User blog comment:IamJakuhoRaikoben/Chapter 371: Tartarus Arc: Part 2 - Song of the Heavenly Dragon, Review./@comment-3424593-20140215225420/@comment-5598380-20140215233023

Dumbledore suspected at first then knew Harry was the seventh Horcrux. The plan was to make Harry go to the Dark Forest to meet his death, instead the one which was destroyed was a piece of Voldemort's soul inside Harry. Dumbledore knew Harry will survive since he is the master of the Deathly Hallows.