Question 1

What is FCM and how does it work?

Accepted Answer

FCM stands for Firebase Cloud Messaging. It is a free service by Google that lets you send push notifications to Android, iOS, and web apps. Your server sends a message to Google's FCM servers, and FCM delivers it to the user's device. The device must have a valid FCM token which your app generates when the user installs it.

Question 2

What is the difference between notification message and data message?

Accepted Answer

Notification messages are handled by the system tray automatically — FCM shows the notification for you when the app is in background. Data messages are handled by your app code, so you get full control over what to do with the message. You can also send both together in one message.

Question 3

How do I get my FCM device token?

Accepted Answer

In your app code, call the FCM SDK method to get the token. On Android (Kotlin) use FirebaseMessaging.getInstance().token, in Flutter use FirebaseMessaging.instance.getToken(), and on web use getToken(messaging, { vapidKey: '...' }). The token is a long string that uniquely identifies the device for push notifications.

Question 4

What is a topic in FCM?

Accepted Answer

A topic is like a channel that devices can subscribe to. When you send a message to a topic, all devices subscribed to that topic will receive it. For example, you can create a topic called 'news' and subscribe all users who want news updates. Topics are managed from your app code using the FCM SDK.

Question 5

Can I send to multiple devices at once?

Accepted Answer

Yes, you can use topics to send to all subscribed devices at once. You can also use conditions to combine topics like sending to devices subscribed to both 'sports' AND 'news'. The old way of sending to a list of tokens (multicast) is available but topics are simpler and more efficient.

Question 6

What is the maximum payload size for FCM?

Accepted Answer

The maximum payload size for an FCM message is 4096 bytes (4 KB). This includes the notification fields (title, body, image) and data fields combined. If your message is too big, FCM will reject it with a MessageTooBig error. Try to keep your payloads small and avoid sending large data through FCM.

Question 7

What happens if the device is offline when I send a notification?

Accepted Answer

FCM stores the message on its servers and delivers it when the device comes back online. This is called store-and-forward. The message will be stored for up to 28 days by default. You can change this duration using the TTL (time to live) field in your message.

Question 8

How long does FCM store undelivered messages?

Accepted Answer

By default, FCM stores undelivered messages for up to 28 days. You can change this by setting the TTL (time to live) field in your message payload. If you set TTL to 0, FCM will attempt to deliver immediately and discard the message if the device is offline.

Question 9

How does FCM HTTP v1 API authentication work?

Accepted Answer

The HTTP v1 API uses OAuth 2.0 authentication with a service account JSON file. You generate a short-lived access token from the service account credentials and include it as a Bearer token in the Authorization header. Our tool handles this token generation automatically — just paste your service account JSON.

Question 10

Is FCM free?

Accepted Answer

Yes, FCM is completely free to use. There is no charge for sending notifications regardless of the volume. Google does not charge for FCM messaging. The only potential cost is if you use other Firebase services like Firestore or Cloud Functions alongside FCM.

Question 11

Why is my FCM notification not showing when app is in foreground?

Accepted Answer

This is the most common FCM confusion. When your app is in the foreground, FCM does not automatically show a notification in the system tray. You need to handle it in your app code. On Android, create a notification channel and show it manually. In Flutter, use flutter_local_notifications package to display it when onMessage fires.

Question 12

Why is notification not received on iOS but works fine on Android?

Accepted Answer

iOS has extra setup steps compared to Android. First, make sure you have uploaded your APNs key or certificate in Firebase Console under Project Settings > Cloud Messaging. Second, check that Push Notifications and Background Modes capabilities are enabled in Xcode. Third, on iOS you must request notification permission from the user before receiving notifications.

Question 13

My FCM token keeps changing — what should I do?

Accepted Answer

FCM tokens can change when the app is reinstalled, data is cleared, or the app is restored on a new device. This is normal behaviour. In your app, listen for token refresh events and update the token on your server whenever it changes. On Android use onNewToken callback, in Flutter use onTokenRefresh stream.

Question 14

Can I send FCM notifications without Firebase (self-hosted)?

Accepted Answer

You still need a Firebase project to use FCM — there is no way around it. However, you do not need to use any other Firebase service. Create a Firebase project, download the service account key, and use the FCM HTTP v1 API to send notifications from your own server. Your server does not need the Firebase SDK.

Question 15

Why am I getting SENDER_ID_MISMATCH error?

Accepted Answer

This error means the FCM token was generated by a different Firebase project than the one you are using to send the notification. Make sure the google-services.json (Android) or GoogleService-Info.plist (iOS) in your app matches the same project as the service account key you are using to send notifications.

Question 16

Do FCM notifications work when phone is in Do Not Disturb mode?

Accepted Answer

FCM will still deliver the notification to the device, but the phone will not show it to the user or make a sound in Do Not Disturb mode. The notification will silently appear in the notification shade. This is controlled by the phone's settings, not by FCM. On Android, high-priority notifications may override DND if the user has allowed it.

Question 17

Is there a limit to how many notifications I can send per day?

Accepted Answer

There is no hard daily limit on FCM messages. However, FCM has rate limits: you can send up to 500 messages per second per project for topic messages, and there are per-device rate limits to prevent spamming a single user. If you exceed these limits, you will get a QUOTA_EXCEEDED or RESOURCE_EXHAUSTED error.

Question 18

Why does my notification show but tapping it does nothing?

Accepted Answer

You need to handle the notification tap in your app code. On Android, set a click_action or use a pending intent. In Flutter, use FirebaseMessaging.onMessageOpenedApp stream and also check getInitialMessage for when the app was terminated. If these handlers are not set up, tapping the notification will just open the app without any action.

Question 19

How do I send silent or background notifications using FCM?

Accepted Answer

To send a silent notification, send a data-only message without the notification field. On Android, this will trigger your FirebaseMessagingService even in the background. On iOS, include content-available: 1 in the APNs payload. Silent notifications are useful for syncing data in the background without showing anything to the user.

Question 20

Flutter FCM not working after adding to pubspec.yaml — what to check first?

Accepted Answer

First make sure you have added both firebase_core and firebase_messaging packages. Second, run flutterfire configure to generate your Firebase config (or manually add google-services.json for Android and GoogleService-Info.plist for iOS). Third, call Firebase.initializeApp(options: DefaultFirebaseOptions.currentPlatform) in main() before any FCM code. Fourth, on Android 13+, request the POST_NOTIFICATIONS permission. On iOS, check that the APNs token is available before calling getToken(). Finally, rebuild the app completely — hot reload does not work for native plugin changes.

Frequently Asked Questions