diff --git a/client/api/ApiCallbackMessage.ts b/client/api/ApiCallbackMessage.ts
index 6f2bc8e..4d29aa1 100644
--- a/client/api/ApiCallbackMessage.ts
+++ b/client/api/ApiCallbackMessage.ts
@@ -1,3 +1,6 @@
+type ErrorCausedBy =
+    'NOT_IN_THIS_CHAT_MEMBER_LIST'
+
 type ApiCallbackMessage = {
     msg: string,
     /**
@@ -8,9 +11,9 @@ type ApiCallbackMessage = {
      * 404: Not Found
      * 500: 伺服器端錯誤
      * 501: 伺服器端不支持請求的功能
-     * -1:  客戶端錯誤
      */
-    code: 200 | 400 | 401 | 403 | 404 | 500 | 501 | -1
-    data?: { [key: string]: unknown }
+    code: 200 | 400 | 401 | 403 | 404 | 500 | 501 | -1,
+    data?: { [key: string]: unknown },
+    caused_by?: ErrorCausedBy,
 }
 export default ApiCallbackMessage
diff --git a/server/api/ApiCallbackMessage.ts b/server/api/ApiCallbackMessage.ts
index e94b2df..d66a0e9 100644
--- a/server/api/ApiCallbackMessage.ts
+++ b/server/api/ApiCallbackMessage.ts
@@ -1,3 +1,6 @@
+type ErrorCausedBy =
+    'NOT_IN_THIS_CHAT_MEMBER_LIST'
+
 type ApiCallbackMessage = {
     msg: string,
     /**
@@ -11,5 +14,6 @@ type ApiCallbackMessage = {
      */
     code: 200 | 400 | 401 | 403 | 404 | 500 | 501,
     data?: { [key: string]: unknown },
+    caused_by?: ErrorCausedBy,
 }
 export default ApiCallbackMessage