'react';
function useTheme() {
const theme = useSyncExternalStore(
// subscribe
(callback) => {
window.addEventListener('storage', callback);
return () => window.removeEventListener('storage', callback);
},
// getSnapshot (client)
() => localStorage.getItem('theme') || 'light',
// getServerSnapshot (SSR)
() => 'light' // Return consistent default for server
);
return theme;
}This prevents mismatches by providing a stable server-side snapshot that won't trigger updates during hydration.
For non-urgent updates, wrap them in startTransition to mark them as low priority:
import { startTransition } from 'react';
function MyComponent() {
useEffect(() => {
// Wrap the update in startTransition
startTransition(() => {
dispatch({ type: 'INIT' });
});
}, []);
}However, note that this alone may not fix the error if the update happens before hydration completes. Combine with hydration checks from step 2.
If a component cannot be fixed to be hydration-safe, disable SSR:
import dynamic from 'next/dynamic';
const ClientOnlyComponent = dynamic(
() => import('./ClientOnlyComponent'),
{ ssr: false }
);
function Page() {
return (
<Suspense fallback={<Loading />}>
<ClientOnlyComponent />
</Suspense>
);
}This prevents the component from rendering on the server, eliminating hydration conflicts. Use sparingly as it reduces SSR benefits.
As a last resort for third-party libraries you cannot modify:
function App({ children }) {
const [mounted, setMounted] = useState(false);
useEffect(() => {
setMounted(true);
}, []);
return (
<div suppressHydrationWarning>
{mounted ? (
<ThirdPartyProvider>
{children}
</ThirdPartyProvider>
) : (
children
)}
</div>
);
}This suppresses the hydration warning but is an escape hatch that should only be used when you cannot fix the root cause.
Understanding React 18 Hydration
React 18 introduced Concurrent Rendering and improved Suspense, which changed how hydration works. Previously, React would hydrate the entire tree before any interactions could happen. Now, React can selectively hydrate parts of the tree and handle updates during hydration more gracefully.
However, when a Suspense boundary receives an update before hydration completes, React must choose between:
1. Waiting for hydration and potentially blocking the update
2. Abandoning SSR for that boundary and client-rendering it
React chooses option 2 for better user experience, but this loses SSR benefits.
Common Library Issues
- Redux: Early versions of react-redux had issues with hydration. Ensure you're on v8+ and use useSyncExternalStore internally.
- Next-themes: Known to cause this error. Use the useHydrated pattern or wait for library updates.
- Auth providers: Many fire state updates immediately on mount. Defer authentication checks until after hydration.
Streaming SSR Considerations
With React 18's streaming SSR, Suspense boundaries can stream HTML progressively. If you're using this feature, ensure that:
- Server and client data fetching logic matches exactly
- You're not fetching different data on server vs client
- Cache headers are consistent between environments
Performance Impact
Switching to client rendering for a Suspense boundary means:
- Loss of First Contentful Paint (FCP) improvement from SSR
- Flash of loading state even though content was server-rendered
- Potential Cumulative Layout Shift (CLS) issues
Monitor your Core Web Vitals to understand the real-world impact.
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