diff --git a/api/services/billing_service.py b/api/services/billing_service.py
index 13f9d1fcf4..0f0c49e7fe 100644
--- a/api/services/billing_service.py
+++ b/api/services/billing_service.py
@@ -37,7 +37,11 @@ class _VectorSpaceQuota(TypedDict):
 
 
 class _KnowledgeRateLimit(TypedDict):
+    # NOTE (hj24):
+    # 1. Return for sandbox users but is null for other plans, it's defined but never used.
+    # 2. Keep it for compatibility for now, can be deprecated in future versions.
     size: NotRequired[int]
+    # NOTE END
     limit: int
 
 
diff --git a/api/services/feature_service.py b/api/services/feature_service.py
index 56d896629c..df653e0ba7 100644
--- a/api/services/feature_service.py
+++ b/api/services/feature_service.py
@@ -336,7 +336,11 @@ class FeatureService:
             features.model_load_balancing_enabled = billing_info["model_load_balancing_enabled"]
 
         if "knowledge_rate_limit" in billing_info:
+            # NOTE (hj24):
+            # 1. knowledge_rate_limit size is nullable, currently it's defined but never used, only limit is used.
+            # 2. So be careful if later we decide to use [size], we cannot assume it is always present.
             features.knowledge_rate_limit = billing_info["knowledge_rate_limit"]["limit"]
+            # NOTE END
 
         if "knowledge_pipeline_publish_enabled" in billing_info:
             features.knowledge_pipeline.publish_enabled = billing_info["knowledge_pipeline_publish_enabled"]