diff --git a/packages/cozeloop-ai/src/prompt/utils.ts b/packages/cozeloop-ai/src/prompt/utils.ts
index 3da7c06..08d2edd 100644
--- a/packages/cozeloop-ai/src/prompt/utils.ts
+++ b/packages/cozeloop-ai/src/prompt/utils.ts
@@ -38,8 +38,8 @@ export function formatPromptTemplate(
   if (!promptTemplate?.messages.length) {
     return [];
   }
-
-  const { messages, template_type, variable_defs } = promptTemplate;
+  // variable_defs may be undefined
+  const { messages, template_type, variable_defs = [] } = promptTemplate;
   const variableMap = buildVariableMap(variable_defs, variables);
   const formattedMessages: Message[] = [];