B737-800 pilot reported an inadvertent partial escape slide deployment prior to pushback for departure. Maintenance was called and passengers deplaned.

Narrative

Just prior to pushback; after all exterior aircraft doors were closed and armed; Flight Attendant A informed me that there was a passenger onboard without an available seat. We contacted the gate agent assigned to the flight to advise her of the situation so the additional passenger could be deplaned.The gate agent began repositioning the jet bridge; and Flight Attendant A asked if the doors could be disarmed. I confirmed that the doors could be disarmed. I recall hearing the Flight Attendant A make the PA to disarm and cross-check the doors. At approximately the same time; the gate agent knocked on the door; which may have created a distraction.My understanding is that during this moment; Flight Attendant A may have become distracted and did not remove the girt bar prior to raising the door flag and giving the gate agent a thumbs-up. The gate agent began opening the door and stopped approximately halfway when she realized something was wrong.I then became aware of a concern at the forward door and immediately went to assess the situation. I observed the slide partially deployed and instructed the flight attendants to remain out of the galley area. I made a PA advising passengers to remain seated and not move toward the forward part of the aircraft while we addressed a mechanical issue with the door.I contacted Operations; followed by Maintenance. Maintenance personnel arrived; safely disarmed the door; and the aircraft was subsequently deplaned without further incident.Causal Factors1. A late passenger issue required reopening an aircraft door after all doors had been closed and armed.2. Door disarming occurred during a high-workload transition phase with multiple tasks and communications at the forward door.3. The door opening sequence began before positive confirmation that the door was fully disarmed; resulting in partial slide deployment.Risk Mitigation Recommendations1. Reinforce positive verbal confirmation that doors are fully disarmed prior to opening.2. Minimize interruptions during door arming/disarming procedures.3. Encourage gate agents to wait for explicit confirmation before opening aircraft doors.4. Emphasize risks associated with reopening armed doors during post-boarding operations.