diff --git a/fieldbehavior/fieldbehavior.go b/fieldbehavior/fieldbehavior.go
index 0f984518cd..e59a520797 100644
--- a/fieldbehavior/fieldbehavior.go
+++ b/fieldbehavior/fieldbehavior.go
@@ -62,13 +62,16 @@ func CopyFields(dst, src proto.Message, behaviorsToCopy ...annotations.FieldBeha
 }
 
 func isMessageFieldPresent(m protoreflect.Message, f protoreflect.FieldDescriptor) bool {
-	return isPresent(m.Get(f), f)
+	return isPresent(m.Get(f), f, m.Has(f))
 }
 
-func isPresent(v protoreflect.Value, f protoreflect.FieldDescriptor) bool {
+func isPresent(v protoreflect.Value, f protoreflect.FieldDescriptor, populated bool) bool {
 	if !v.IsValid() {
 		return false
 	}
+	if f.HasOptionalKeyword() && populated {
+		return true
+	}
 	if f.IsList() {
 		return v.List().Len() > 0
 	}
diff --git a/fieldbehavior/fieldbehavior_test.go b/fieldbehavior/fieldbehavior_test.go
index 9934f43cd5..2857317efe 100644
--- a/fieldbehavior/fieldbehavior_test.go
+++ b/fieldbehavior/fieldbehavior_test.go
@@ -100,6 +100,17 @@ func TestValidateRequiredFieldsWithMask(t *testing.T) {
 			"missing required field: shipment.origin_site",
 		)
 	})
+	t.Run("missing optional field annotated with required", func(t *testing.T) {
+		t.Parallel()
+		assert.Error(
+			t,
+			ValidateRequiredFieldsWithMask(
+				&examplefreightv1.Site{},
+				&fieldmaskpb.FieldMask{Paths: []string{"personnel_count"}},
+			),
+			"missing required field: personnel_count",
+		)
+	})
 	t.Run("missing nested not in mask", func(t *testing.T) {
 		t.Parallel()
 		assert.NilError(
diff --git a/proto/einride/example/freight/v1/site.proto b/proto/einride/example/freight/v1/site.proto
index 370e67344f..0193098cad 100644
--- a/proto/einride/example/freight/v1/site.proto
+++ b/proto/einride/example/freight/v1/site.proto
@@ -30,4 +30,6 @@ message Site {
   string display_name = 5 [(google.api.field_behavior) = REQUIRED];
   // The geographic location of the site.
   google.type.LatLng lat_lng = 6;
+  // Personnel count on site.
+  optional int64 personnel_count = 7 [(google.api.field_behavior) = REQUIRED];
 }
diff --git a/proto/gen/einride/example/freight/v1/site.pb.go b/proto/gen/einride/example/freight/v1/site.pb.go
index e52c1c3a1b..20e2ca2e86 100644
--- a/proto/gen/einride/example/freight/v1/site.pb.go
+++ b/proto/gen/einride/example/freight/v1/site.pb.go
@@ -44,6 +44,8 @@ type Site struct {
 	DisplayName string `protobuf:"bytes,5,opt,name=display_name,json=displayName,proto3" json:"display_name,omitempty"`
 	// The geographic location of the site.
 	LatLng *latlng.LatLng `protobuf:"bytes,6,opt,name=lat_lng,json=latLng,proto3" json:"lat_lng,omitempty"`
+	// Personnel count on site.
+	PersonnelCount *int64 `protobuf:"varint,7,opt,name=personnel_count,json=personnelCount,proto3,oneof" json:"personnel_count,omitempty"`
 }
 
 func (x *Site) Reset() {
@@ -120,6 +122,13 @@ func (x *Site) GetLatLng() *latlng.LatLng {
 	return nil
 }
 
+func (x *Site) GetPersonnelCount() int64 {
+	if x != nil && x.PersonnelCount != nil {
+		return *x.PersonnelCount
+	}
+	return 0
+}
+
 var File_einride_example_freight_v1_site_proto protoreflect.FileDescriptor
 
 var file_einride_example_freight_v1_site_proto_rawDesc = []byte{
@@ -134,7 +143,7 @@ var file_einride_example_freight_v1_site_proto_rawDesc = []byte{
 	0x1f, 0x67, 0x6f, 0x6f, 0x67, 0x6c, 0x65, 0x2f, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x62, 0x75, 0x66,
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 	0x69, 0x74, 0x65, 0x12, 0x12, 0x0a, 0x04, 0x6e, 0x61, 0x6d, 0x65, 0x18, 0x01, 0x20, 0x01, 0x28,
 	0x09, 0x52, 0x04, 0x6e, 0x61, 0x6d, 0x65, 0x12, 0x40, 0x0a, 0x0b, 0x63, 0x72, 0x65, 0x61, 0x74,
 	0x65, 0x5f, 0x74, 0x69, 0x6d, 0x65, 0x18, 0x02, 0x20, 0x01, 0x28, 0x0b, 0x32, 0x1a, 0x2e, 0x67,
@@ -154,28 +163,33 @@ var file_einride_example_freight_v1_site_proto_rawDesc = []byte{
 	0x79, 0x4e, 0x61, 0x6d, 0x65, 0x12, 0x2c, 0x0a, 0x07, 0x6c, 0x61, 0x74, 0x5f, 0x6c, 0x6e, 0x67,
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+	0x72, 0x6f, 0x74, 0x6f, 0x33,
 }
 
 var (
@@ -227,6 +241,7 @@ func file_einride_example_freight_v1_site_proto_init() {
 			}
 		}
 	}
+	file_einride_example_freight_v1_site_proto_msgTypes[0].OneofWrappers = []interface{}{}
 	type x struct{}
 	out := protoimpl.TypeBuilder{
 		File: protoimpl.DescBuilder{