Flight Attendant reported the gate agent opened the 1L door before the door unarmed signal was given. The door automatically disarmed during opening; preventing an escape slide deployment.

Narrative

Upon arrival in ZZZZ on flight ABCD; once the seatbelt sign was turned off; I disarmed my assigned door; 1R. As I turned around to walk over and crosscheck; I noticed that door 1L was being opened. This prevented me from completing my crosscheck responsibilities for that door.I was unsure of what had happened prior to my crosscheck duties; but I heard the Purser telling Flight Attendant A that she should not have made any hand gestures and that it was his responsibility to give a thumbs-up. The gate agent who met the flight and opened the door seemed confused and apologized; mistakenly thinking he had seen a thumbs-up from his vantage point.Flight Attendant A explained to the gate agent that she was waving her hands urgently to communicate that the door was not disarmed and was not ready to be opened. The Purser stated that he had tried to tell her to stop gesturing and to step back. He also mentioned that he never had a chance to disarm the door before the gate agent opened it.The Purser did complete the crosscheck for the 1R door and all call procedures for the remaining doors.In summary; door 1L was never disarmed from inside the aircraft. When the agent manually opened the door from the outside; the door mechanism for this particular door and aircraft automatically disarmed itself; thankfully preventing an inadvertent slide deployment.