diff --git a/google/pubsub/v1/pubsub.proto b/google/pubsub/v1/pubsub.proto
index 2792133a4e..5a8bb07687 100644
--- a/google/pubsub/v1/pubsub.proto
+++ b/google/pubsub/v1/pubsub.proto
@@ -553,6 +553,10 @@ message IngestionFailureEvent {
   // validation violation.
   message SchemaViolationReason {}
 
+  // Set when a Pub/Sub message fails to get published due to a message
+  // transformation error.
+  message MessageTransformationFailureReason {}
+
   // Failure when ingesting from a Cloud Storage source.
   message CloudStorageFailure {
     // Optional. Name of the Cloud Storage bucket used for ingestion.
@@ -580,6 +584,11 @@ message IngestionFailureEvent {
       // Optional. The Pub/Sub message failed schema validation.
       SchemaViolationReason schema_violation_reason = 7
           [(google.api.field_behavior) = OPTIONAL];
+
+      // Optional. Failure encountered when applying a message transformation to
+      // the Pub/Sub message.
+      MessageTransformationFailureReason message_transformation_failure_reason =
+          8 [(google.api.field_behavior) = OPTIONAL];
     }
   }
 
@@ -608,6 +617,11 @@ message IngestionFailureEvent {
       // Optional. The Pub/Sub message failed schema validation.
       SchemaViolationReason schema_violation_reason = 6
           [(google.api.field_behavior) = OPTIONAL];
+
+      // Optional. Failure encountered when applying a message transformation to
+      // the Pub/Sub message.
+      MessageTransformationFailureReason message_transformation_failure_reason =
+          7 [(google.api.field_behavior) = OPTIONAL];
     }
   }
 
@@ -636,6 +650,11 @@ message IngestionFailureEvent {
       // Optional. The Pub/Sub message failed schema validation.
       SchemaViolationReason schema_violation_reason = 6
           [(google.api.field_behavior) = OPTIONAL];
+
+      // Optional. Failure encountered when applying a message transformation to
+      // the Pub/Sub message.
+      MessageTransformationFailureReason message_transformation_failure_reason =
+          7 [(google.api.field_behavior) = OPTIONAL];
     }
   }
 
@@ -664,6 +683,11 @@ message IngestionFailureEvent {
       // Optional. The Pub/Sub message failed schema validation.
       SchemaViolationReason schema_violation_reason = 6
           [(google.api.field_behavior) = OPTIONAL];
+
+      // Optional. Failure encountered when applying a message transformation to
+      // the Pub/Sub message.
+      MessageTransformationFailureReason message_transformation_failure_reason =
+          7 [(google.api.field_behavior) = OPTIONAL];
     }
   }
 
@@ -683,6 +707,11 @@ message IngestionFailureEvent {
       // Optional. The Pub/Sub message failed schema validation.
       SchemaViolationReason schema_violation_reason = 4
           [(google.api.field_behavior) = OPTIONAL];
+
+      // Optional. Failure encountered when applying a message transformation to
+      // the Pub/Sub message.
+      MessageTransformationFailureReason message_transformation_failure_reason =
+          5 [(google.api.field_behavior) = OPTIONAL];
     }
   }
 
@@ -2154,12 +2183,12 @@ message StreamingPullResponse {
       [(google.api.field_behavior) = OPTIONAL];
 
   // Optional. This field will only be set if `enable_exactly_once_delivery` is
-  // set to `true`.
+  // set to `true` and is not guaranteed to be populated.
   AcknowledgeConfirmation acknowledge_confirmation = 5
       [(google.api.field_behavior) = OPTIONAL];
 
   // Optional. This field will only be set if `enable_exactly_once_delivery` is
-  // set to `true`.
+  // set to `true` and is not guaranteed to be populated.
   ModifyAckDeadlineConfirmation modify_ack_deadline_confirmation = 3
       [(google.api.field_behavior) = OPTIONAL];