Android Studio suddenly (and for no apparent reason) started complaining that it could not find org.apache.http.legacy (which was added to build.gradle as a legacy library).

Warning: Unable to find optional library: org.apache.http.legacy

The issue was caused by a missing optional.json file that was supposed to be located in Android SDK folder …AndroidSDK\platforms\android-23\optional

The error was resolved by re-installing Android SDK (v23 – Android 6.0).

Content of the missing optional.json file was:

[
 {
  "name": "org.apache.http.legacy",
  "jar": "org.apache.http.legacy.jar",
  "manifest": false
 }
]

Android Studio 1.5.1
Android SDK 23