diff --git a/google/api/field_behavior.proto b/google/api/field_behavior.proto
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+// Copyright 2019 Google LLC.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+syntax = "proto3";
+
+package google.api;
+
+import "google/protobuf/descriptor.proto";
+
+option go_package = "google.golang.org/genproto/googleapis/api/annotations;annotations";
+option java_multiple_files = true;
+option java_outer_classname = "FieldBehaviorProto";
+option java_package = "com.google.api";
+option objc_class_prefix = "GAPI";
+
+extend google.protobuf.FieldOptions {
+  // A designation of a specific field behavior (required, output only, etc.)
+  // in protobuf messages.
+  //
+  // Examples:
+  //
+  //   string name = 1 [(google.api.field_behavior) = REQUIRED];
+  //   State state = 1 [(google.api.field_behavior) = OUTPUT_ONLY];
+  //   google.protobuf.Duration ttl = 1
+  //     [(google.api.field_behavior) = INPUT_ONLY];
+  //   google.protobuf.Timestamp expire_time = 1
+  //     [(google.api.field_behavior) = OUTPUT_ONLY,
+  //      (google.api.field_behavior) = IMMUTABLE];
+  repeated google.api.FieldBehavior field_behavior = 1052;
+}
+
+// An indicator of the behavior of a given field (for example, that a field
+// is required in requests, or given as output but ignored as input).
+// This **does not** change the behavior in protocol buffers itself; it only
+// denotes the behavior and may affect how API tooling handles the field.
+//
+// Note: This enum **may** receive new values in the future.
+enum FieldBehavior {
+  // Conventional default for enums. Do not use this.
+  FIELD_BEHAVIOR_UNSPECIFIED = 0;
+
+  // Specifically denotes a field as optional.
+  // While all fields in protocol buffers are optional, this may be specified
+  // for emphasis if appropriate.
+  OPTIONAL = 1;
+
+  // Denotes a field as required.
+  // This indicates that the field **must** be provided as part of the request,
+  // and failure to do so will cause an error (usually `INVALID_ARGUMENT`).
+  REQUIRED = 2;
+
+  // Denotes a field as output only.
+  // This indicates that the field is provided in responses, but including the
+  // field in a request does nothing (the server *must* ignore it and
+  // *must not* throw an error as a result of the field's presence).
+  OUTPUT_ONLY = 3;
+
+  // Denotes a field as input only.
+  // This indicates that the field is provided in requests, and the
+  // corresponding field is not included in output.
+  INPUT_ONLY = 4;
+
+  // Denotes a field as immutable.
+  // This indicates that the field may be set once in a request to create a
+  // resource, but may not be changed thereafter.
+  IMMUTABLE = 5;
+}
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