Air carrier flight attendant reported a near escape slide deployment after attempting to assist the ramp agent in opening a door that was not disarmed. The escape slide remained in its case between the door and fuselage.

Narrative

Upon arrival at ZZZ when the seatbelt sign was turned off by the Captain; I got out of my jump seat and turned on the lights. I looked out my portal window and saw the ramp in place. I picked up the phone to make the announcement; 'Prepare doors for arrival and cross check.'As I made the announcement; the ramp agent came to the door and the door began to open. I had not given the thumbs up signal to open the door. I had not yet disarmed the door. The door began to move. The agent backed away. I immediately went to open the door further with the main handle trying to assist him. I realize now that I should not have done that but at the time my only thought was to assist. Although I can't recall exactly when there was an unfamiliar sound. I heard something 'hiss'. When the door was opened to the extent that the auto assist kicked in; the full realization of the slide having been triggered was in front of me. The slide was in this case sandwiched between the door and the fuselage.