Android Studio: Client not ready yet

The activity has to be exported to be accessible from adb. If the Activity has an intent-filter it is exported by default. Otherwise add the exported flag:

<activity    android:name=".my.MyActivity"    android:exported="true"/>

This is the error you get when the activity is not exported and you run

adb shell am start -n "com.example/com.example.my.MyActivity" -a android.intent.action.MAIN -c android.intent.category.LAUNCHER

Error:

Permission Denial: starting Intent { act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] flg=0x10000000 cmp=com.example/.my.MyActiviy } from null (pid=26721, uid=2000) not exported from uid 10304    at android.os.Parcel.readException(Parcel.java:1620)    at android.os.Parcel.readException(Parcel.java:1573)    at android.app.ActivityManagerProxy.startActivityAsUser(ActivityManagerNative.java:2694)    at com.android.commands.am.Am.runStart(Am.java:875)    at com.android.commands.am.Am.onRun(Am.java:359)    at com.android.internal.os.BaseCommand.run(BaseCommand.java:47)    at com.android.commands.am.Am.main(Am.java:100)    at com.android.internal.os.RuntimeInit.nativeFinishInit(Native Method)    at com.android.internal.os.RuntimeInit.main(RuntimeInit.java:251)

[This answer is obsolete for latest versions of Android Studio]

This worked for me :

Tools > Android > Enable ADB Integration

You need to tick Enable ADB Integration

If this is a transient issue then in most cases I have found restarting Android Studio fixed the issue.

In one case a Windows restart seemed to fix the problem. A reboot of the device did not.

It was a transient issue which happened just occasionally for a few days but eventually started happening every time and therefore stopped all work.

I was working with two different Android Studio projects open at the same time, both running (different) apps on the same device (but not at the same time).

I may have opened the Device File Explorer on both projects.