The Special Stage Key (スペシャルステージキー,Supesharu Sutēji Kī?), also called the Special Key, is an item found in stages of Sonic Heroes and Sonic Advance 3. It is a golden, sparkling key, that by collecting it, allows entrance to the Special Stage.
In Sonic Heroes, the Special Stage Keys can be found and collected in a similar way as the Chao Key from Sonic Adventure 2, but instead of being in a box, it is usually contained in a metallic cage (The cage can be broken with one attack from the power formation character). To keep this key, the leading character must not be hurt (doesn't disappear if the leading character had a Shield) throughout the rest of the stage or lose a life (e.g. falling into a bottomless pit.) After completing the stage, the team is taken to the Special Stage. In the Super Hard Mode, all the Special Keys are replaced with Power Cores instead.
Sonic Advance 3
A similar key item called the "Special Key" appears in Sonic Advance 3, which can be found in certain areas in an act of a zone, when the player has found all ten Chao from the each zone. The player can collect the key, clear the act and then go to the locked Special Spring on the hub world to enter the Special Stage. Whether the player passes/fails to complete the Special Stage, the player has to go back to one of the zone's acts and get another Special Key.
In the early versions of Sonic Advance 3, the Special Stage does not need to be finished; the players could just exit the stage while retaining the Special Key in their possession.