diff --git a/api/envoy/config/core/v3/base.proto b/api/envoy/config/core/v3/base.proto
index 52a3b8a2c54e4..383b3d96398ae 100644
--- a/api/envoy/config/core/v3/base.proto
+++ b/api/envoy/config/core/v3/base.proto
@@ -358,6 +358,11 @@ message HeaderValueOption {
   // existing values. Otherwise it replaces any existing values.
   // This field is deprecated and please use
   // :ref:`append_action <envoy_v3_api_field_config.core.v3.HeaderValueOption.append_action>` as replacement.
+  //
+  // .. note::
+  //   The :ref:`external authorization service <envoy_v3_api_msg_service.auth.v3.CheckResponse>` and
+  //   :ref:`external processor service <envoy_v3_api_msg_service.ext_proc.v3.ProcessingResponse>` have
+  //   default value (``false``) for this field.
   google.protobuf.BoolValue append = 2
       [deprecated = true, (envoy.annotations.deprecated_at_minor_version) = "3.0"];