Custom JsonConverter causes OpenAPI request schema to be empty

[Archanian]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

In a .NET 9 (also happens in .NET 10 preview) web app (minimal API), when implementing a custom JsonConverter for a request model, the generated OpenAPI spec contains an empty object for the request body schema. If the converter is not registered, the request body schema is defined correctly.

Expected Behavior

The OpenAPI spec should contain a correctly defined schema for the request model in cases where a custom JsonConverter is defined.

Steps To Reproduce

Simple repro project repository can be found here:. Running this app will generate the following OpenAPI spec (note the empty schema for CreateChannelRequest):

{
  "openapi": "3.0.1",
  "info": {
    "title": "open-api-test | v1",
    "version": "1.0.0"
  },
  "servers": [
    {
      "url": "http://localhost:5161/"
    }
  ],
  "paths": {
    "/channels": {
      "post": {
        "tags": [
          "open-api-test"
        ],
        "requestBody": {
          "content": {
            "application/json": {
              "schema": {
                "$ref": "#/components/schemas/CreateChannelRequest"
              }
            }
          },
          "required": true
        },
        "responses": {
          "200": {
            "description": "OK",
            "content": {
              "text/plain": {
                "schema": {
                  "type": "string"
                }
              }
            }
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
      "CreateChannelRequest": { }
    }
  },
  "tags": [
    {
      "name": "open-api-test"
    }
  ]
}

Removing the JsonConverter registration results in the following (correct) schema for CreateChannelRequest:

{
  "openapi": "3.0.1",
  "info": {
    "title": "open-api-test | v1",
    "version": "1.0.0"
  },
  "servers": [
    {
      "url": "http://localhost:5161/"
    }
  ],
  "paths": {
    "/channels": {
      "post": {
        "tags": [
          "open-api-test"
        ],
        "requestBody": {
          "content": {
            "application/json": {
              "schema": {
                "$ref": "#/components/schemas/CreateChannelRequest"
              }
            }
          },
          "required": true
        },
        "responses": {
          "200": {
            "description": "OK",
            "content": {
              "text/plain": {
                "schema": {
                  "type": "string"
                }
              }
            }
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
      "CreateChannelRequest": {
        "required": [
          "name",
          "enabled"
        ],
        "type": "object",
        "properties": {
          "name": {
            "type": "string"
          },
          "enabled": {
            "type": "boolean"
          }
        }
      }
    }
  },
  "tags": [
    {
      "name": "open-api-test"
    }
  ]
}

Exceptions (if any)

No response

.NET Version

9.0.7 (but also fails in 10.0.100-preview.6.25358.103)

Anything else?

No response

[MihuBot]

Possible related and/or duplicate issue:

• OpenApi generates the same schema for generic types with a JsonConverterFactory. #59172

[Archanian]

I've managed to work around this issue by defining a custom schema transformer (IOpenApiSchemaTransformer), but it is rather inconvenient as now the spec can diverge from reality, given these schemas are no longer autogenerated and have to be hand crafted. In my project I have quite a lot of request models that are like this, as we are using a custom JsonConverterFactory to perform schema validation an all JSON bodies during deserialization (via. JsonSchema.Net), the actual bindings are all still exactly matching the type definitions.

On the one hand, I can see how there might be some justification for not generating a schema for the type - if you're defining a custom converter there is a likelihood that the request model can be quite different from the basic type definition to which it is being bound ... but still it might be nice for the framework to default to using the underlying type info, with an option to override it, rather requiring overriding with a schema transformer just to get any schema definition at all.

[captainsafia]

This is a dupe of dotnet/runtime#105769.

We rely on STJ's schema generation APIs to provide the base schemas they extend. They don't currently have an API for registering schemas for custom converters. The only customize option that exists is the IOpenApiSchemaTransformer.

In an ideal world, STJ exposes an API directly on the JsonConverter implementation that allows custom converters to describe their schema and all components would pick this up.