dio by default convert the headers to small case, which create issues in communicating older iOT devices

[sulaimankhan7]

Package

dio

Version

5.9.0

Operating-System

iOS

Adapter

Default Dio

Output of flutter doctor -v

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[✓] Connected device (4 available) [6.0s]
    • Khan (wireless) (mobile) • 00008130-0010043E0A50001C • ios            • iOS 26.2.1 23C71
    • LDC iPhone (mobile)      • 00008101-001159680E85001E • ios            • iOS 26.2 23C55
    • macOS (desktop)          • macos                     • darwin-arm64   • macOS 26.2 25C56 darwin-arm64
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      The device must be opted into Developer Mode to connect wirelessly. (code -27)

[✓] Network resources [270ms]
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• No issues found!

Dart Version

3.10.4

Steps to Reproduce

1. Create a put request
2. Add headers(ex: 'Content-Type': 'application/json') and give something in the body
3. Set dio.options.preserveHeaderCase = true;
4. Send the request
5. You will see that dio only set this Content-Type to capital but Content-Length and other defaults header are small letter 'content-length'
6. My expectation was it should be 'Content-Length'

Expected Result

Content-Type': 'application/json'
Content-Length': x

Actual Result

'Content-Type': 'application/json'
'content-length': x

Codes

 @singleton
  @Named('sdb')
  Dio sdbDio(AppLogInterceptor logInterceptor, SdbContentLengthInterceptor sdbContentLengthInterceptor) {
    final dio = Dio();
    dio.options.headers.clear();
    dio.interceptors.addAll([logInterceptor]);
    dio.options.baseUrl = 'http://192.168.42.1';
    dio.options.preserveHeaderCase = true;
    return dio;
  }


@RestApi(baseUrl: '')
abstract class SdbConnectivityRemoteDataSourceImpl extends SdbConnectivityRemoteDataSource {
  @factoryMethod
  factory SdbConnectivityRemoteDataSourceImpl(@Named('sdb') Dio dio) = _SdbConnectivityRemoteDataSourceImpl;

  @override
  @Headers({'Content-Type': 'application/json'})
  @PUT('/wlan/config')
  Future<void> sendCredentials(@Body() SdbWifiCredentialsRequestDto credentials);

  @override
  @GET('/wlan/state')
  Future<OnboardingSdbConfigurationStateResponseDto> getConfigurationState();

  @override
  @Headers({'Content-Type': 'application/json'})
  @PUT('/wlan/scan')
  Future<void> scanForNetworks({@Body() Map<String, dynamic> body = const {}});

  @override
  @GET('/wlan/list')
  Future<SdbWifiNetworksResponseDto> getAvailableWifiNetworks();
}

Logs from Insomnia:

> PUT /wlan/scan HTTP/1.1
> Host: 192.168.42.1
> Content-Type: application/json
> User-Agent: insomnia/12.3.0
> Accept: */*
> Content-Length: 0

Logs from flutter devtools :

Request header: 
user-agent: [Dart/3.10 (dart:io)]
Content-Type: [application/json] -> i explicitly defined and set preserveHeader = true
accept-encoding: [gzip]
content-length: [2]
host: [192.168.42.1]

*** Observations
As i set dio.options.preserveHeaderCase = true; , only Content-Type is kept as it is. Rest of the HTTP Header properties which dio added are used as small letter. Though the standard is camel case. as an example 'Content-Length'.
Note: This doesn't have any issue with standard server side. As i am connecting to iOT which use http 1.1 protocol and there they handle everything manually by cpp codes, there they check the header's key as string and you by default convert the properties to small letter words, i am getting http error from their end(Note: that project is not maintained anymore, can't change there)

*** Solution/Workaround I used:

class SdbContentLengthInterceptor extends Interceptor {
  @override
  void onRequest(RequestOptions options, RequestInterceptorHandler handler) {

    if (options.data != null && (options.data is Map)) {
      final rawJson = jsonEncode(options.data);
      final bytes = utf8.encode(rawJson);

      options.data = bytes;
      options.headers.remove('content-length');
      options.headers['Content-Length'] = bytes.length;
    }

    handler.next(options);
  }
}

[AlexV525]

Could you create a minimal reproducible example?

[sulaimankhan7]

Ok, I am creating an example and will upload the code soon.

[sulaimankhan7]

@AlexV525 Updated the ticket descriptions.

[AlexV525]

It looks like your @Headers annotation is affecting the result. Could you use a simple dio implementation without retrofit to reproduce?