Module Federation: Mixing dev/prod React builds breaks shared actQueue in React 19

[01-binary]

• Context: Vite + @module-federation/vite host app running in dev mode, pulling a remote platform app that is served as a production build. Both share react/[email protected] as singletons.

• Symptom: Loading the remote in the host triggers TypeError: Cannot read properties of undefined (reading 'push') inside react-dom_client.js. Dev React expects ReactSharedInternals.actQueue, but the prod bundle (remote) lacks it. The console also prints “The current testing environment is not configured to support act(...)”.

Repro Steps:

1. Start host with vite dev in an environment where the host shares React with a production-built remote (remoteEntry.js).
2. Navigate to a route that lazy-loads the remote (React.lazy(() => import('platform/App'))).
3. Observe the runtime error before any UI renders.

• Workaround Implemented: In main.tsx, during dev builds we check __CLIENT_INTERNALS_DO_NOT_USE_OR_WARN_USERS_THEY_CANNOT_UPGRADE; if actQueue is missing we define it (reactInternals.actQueue = null). A feature flag (VITE_ENABLE_REACT_ACT_PATCH) scopes this to local dev only.

if (
  import.meta.env.DEV &&
  import.meta.env.VITE_ENABLE_REACT_ACT_PATCH === 'true'
) {
  console.log('patching react act')
  const reactInternals = (
    React as unknown as {
      __CLIENT_INTERNALS_DO_NOT_USE_OR_WARN_USERS_THEY_CANNOT_UPGRADE?: {
        actQueue?: unknown
      }
    }
  ).__CLIENT_INTERNALS_DO_NOT_USE_OR_WARN_USERS_THEY_CANNOT_UPGRADE

  if (reactInternals && !('actQueue' in reactInternals)) {
    // Remote production bundles share a React build without actQueue; define it to keep dev scheduling intact.
    reactInternals.actQueue = null
  }
}

• Other Considered Options: Run the remote as a dev build, proxy the remote through a dev server, or force both host and remote to share the same production React build (losing dev diagnostics). None are ideal for running the host against a production remote during local dev.

• Clarification Needed: Is there an official Module Federation recommendation for mixing dev/prod React builds? Is there a safer pattern than patching internals, or should MF prevent sharing when builds mismatch?

[gioboa]

Solved by module-federation/vite#491