diff --git a/backend/mpc-system/services/account/adapters/input/http/co_managed_handler.go b/backend/mpc-system/services/account/adapters/input/http/co_managed_handler.go
index 630c18de..a70dceb6 100644
--- a/backend/mpc-system/services/account/adapters/input/http/co_managed_handler.go
+++ b/backend/mpc-system/services/account/adapters/input/http/co_managed_handler.go
@@ -499,10 +499,10 @@ func (h *CoManagedHTTPHandler) CreateSignSession(c *gin.Context) {
 		return
 	}
 
-	// Validate party count >= threshold + 1
-	if len(req.Parties) < req.ThresholdT+1 {
+	// Validate party count == threshold_t (for t-of-n signing, exactly t parties are needed)
+	if len(req.Parties) != req.ThresholdT {
 		c.JSON(http.StatusBadRequest, gin.H{
-			"error": fmt.Sprintf("need at least %d parties for threshold %d", req.ThresholdT+1, req.ThresholdT),
+			"error": fmt.Sprintf("need exactly %d parties for threshold %d, got %d", req.ThresholdT, req.ThresholdT, len(req.Parties)),
 		})
 		return
 	}