diff --git a/backend/mpc-system/services/session-coordinator/adapters/output/postgres/session_postgres_repo.go b/backend/mpc-system/services/session-coordinator/adapters/output/postgres/session_postgres_repo.go
index 22ca2c4c..323a9089 100644
--- a/backend/mpc-system/services/session-coordinator/adapters/output/postgres/session_postgres_repo.go
+++ b/backend/mpc-system/services/session-coordinator/adapters/output/postgres/session_postgres_repo.go
@@ -265,34 +265,66 @@ func (r *SessionPostgresRepo) Update(ctx context.Context, session *entities.MPCS
 		return err
 	}
 
-	// Upsert participants (insert or update)
+	// Update each participant individually using UPDATE to avoid lost updates
+	// Using individual UPDATE statements ensures concurrent updates to different participants don't conflict
 	for _, p := range session.Participants {
-		_, err = tx.ExecContext(ctx, `
-			INSERT INTO participants (
-				id, session_id, party_id, party_index, status,
-				device_type, device_id, platform, app_version, public_key, joined_at, completed_at
-			) VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12)
-			ON CONFLICT (session_id, party_id) DO UPDATE SET
-				status = EXCLUDED.status,
-				public_key = EXCLUDED.public_key,
-				completed_at = EXCLUDED.completed_at
+		// Try UPDATE first
+		result, err := tx.ExecContext(ctx, `
+			UPDATE participants SET
+				status = $1,
+				public_key = $2,
+				completed_at = $3,
+				device_type = $4,
+				device_id = $5,
+				platform = $6,
+				app_version = $7
+			WHERE session_id = $8 AND party_id = $9
 		`,
-			uuid.New(),
-			session.ID.UUID(),
-			p.PartyID.String(),
-			p.PartyIndex,
 			p.Status.String(),
+			p.PublicKey,
+			p.CompletedAt,
 			p.DeviceInfo.DeviceType,
 			p.DeviceInfo.DeviceID,
 			p.DeviceInfo.Platform,
 			p.DeviceInfo.AppVersion,
-			p.PublicKey,
-			p.JoinedAt,
-			p.CompletedAt,
+			session.ID.UUID(),
+			p.PartyID.String(),
 		)
 		if err != nil {
 			return err
 		}
+
+		// If no rows affected, participant doesn't exist yet, INSERT it
+		rowsAffected, err := result.RowsAffected()
+		if err != nil {
+			return err
+		}
+
+		if rowsAffected == 0 {
+			// Participant doesn't exist, INSERT it
+			_, err = tx.ExecContext(ctx, `
+				INSERT INTO participants (
+					id, session_id, party_id, party_index, status,
+					device_type, device_id, platform, app_version, public_key, joined_at, completed_at
+				) VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12)
+			`,
+				uuid.New(),
+				session.ID.UUID(),
+				p.PartyID.String(),
+				p.PartyIndex,
+				p.Status.String(),
+				p.DeviceInfo.DeviceType,
+				p.DeviceInfo.DeviceID,
+				p.DeviceInfo.Platform,
+				p.DeviceInfo.AppVersion,
+				p.PublicKey,
+				p.JoinedAt,
+				p.CompletedAt,
+			)
+			if err != nil {
+				return err
+			}
+		}
 	}
 
 	return tx.Commit()