diff --git a/backend/mpc-system/services/server-party/application/use_cases/participate_signing.go b/backend/mpc-system/services/server-party/application/use_cases/participate_signing.go
index da81862d..585ae2c3 100644
--- a/backend/mpc-system/services/server-party/application/use_cases/participate_signing.go
+++ b/backend/mpc-system/services/server-party/application/use_cases/participate_signing.go
@@ -156,6 +156,16 @@ func (uc *ParticipateSigningUseCase) Execute(
 		return nil, err
 	}
 
+	// Wait for all parties to subscribe before starting TSS protocol
+	// This prevents race condition where some parties send broadcast messages
+	// before others have subscribed to the session
+	expectedParties := len(sessionInfo.Participants)
+	logger.Info("Waiting for all parties to subscribe",
+		zap.String("session_id", input.SessionID.String()),
+		zap.String("party_id", input.PartyID),
+		zap.Int("expected_parties", expectedParties))
+	time.Sleep(500 * time.Millisecond)
+
 	// Use message hash from session if not provided
 	messageHash := input.MessageHash
 	if len(messageHash) == 0 {