SwaggerGen not handling `application/json; charset=utf-8` properly

[ancailliau]

I'm using version 6.4.0 from Nuget.

The generated swagger.json file exposes responses with application/json although the application returns application/json; charset=utf-8. This cause strict clients to reject the response as the specification does not match the returned value. See https://github.com/OpenAPITools/openapi-generator/issues/13467#issuecomment-1297215358 for example.

For example, my API generates the following

{
  "openapi": "3.0.1",
  "info": {
    "title": "DocIntel API",
    "description": "API for DocIntel",
    "version": "v2.2"
  },
  "paths": {
    "/API/Authentication/Login": {
      "post": {
        "tags": [
          "Authentication"
        ],
        "operationId": "LoginAuthentication",
        "requestBody": {
          "content": {
            "application/json-patch+json": {
              "schema": {
                "$ref": "#/components/schemas/LoginInformation"
              }
            },
            "application/json": {
              "schema": {
                "$ref": "#/components/schemas/LoginInformation"
              }
            },
            "text/json": {
              "schema": {
                "$ref": "#/components/schemas/LoginInformation"
              }
            },
            "application/*+json": {
              "schema": {
                "$ref": "#/components/schemas/LoginInformation"
              }
            }
          }
        },
        "responses": {
          "200": {
            "description": "Success",
            "content": {
              "text/plain": {
                "schema": {
                  "$ref": "#/components/schemas/LoginResult"
                }
              },
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/LoginResult"
                }
              },
              "text/json": {
                "schema": {
                  "$ref": "#/components/schemas/LoginResult"
                }
              }
            }
          },
          "400": {
            "description": "Bad Request",
            "content": {
              "text/plain": {
                "schema": {
                  "$ref": "#/components/schemas/ProblemDetails"
                }
              },
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/ProblemDetails"
                }
              },
              "text/json": {
                "schema": {
                  "$ref": "#/components/schemas/ProblemDetails"
                }
              }
            }
          }
        }
      }
    },
...

And the endpoint returns

< HTTP/1.1 200 OK
< Content-Length: 2800
< Content-Type: application/json; charset=utf-8
< Date: Mon, 31 Oct 2022 09:33:50 GMT
< Server: Kestrel

Adding an annotation [Produces("application/json; charset=utf-8")] to the method produces the expected specification but prevent the application to work, as it will refuse to serve a response with No output formatter was found for content types 'application/json; charset=utf-8, application/json; charset=utf-8' to write the response. error.

{
  "openapi": "3.0.1",
  "info": {
    "title": "DocIntel API",
    "description": "API for DocIntel",
    "version": "v2.2"
  },
  "paths": {
    "/API/Authentication/Login": {
      "post": {
        "tags": [
          "Authentication"
        ],
        "operationId": "LoginAuthentication",
        "requestBody": {
          "content": {
            "application/json-patch+json": {
              "schema": {
                "$ref": "#/components/schemas/LoginInformation"
              }
            },
            "application/json": {
              "schema": {
                "$ref": "#/components/schemas/LoginInformation"
              }
            },
            "text/json": {
              "schema": {
                "$ref": "#/components/schemas/LoginInformation"
              }
            },
            "application/*+json": {
              "schema": {
                "$ref": "#/components/schemas/LoginInformation"
              }
            }
          }
        },
        "responses": {
          "200": {
            "description": "Success",
            "content": {
              "application/json; charset=utf-8": {
                "schema": {
                  "$ref": "#/components/schemas/LoginResult"
                }
              }
            }
          },
          "400": {
            "description": "Bad Request",
            "content": {
              "application/json; charset=utf-8": {
                "schema": {
                  "$ref": "#/components/schemas/ProblemDetails"
                }
              }
            }
          }
        }
      }
    },
...

The auto-generated file should include the charset by default, or offer an option to do so.

[domaindrivendev]

Hmmm - I'm not convinced the client is behaving appropriately in this case TBH. But, if you want a quick fix, I think you could make the generated response media types more explicit by applying a simple operation filter:

// Startup.cs
builder.Services.AddSwaggerGen(c =>
{
    ...

    c.OperationFilter<AddMediaTypeEncoding>();
});

// AddMediaTypeEncoding.cs
internal class AddMediaTypeEncoding : IOperationFilter
{
    public void Apply(OpenApiOperation operation, OperationFilterContext context)
    {
        foreach (var response in operation.Responses.Values)
        {
            response.Content = response.Content
                .ToDictionary(
                    entry => $"{entry.Key}; charset=utf-8",
                    entry => entry.Value
                );
        }
    }
}

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