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 44bef0bd..3f274359 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
@@ -32,6 +32,7 @@ type PendingSession struct {
 	SessionID       uuid.UUID
 	JoinToken       string
 	MessageHash     []byte
+	KeygenSessionID uuid.UUID // For sign sessions: the keygen session that created the keys
 	ThresholdN      int
 	ThresholdT      int
 	SelectedParties []string
@@ -392,7 +393,7 @@ func createCoManagedSessionEventHandler(
 
 			// Immediately call JoinSession (this is required to trigger session_started)
 			joinCtx, joinCancel := context.WithTimeout(ctx, 30*time.Second)
-			_, err := messageRouter.JoinSession(joinCtx, sessionID, partyID, joinToken)
+			sessionInfo, err := messageRouter.JoinSession(joinCtx, sessionID, partyID, joinToken)
 			joinCancel()
 			if err != nil {
 				logger.Error("Failed to join session",
@@ -404,13 +405,15 @@ func createCoManagedSessionEventHandler(
 
 			logger.Info("Successfully joined session, waiting for session_started",
 				zap.String("session_id", event.SessionId),
-				zap.String("party_id", partyID))
+				zap.String("party_id", partyID),
+				zap.String("keygen_session_id", sessionInfo.KeygenSessionID.String()))
 
 			// Store pending session for later use when session_started arrives
 			pendingSessionCache.Store(event.SessionId, &PendingSession{
 				SessionID:       sessionID,
 				JoinToken:       joinToken,
 				MessageHash:     event.MessageHash,
+				KeygenSessionID: sessionInfo.KeygenSessionID, // CRITICAL: Save the correct keygen session ID from JoinSession
 				ThresholdN:      int(event.ThresholdN),
 				ThresholdT:      int(event.ThresholdT),
 				SelectedParties: event.SelectedParties,
@@ -464,7 +467,8 @@ func createCoManagedSessionEventHandler(
 					// Execute signing protocol
 					logger.Info("Auto-participating in co_managed_sign session",
 						zap.String("session_id", event.SessionId),
-						zap.String("party_id", partyID))
+						zap.String("party_id", partyID),
+						zap.String("keygen_session_id", pendingSession.KeygenSessionID.String()))
 
 					sessionInfo := &use_cases.SessionInfo{
 						SessionID:        pendingSession.SessionID,
@@ -472,7 +476,7 @@ func createCoManagedSessionEventHandler(
 						ThresholdN:       int(event.ThresholdN),
 						ThresholdT:       int(event.ThresholdT),
 						MessageHash:      pendingSession.MessageHash,
-						KeygenSessionID:  uuid.Nil, // Use nil to trigger fallback logic (load most recent share)
+						KeygenSessionID:  pendingSession.KeygenSessionID, // CRITICAL: Use the correct keygen session ID from JoinSession
 						Participants:     participants,
 					}