Although the Sukhoi Su-57 (T-50 PAK-FA) is still in development status, but Russia has brought in two prototypes of the fifth generation fighter aircraft based on Hwmeymim, Syria.
The emergence of Su-57 in the Syrian war could be an operational test event, considering that Russia itself now has 12 Su-57 prototypes used for various purposes. Even the Russian government has ordered 12 Su-57 units for the first phase in combat test.
Source : https://amp.theguardian.com
Nevertheless, lowering the Su-57 at the forefront of Syrian warfare is something extraordinary, considering the Syrian war is not very friendly for fighter aircraft. Until now Russia has lost some Su-24 and Su-25 plus a number of helicopters.
Is the Su-57 imported to compensate for the United States air force fighter power, as well as challenging the US Air Force's F-22 Raptor duel that disrupts Russian missions?