You can apply this same idea to serialization formats. JSON doesn't have a schema. On the other hand, you can think of all JSON values as belonging to this Protocol Buffers schema:

    message JsonArray {
      repeated JsonValue value = 1;
    }

    message JsonObject {
      map<string, JsonValue> properties = 1;
    }

    message JsonValue {
      oneof value {
        JsonObject object_value = 1;
        JsonArray array_value = 2;
        bool is_null = 3;
        bool boolean_value = 4;
        string string_value = 5;

        // Represented as a string because JSON doesn't restrict the
        // range/precision of numbers.
        string number_value = 6;
      }
    }