Converters are used to apply transformations on request and/or response bodies, for example, transforming a Dart object to a Map<String, dynamic> or vice versa.

Both converter and errorConverter are called before request and response interceptors.

final chopper = ChopperClient(
   converter: JsonConverter(),
   errorConverter: JsonConverter()

The errorConverter is called only on error responses (statusCode < 200 || statusCode >= 300).

The built-in JSON converter

Chopper provides a JsonConverter that is able to encode data to JSON and decode JSON strings. It will also apply the correct header to the request (application/json).

However, if content type header is modified (for example by using @Post(headers: {'content-type': '...'})), JsonConverter won't add the header and it won't call json.encode if content type is not JSON.

JsonConverter itself won't convert a Dart object into a Map<String, dynamic> or a List, but it will convert a Map<String, dynamic> into a JSON string.

Implementing custom converters

You can implement custom converters by implementing the Converter class.

class MyConverter implements Converter {
  Response<BodyType> convertResponse<BodyType, InnerType>(Response response) {
    var body = response.body;
    // Convert body to BodyType however you like
    response.copyWith<BodyType>(body: body);

  Request convertRequest(Request request) {
    var body = request.body;
    // Convert body to String however you like    
    return request.copyWith(body: body);

BodyTypeis the expected type of the response body (e.g., String or CustomObject).

If BodyType is a List or a BuiltList, InnerType is the type of the generic parameter (e.g., convertResponse<List<CustomObject>, CustomObject>(response)).

Using different converters for specific endpoints

If you want to apply specific converters only to a single endpoint, you can do so by using the @FactoryConverter annotation:

@ChopperApi(baseUrl: "/todos")
abstract class TodosListService extends ChopperService {

      request: FormUrlEncodedConverter.requestFactory,
      response: convertResponse,
  @Post(path: '/')
  Future<Response> post(@Field() String foo, @Field() int bar);


Response<T> convertResponse<T>(Response res) =>

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