diff --git a/pkg/tools/cron.go b/pkg/tools/cron.go
index 31ac9ab88..6af0aa9e1 100644
--- a/pkg/tools/cron.go
+++ b/pkg/tools/cron.go
@@ -141,6 +141,12 @@ func (t *CronTool) addJob(ctx context.Context, args map[string]any) *ToolResult
 	everySeconds, hasEvery := args["every_seconds"].(float64)
 	cronExpr, hasCron := args["cron_expr"].(string)
 
+	// Fix: type assertions return true for zero values, need additional validity checks
+	// This prevents LLMs that fill unused optional parameters with defaults (0) from triggering wrong type
+	hasAt = hasAt && atSeconds > 0
+	hasEvery = hasEvery && everySeconds > 0
+	hasCron = hasCron && cronExpr != ""
+
 	// Priority: at_seconds > every_seconds > cron_expr
 	if hasAt {
 		atMS := time.Now().UnixMilli() + int64(atSeconds)*1000