Flight Attendant reported injuring their hand after banging on main cabin door to preclude an inadvertent escape slide deployment. Captain advised Flight Attendant that he had informed ground personnel of tail tip potential but jetway driver was unaware.

Narrative

Upon arrival at the gate; captain kept the seatbelt sign on and confirmed we should keep the doors armed because we were in a tail-tip scenario and the jetbridge would not be attached for a few more minutes. When I saw the bridge start moving toward us; I called the captain and he said they wouldn't open the door because he had informed ramp.A minute later; the agent knocked on the door. Per the training I just received at training; initially I didn't respond at all (no thumbs up); but when she knocked again; Person B and I both made X signs with our arms; attempting to communicate that they should not open the door.We then heard the agent start to move the handle so I held our side of the handle down and started banging on the door in an attempt to get her attention. In my panic; I punched the window; injuring my hand.I had Person B call the flight deck and he said to 'go ahead and open the door;' but we hadn't followed our door disarming procedures yet because we had been instructed not to. I quickly disarmed the boarding door; then; while the agent opened the door; I made the announcement and proceeded with the all call. We confronted the agent; saying she could have blown a slide because she wasn't waiting for the thumbs up; and she just sort of shrugged and said sorry.