Buddy Spy, in the past, known as Open Source Software of Yahoo Tool to make you able to detect any Invisible Status of Yahoo Messenger ID / Username (for example you wanna know the availability of your friend in Yahoo). But due to many of Internet User grab the Source Code without leave any Credit to the Software, right now the official release of Buddy Spy would be no longer available.
By the unavailability of Buddy Spy, there no maintenance and continuation of developing this YM Invisible Detector. That why right now if you got Buddy Spy software from mirror links in the net (you can also get it in this blog), there no guarantee that the Invisible Feature would works properly.
Instead of facing uncertainty of Buddy Spy, we suggest you to use the Newest Yahoo Messenger Invisible Detector (just click the link: available in web base and also desktop software), that been proved worked well.