does anyone know how to debug a FCM notification while app is closed

App isn’t closed when coming notification, FCM service is running on background. You can select your app in “choose process” screen on android studio.

how you will do that

Just log incoming message to Logcat

Send notification to yourself while app is closed and FCM Service start to running on background in this moment. You can select your app in “choose process” screen

True, but it may be tricky to catch particular moment between app start and moment when you attach it to debugger, waitForDebugger() may help debug such cases

In my case, App will stop at breakpoint while you click to notification.

yes, but it only if you debug some code that runs after click

Right. Two case can work for you, @Mohamed Ibrahim should try these.

thank you guys I'll try that