Bug: [StrictMode] `getSnapshot` is not called after subscribing to external store when used in Suspense

[jzhan-canva]

in useSyncExternalStore, subscribe is called as a passive effect, so it's possible the store value has changed between render and effect. As a solution, I can see react calls getSnapshot right after subscribing to store, to check if value is stale.

However, when using Suspense in StrictMode, getSnapshot is not called after subscribing to store

React version: 19.2.3

Steps To Reproduce

Consider below code (codesandbox)

const subscribe = (onStoreChange: () => void) => {
  console.log("subscribe called");
  return () => {
    console.log("unsubscribe called");
  };
};
const getSnapshost = () => {
  console.log("getSnapshot called");
  return 1;
};

const Content = ({ promise }: { promise: Promise<any> }) => {
  React.use(promise);
  const value = React.useSyncExternalStore(subscribe, getSnapshost);
  console.log("rendered with value", value);
  return <div>Value {value}</div>;
};

export default function App() {
  const [, reRender] = React.useReducer((state: number) => state + 1, 0);
  const promise = React.useRef(Promise.resolve());
  return (
    <div>
      <button
        onClick={() => {
          console.log("button clicked");
          promise.current = new Promise<void>((resolve) => {
            setTimeout(() => {
              resolve();
            }, 500);
          });
          reRender();
        }}
      >
        Trigger Suspense
      </button>

      <Suspense fallback={<p>Loading...</p>}>
        <Content promise={promise.current} />
      </Suspense>
    </div>
  );
}

The current behavior

In StrictMode, after I click the button to trigger suspense, when it's resolved, it will re-mount the Content component, unsubscribe store, subscribe store, without calling getSnapshot again
my console is

button clicked
getSnapshot called
getSnapshot called
rendered with value 1
getSnapshot called
getSnapshot called
rendered with value 1
unsubscribe called
subscribe called

The expected behavior

The expected behavior is that getSnapshot should be always called after subscribe is called, i.e. in console I should see

subscribe called
getSnapshot called <-- after last subscribe

[Ranamalsingh12]

Thanks for confirming.

This behavior seems to break the core safety guarantee of useSyncExternalStore.

Even though getSnapshot is called during render, the RFC and documentation imply that React should validate the snapshot after subscribing, since subscribe runs as a passive effect and the store could change between render and effect.

In this Suspense + StrictMode scenario, subscribe is called without a subsequent getSnapshot check, which can lead to missed updates or tearing for external stores.

The expectation that getSnapshot runs after subscribe feels reasonable and consistent with the hook’s design goals.

If this is intentional, it would be helpful to clarify this exception in the documentation. Otherwise, this appears to be a correctness issue specific to Suspense replay semantics.