Why do I get "MismatchSenderId" from GCM server side?

Did your server use the new registration ID returned by the GCM server to your app? I had this problem, if trying to send a message to registration IDs that are given out by the old C2DM server.

And also double check the Sender ID and API_KEY, they must match or else you will get that MismatchSenderId error. In the Google API Console, look at the URL of your project:

https://code.google.com/apis/console/#project:xxxxxxxxxxx

The xxxxxxxxx is the project ID, which is the sender ID.

And make sure the API Key belongs to 'Key for server apps (with IP locking)'

I encountered the same issue recently and I tried different values for "gcm_sender_id" based on the project ID. However, the "gcm_sender_id" value must be set to the "Project Number".

You can find this value under: Menu > IAM & Admin > Settings.

See screenshot: GCM Project Number

Please run below script in your terminal

curl -X POST \
-H "Authorization: key=  write here api_key" \
-H "Content-Type: application/json" \
-d '{ 
"registration_ids": [ 
"write here reg_id generated by gcm"
], 
"data": { 
"message": "Manual push notification from Rajkumar"
},
"priority": "high"
}' \
https://android.googleapis.com/gcm/send

it will give the message if it is succeeded or failed

Mismatch happens when you don't use the numeric ID. Project ID IS NOT SENDER ID!! It took me 9 hours to figure this out. For all the confusion created by google, check the following link to get numeric id.

https://console.cloud.google.com

instead of

https://console.developers.google.com

Hope it helps!!

Update:- Things have changed again. Now the sender id is with firebase.

Go to https://console.firebase.google.com and select your project. Under settings -> cloud messaging, you can find the sender id.

And it works!