diff --git a/backend/mpc-system/api/proto/session_coordinator.proto b/backend/mpc-system/api/proto/session_coordinator.proto
index c1aceb1c..506c41df 100644
--- a/backend/mpc-system/api/proto/session_coordinator.proto
+++ b/backend/mpc-system/api/proto/session_coordinator.proto
@@ -82,6 +82,7 @@ message JoinSessionResponse {
   bool success = 1;
   SessionInfo session_info = 2;
   repeated PartyInfo other_parties = 3;
+  int32 party_index = 4;                   // This party's assigned index
 }
 
 // SessionInfo contains session information
diff --git a/backend/mpc-system/services/message-router/adapters/input/grpc/message_grpc_handler.go b/backend/mpc-system/services/message-router/adapters/input/grpc/message_grpc_handler.go
index 7d51fd19..042751d1 100644
--- a/backend/mpc-system/services/message-router/adapters/input/grpc/message_grpc_handler.go
+++ b/backend/mpc-system/services/message-router/adapters/input/grpc/message_grpc_handler.go
@@ -518,6 +518,9 @@ func (s *MessageRouterServer) JoinSession(
 		}
 	}
 
+	// Use party_index directly from coordinator response
+	resp.PartyIndex = coordResp.PartyIndex
+
 	if len(coordResp.OtherParties) > 0 {
 		resp.OtherParties = make([]*pb.PartyInfo, len(coordResp.OtherParties))
 		for i, p := range coordResp.OtherParties {
@@ -525,16 +528,13 @@ func (s *MessageRouterServer) JoinSession(
 				PartyId:    p.PartyId,
 				PartyIndex: p.PartyIndex,
 			}
-			// Find this party's index
-			if p.PartyId == req.PartyId {
-				resp.PartyIndex = p.PartyIndex
-			}
 		}
 	}
 
 	logger.Info("Proxied JoinSession to coordinator",
 		zap.String("session_id", req.SessionId),
 		zap.String("party_id", req.PartyId),
+		zap.Int32("party_index", resp.PartyIndex),
 		zap.Bool("success", resp.Success))
 
 	return resp, nil
diff --git a/backend/mpc-system/services/session-coordinator/adapters/input/grpc/session_grpc_handler.go b/backend/mpc-system/services/session-coordinator/adapters/input/grpc/session_grpc_handler.go
index 2f986ed9..8fa0762e 100644
--- a/backend/mpc-system/services/session-coordinator/adapters/input/grpc/session_grpc_handler.go
+++ b/backend/mpc-system/services/session-coordinator/adapters/input/grpc/session_grpc_handler.go
@@ -251,6 +251,7 @@ func (s *SessionCoordinatorServer) JoinSession(
 			Status:      output.SessionInfo.Status,
 		},
 		OtherParties: otherParties,
+		PartyIndex:   int32(output.PartyIndex),
 	}, nil
 }