diff --git a/backend/mpc-system/services/server-party-co-managed/cmd/server/main.go b/backend/mpc-system/services/server-party-co-managed/cmd/server/main.go
index cef97a68..c3c926b4 100644
--- a/backend/mpc-system/services/server-party-co-managed/cmd/server/main.go
+++ b/backend/mpc-system/services/server-party-co-managed/cmd/server/main.go
@@ -416,11 +416,13 @@ func createCoManagedSessionEventHandler(
 					zap.String("session_id", event.SessionId),
 					zap.String("party_id", partyID))
 
-				// Build SessionInfo from pending session and event data
+				// Build SessionInfo from session_started event (NOT from pendingSession cache)
+				// session_started event contains ALL participants who have joined,
+				// including external parties that joined dynamically after session_created
 				// Note: We already called JoinSession in session_created phase,
 				// so we use ExecuteWithSessionInfo to skip the duplicate JoinSession call
-				participants := make([]use_cases.ParticipantInfo, len(pendingSession.SelectedParties))
-				for i, p := range pendingSession.SelectedParties {
+				participants := make([]use_cases.ParticipantInfo, len(event.SelectedParties))
+				for i, p := range event.SelectedParties {
 					participants[i] = use_cases.ParticipantInfo{
 						PartyID:    p,
 						PartyIndex: i,
@@ -430,8 +432,8 @@ func createCoManagedSessionEventHandler(
 				sessionInfo := &use_cases.SessionInfo{
 					SessionID:    pendingSession.SessionID,
 					SessionType:  "co_managed_keygen",
-					ThresholdN:   pendingSession.ThresholdN,
-					ThresholdT:   pendingSession.ThresholdT,
+					ThresholdN:   int(event.ThresholdN),
+					ThresholdT:   int(event.ThresholdT),
 					MessageHash:  pendingSession.MessageHash,
 					Participants: participants,
 				}