Mismatched Users is thrown when Google returns a MismatchSenderId error. This is because some users' FCM registration tokens are tied to a Google Server API key that no longer matches their current project's Google Server API key.

This generally occurs if...

1. your Firebase project was deleted
2. you changed the Firebase Project Id and/or Firebase Server API key in your OneSignal settings
3. you are still using the GCM keys and need to switch over to FCM, see Updating GCM To FCM

Steps to Resolve

1. Open your project's Firebase Developer Console and navigate to the Firebase Project Number (Sender ID) and the Server API Key.

2. Make sure these keys match in your OneSignal Settings > Platforms > Android configuration Firebase Server Key and Firebase Sender ID

3. If your keys match, then the listed devices are likely old test devices that can be deleted or open the app again on these devices to be moved to the new keys. Press SAVE on the OneSignal Android modal to clear the warning.

If you're using another SDK that uses push notifications, make sure the same Google Project Number is used both in our SDK and the other SDKs.

If your Firebase keys were deleted or have changed, you can use new keys and update the OneSignal configuration by following these steps:

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Android Messaging Warning

Keep in mind that changing your current setup will cause all devices that subscribed to the old project number to become invalid and will not be able to receive notifications until they re-open the app. Upon which, our SDK will update them to the new project number. No app update required.

Steps to change Android FCM Keys

Contact Support with the following data:

1. Your OneSignal App Id. See Keys & IDs to find this.
2. Your new Firebase Sender ID
3. Your new Firebase Server API key

We will update it as soon as possible.

Update GCM to FCM

If you are changing from GCM to FCM you can import the GCM keys into Firebase which will keep the same project number / sender ID but will make it FCM.

Within https://console.firebase.google.com press "Add project" and select your existing Google project to import it to FCM.

You will see your GCM projects listed when you click the down arrow like this: https://i.imgur.com/ztLmdYJ.png

Your Sender ID should stay the same. Your Server API key may change. Follow the steps to change Android FCM Keys.