Firebase Local Emulator Suite FAQ and troubleshooting

Why do Local Emulator Suite logs show an error starting with "Multiple projectIds are not recommended in single project mode"?

This message means the Local Emulator Suite has detected it may be running a particular product emulator using different project IDs. This may indicate a misconfiguration, and can cause issues when emulators try to communicate with one another, and when you try to interact with emulators from your code. If project IDs don't match, it often appears that data is missing, because data stored in emulators is keyed to projectID, and interoperability depends on matching project IDs.

This has been a common source of confusion among developers, so by default the Local Emulator Suite will only allow running with a single project ID, unless you specify otherwise in the firebase.json configuration file. If an emulator detects more than one project ID, it will log a warning and potentially throw a fatal error.

Check your project ID declarations for mismatches in:

• The default project set at the command line. By default, the project ID will be taken on startup from the project selected with firebase init or firebase use. To view the list of projects (and see which one is selected) use firebase projects:list.
• Unit tests. The project ID is often specified in calls to the Rules Unit Testing library methods initializeTestEnvironment or initializeTestApp. Other testing code may initialize with initializeApp(config).
• The command line --project flag. Passing the Firebase CLI --project flag overrides the default project. You'll need to ensure the value of the flag matches the project ID in unit tests and app initialization.

Platform-specific places to check:

To disable single project mode, update firebase.json with the singleProjectMode key:

{
  "firestore": {
    ...
  },
  "functions": {
    ...
  },
  "hosting": {
    ...
  },
  "emulators": {
    "singleProjectMode": false,
    "auth": {
      "port": 9099
    },
    "functions": {
      "port": 5001
    },
    ...
  }
}