Sending and receiving SMS messages are fundamental features on mobile devices and many developers have built successful apps that enhance this experience on Android. Some of you have built SMS apps using hidden APIs—a practice we discourage because hidden APIs may be changed or removed and new devices are not tested against them for compatibility. So, to provide you with a fully supported set of APIs for building SMS apps and to make the user experience for messaging more predictable, Android 4.4 (KitKat) makes the existing APIs public and adds the concept of a default SMS app, which the user can select in system settings.

This means that if you are using the hidden SMS APIs on previous platform versions, you need to make some adjustments so your app continues to work when Android 4.4 is released later this year.Google Android 4.4 KitKat Hidden APIs For SMS Now Made Public [android-developers.blogspot.com]