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 3f274359..1a546f32 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
@@ -36,6 +36,7 @@ type PendingSession struct {
 	ThresholdN      int
 	ThresholdT      int
 	SelectedParties []string
+	Participants    []use_cases.ParticipantInfo // CRITICAL: Correct PartyIndex from database (via JoinSession)
 	CreatedAt       time.Time
 }
 
@@ -417,6 +418,7 @@ func createCoManagedSessionEventHandler(
 				ThresholdN:      int(event.ThresholdN),
 				ThresholdT:      int(event.ThresholdT),
 				SelectedParties: event.SelectedParties,
+				Participants:    sessionInfo.Participants, // CRITICAL: Save participants with correct PartyIndex from database
 				CreatedAt:       time.Now(),
 			})
 
@@ -450,18 +452,10 @@ func createCoManagedSessionEventHandler(
 				participateCtx, cancel := context.WithTimeout(ctx, 10*time.Minute)
 				defer cancel()
 
-				// 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(event.SelectedParties))
-				for i, p := range event.SelectedParties {
-					participants[i] = use_cases.ParticipantInfo{
-						PartyID:    p,
-						PartyIndex: i,
-					}
-				}
+				// CRITICAL: Use participants from pendingSession (which came from JoinSession response)
+				// These contain the correct PartyIndex values from the database, NOT loop indices
+				// The JoinSession response already includes all participants with their assigned indices
+				participants := pendingSession.Participants
 
 				if isSignSession {
 					// Execute signing protocol