Even though Protobuf mainly relies on tag numbers—not field names—there’s a catch when working with systems like gRPC that convert Protobuf messages into JSON (or other text-based formats) for interoperability. message MetricData { string service_name = 1 ; string metric_name = 2 ; double value = 3 ; extensions 100 to 199 [ declaration = { number : 100 , full_name : ".monitoring.extensions.error_code" , type : "int32" }, declaration = { number : 101 , full_name : ".monitoring.extensions.logs" , type : "string" , repeated : true } ]; } Instead of reserving field numbers ahead of time and defining extensions separately, Any lets you attach any message directly inside another message: