diff --git a/backend/mpc-system/services/session-coordinator/application/use_cases/report_completion.go b/backend/mpc-system/services/session-coordinator/application/use_cases/report_completion.go
index 60c77cfd..abcb87e8 100644
--- a/backend/mpc-system/services/session-coordinator/application/use_cases/report_completion.go
+++ b/backend/mpc-system/services/session-coordinator/application/use_cases/report_completion.go
@@ -53,7 +53,25 @@ func (uc *ReportCompletionUseCase) Execute(
 		return nil, err
 	}
 
-	// 3. Update participant status to completed
+	// 3. Get participant and check current status
+	participant, err := session.GetParticipant(partyID)
+	if err != nil {
+		return nil, err
+	}
+
+	// 3.1 Ensure participant is in Ready state before marking as Completed
+	// The status flow is: Joined -> Ready -> Completed
+	if participant.Status == value_objects.ParticipantStatusJoined {
+		// Auto-transition to Ready if currently Joined
+		if err := session.UpdateParticipantStatus(partyID, value_objects.ParticipantStatusReady); err != nil {
+			return nil, err
+		}
+		logger.Debug("auto-transitioned participant to ready",
+			zap.String("session_id", session.ID.String()),
+			zap.String("party_id", inputData.PartyID))
+	}
+
+	// 3.2 Update participant status to completed
 	if err := session.UpdateParticipantStatus(partyID, value_objects.ParticipantStatusCompleted); err != nil {
 		return nil, err
 	}
@@ -67,10 +85,6 @@ func (uc *ReportCompletionUseCase) Execute(
 	}
 
 	// 4. Update participant's public key if provided
-	participant, err := session.GetParticipant(partyID)
-	if err != nil {
-		return nil, err
-	}
 	if len(inputData.PublicKey) > 0 {
 		participant.SetPublicKey(inputData.PublicKey)
 	}