How to verify FCM registration token on server?

When sending to an invalid registration token, you'll should receive 200 + error:InvalidRegistration:

Check the format of the registration token you pass to the server. Make sure it matches the registration token the client app receives from registering with Firebase Notifications. Do not truncate or add additional characters.

This is the response when you try to send a simple cURL request where the registration token is just randomly made:

curl --header "Authorization: key=$[your_server_key_here]" \
       --header Content-Type:"application/json" \
       https://fcm.googleapis.com/fcm/send \
       -d "{\"registration_ids\":[\"ABC\"]}"

Notice that I added in "ABC", in the registration_ids parameter. If ever it is a valid registration token, but is not associated to your project, you'll probably receive 200 + error:NotRegistered.

You can try sending a test message from your server to see the response without sending an actual message towards the device by using the dry_run parameter:

This parameter, when set to true, allows developers to test a request without actually sending a message.

Using Node Admin SDK

If anyone using firebase Admin SDK for node.js, there is no need to manually send the request using the server_key explicitly. Admin SDK provide sending dry_run push message to verify the fcm_token.

function verifyFCMToken (fcmToken) => {
    return admin.messaging().send({
        token: fcmToken
    }, true)
}

Use this method like following

verifyFCMToken("YOUR_FCM_TOKEN_HERE")
.then(result => {
    // YOUR TOKEN IS VALID
})
.catch(err => {
    // YOUR TOKEN IS INVALID
})

Using Java Admin SDK

You can use following function

public Boolean isValidFCMToken(String fcmToken) {
        Message message = Message.builder().setToken(fcmToken).build();
        try {
            FirebaseMessaging.getInstance().send(message);
            return true;
        } catch (FirebaseMessagingException fme) {
            logger.error("Firebase token verification exception", fme);
            return false;
        }
    }