facebook/react · error · Error

Missing getServerSnapshot, which is required for…

Error message

Missing getServerSnapshot, which is required for server-rendered content. Will revert to client rendering.

What it means

useSyncExternalStore requires a third argument (getServerSnapshot) whenever the component can render on the server: during SSR there is no external store to subscribe to, so React needs a deterministic value to render. When the argument is missing, Fizz throws this error and the framework falls back to client rendering (or the SSR request fails).

Solutions

1. Pass the third argument with a deterministic server value: useSyncExternalStore(subscribe, getSnapshot, () => initialState)
2. Upgrade the store library/binding to an SSR-aware version that supplies getServerSnapshot (react-redux v8+, zustand v4+)
3. If no meaningful server value exists, render a loading shell on the server and read the store only after hydration

Example fix

// before
const width = useSyncExternalStore(subscribeWindow, () => window.innerWidth); // throws on server

// after
const width = useSyncExternalStore(
  subscribeWindow,
  () => window.innerWidth,
  () => 0 // deterministic server snapshot
);

// Wrapper that refuses the two-argument form before SSR can hit it.
function useServerSafeSyncExternalStore(subscribe, getSnapshot, getServerSnapshot) {
  if (!getServerSnapshot) {
    throw new Error('getServerSnapshot is required for SSR — pass a deterministic server value');
  }
  return useSyncExternalStore(subscribe, getSnapshot, getServerSnapshot);
}

When it happens

Trigger: useSyncExternalStore(subscribe, getSnapshot) — two arguments — inside any component that gets server-rendered; older store bindings (zustand/redux/jotai) that do not forward a server snapshot internally; components reading window/document/location-derived stores during SSR.

Common situations: React 18 upgrades surfacing store code that never SSR'd before; third-party hooks calling useSyncExternalStore without the third argument; components using browser-only external state rendered in a server component tree.

Related errors

• 407
• 318
• 538
• 558
• A function wrapped in useEffectEvent can't be called during…

AI-assisted analysis of facebook/react@eafeac097b (2026-08-21). Data as JSON: /api/errors/2599e838b2358c34. Report an issue: GitHub.

Appendix: source

Thrown at packages/react-server/src/ReactFizzHooks.js:628

  throw new Error(
    "A function wrapped in useEffectEvent can't be called during rendering.",
  );
}

export function useEffectEvent<Args, Return, F: (...Array<Args>) => Return>(
  callback: F,
): F {
  // $FlowFixMe[incompatible-type]
  return throwOnUseEffectEventCall;
}

function useSyncExternalStore<T>(
  subscribe: (() => void) => () => void,
  getSnapshot: () => T,
  getServerSnapshot?: () => T,
): T {
  if (getServerSnapshot === undefined) {
    throw new Error(
      'Missing getServerSnapshot, which is required for ' +
        'server-rendered content. Will revert to client rendering.',
    );
  }
  return getServerSnapshot();
}

function useDeferredValue<T>(value: T, initialValue?: T): T {
  resolveCurrentlyRenderingComponent();
  return initialValue !== undefined ? initialValue : value;
}

function unsupportedStartTransition() {
  throw new Error('startTransition cannot be called during server rendering.');
}

function useTransition(): [
  boolean,

View on GitHub (pinned to eafeac097b)