diff --git a/backend/mpc-system/services/session-coordinator/domain/services/session_coordinator.go b/backend/mpc-system/services/session-coordinator/domain/services/session_coordinator.go
index f13331b3..333bd3ec 100644
--- a/backend/mpc-system/services/session-coordinator/domain/services/session_coordinator.go
+++ b/backend/mpc-system/services/session-coordinator/domain/services/session_coordinator.go
@@ -29,12 +29,14 @@ func (s *SessionCoordinatorService) ValidateSessionCreation(
 
 	// Validate participant count based on session type
 	// For keygen: all n parties must participate (participantCount == n or 0 for dynamic)
-	// For sign: only t parties participate (participantCount == t or 0 for dynamic)
+	// For sign: at least t parties required, can have up to n (participantCount >= t && <= n)
+	//   - Co-managed sign uses exactly T parties
+	//   - Persistent sign uses T+1 parties
 	if participantCount != 0 {
 		if sessionType == entities.SessionTypeSign {
-			// Signing session: participant count should equal threshold t
-			// (the minimum number of parties needed to sign)
-			if participantCount != threshold.T() {
+			// Signing session: participant count must be at least t (threshold)
+			// and at most n (total parties from keygen)
+			if participantCount < threshold.T() || participantCount > threshold.N() {
 				return entities.ErrSessionFull
 			}
 		} else {