System.Text.Json + NSwag: Duplicate _discriminator property in derived classes causes deserialization issues

[TebohoMBC]

I'm encountering an issue with System.Text.Json polymorphic serialization in combination with NSwag code generation for an Angular frontend.

C# Class Hierarchy:

[JsonPolymorphic]
[JsonDerivedType(typeof(B), nameof(B))]
[JsonDerivedType(typeof(C), nameof(C))]
public abstract class A { }

[JsonPolymorphic]
[JsonDerivedType(typeof(C), nameof(C))]
public abstract class B : A { }

public class C : B { }

Angular Code Generated by NSwag:

export abstract class A {
    _discriminator!: string;
}

export abstract class B extends A {
    _discriminator!: string; // Duplicate
}

export class C extends B { }

The duplicate _discriminator property in B causes issues. When I manually remove the duplicate from B, everything works as expected.

However, if I remove the [JsonPolymorphic] and [JsonDerivedType] annotations from class B to avoid the duplication, I get the following runtime error when posting to a controller expecting B:

System.NotSupportedException: Deserialization of interface or abstract types is not supported.

This exception does not occur when the _discriminator is manually removed and the annotations are kept.

Setup:

• Using System.Text.Json
• NSwag via builder.Services.AddOpenApiDocument();
• Controllers accept A, B, or C as parameters

What I’ve Tried:

-Removing annotations from B: causes deserialization error
-Keeping annotations: causes duplicate _discriminator in generated TypeScript
-Manually removing _discriminator from B: works, but not ideal

Questions:

1. Is this duplication of _discriminator expected behavior?
2. Is there a way to configure NSwag or System.Text.Json to avoid this duplication?
3. Is there a recommended workaround that doesn’t involve switching to Newtonsoft.Json?