Document in OpenAPI

dotnet add package AdCodicem.ValueObjects.OpenApi

builder.Services.AddOpenApi(options => options.AddValueObjects());

That registers a schema transformer on the built-in .NET OpenAPI stack (Microsoft.AspNetCore.OpenApi). A value object is then documented as what it is on the wire — its underlying type — carrying every rule declared on it:

Declared on the type	In the schema
The underlying type	type
SchemaFormat, or the natural format of the type (uuid, date, int64…)	format
MinLength, MaxLength	minLength, maxLength
Pattern	pattern
Minimum, Maximum	minimum, maximum
[KnownValue] on a closed set	enum
Example	an example
Description, or the type's XML <summary>	description

So this declaration:

[ValueObject<string>(
    MinLength = 15,
    MaxLength = 34,
    Pattern = "^[A-Z]{2}[0-9]{2}[A-Z0-9]{11,30}$",
    SchemaFormat = "iban",
    Example = "FR7630006000011234567890189")]
public readonly partial struct Iban;

is documented as a string of format iban, between 15 and 34 characters, matching the pattern — never as an object with a value property. There is nothing to restate in an annotation, and nothing to keep in sync: the schema comes from the declaration that validates.

The transformer targets the built-in OpenAPI stack. Swashbuckle is not supported.