Default ASP JSON error response ignores JSON setting and is always in CamelCase

[svdHero]

Describe the bug

I want my API to return PascalCase JSON. I can achieve that either through

services
    .AddControllers()
    .AddJsonOptions(options => options.JsonSerializerOptions.PropertyNamingPolicy = null);

or, using Newtonsoft, through

services
    .AddControllers()
    .AddNewtonsoftJson(options =>((options.SerializerSettings.ContractResolver as DefaultContractResolver)!).NamingStrategy = new DefaultNamingStrategy() );

No matter which way I go, ASP.NET seems to ignore this when it comes to the default error response, e.g. for model validation errors. There I always get CamelCase such as:

{
    "errors": {
        "myValue": [
            "The value 'foo' is not valid."
        ]
    },
    "type": "https://tools.ietf.org/html/rfc7231#section-6.5.1",
    "title": "One or more validation errors occurred.",
    "status": 400,
    "traceId": "00-1958ab77e30b474b9b3fb63a9644756f-3731b87868e4d345-00"
}

To Reproduce

1. Download and run this example Web API: https://github.com/svdHero/aspnet-camelcase-error-app
2. GET this resource: https://localhost:5001/WeatherForecast/temperature which returns PascalCase.
3. GET this resource: https://localhost:5001/WeatherForecast/error which returns CamelCase.

Further technical details

.NET SDK (reflecting any global.json):
 Version:   5.0.101
 Commit:    d05174dc5a

Runtime Environment:
 OS Name:     Windows
 OS Version:  6.3.9600
 OS Platform: Windows
 RID:         win81-x64
 Base Path:   C:\Program Files\dotnet\sdk\5.0.101\

Host (useful for support):
  Version: 5.0.1
  Commit:  b02e13abab

.NET SDKs installed:
  2.2.203 [C:\Program Files\dotnet\sdk]
  2.2.207 [C:\Program Files\dotnet\sdk]
  2.2.402 [C:\Program Files\dotnet\sdk]
  3.1.100 [C:\Program Files\dotnet\sdk]
  5.0.101 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.All 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.2.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.2.7 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.2.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.2.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.2.7 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.2.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.10 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.2.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.2.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.2.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.10 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.1.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 3.1.10 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

I am using Visual Studio 2019 on Windows.

[javiercn]

@svdHero thanks for contacting us.

@pranavkm do you have any thoughts on what's going on here?

[mkArtakMSFT]

Thanks for contacting us. ProblemDetails follow the RFC 7807 spec which calls out for these to be in lower case. That's why we ignore your choice of NamingStrategy options.

[mkArtakMSFT]

@Rick-Anderson can we get my last response above added to the documentation for ProblemDetails please?

[svdHero]

@mkArtakMSFT Thanks for clarifying.

Although, I personally think that for an API-consumer it probably would be more convenient if ALL reponses from a given API have the same case. Also, it makes an API look odd when it returns mixed case.

[javiercn]

@svdHero the problem is that if we did that, we would not be following the spec and standard clients could fail to interop with the API because of the casing.

[Rick-Anderson]

@Rick-Anderson can we get my last response above added to the documentation for ProblemDetails please?

https://github.com/dotnet/AspNetCore.Docs/issues/21318