I have a property that is annotated with a System.ComponentModel.DataAnnotations.MaxLengthAttribute. I'd like this length restriction to be reflected in the Open API spec for the service. Is there a way to achieve this with the available filters? I think I'd need the original type and property name to get the attribute.

public class UpdateCustomer : IReturn<UpdateCustomerResponse>, IPost
{
    [MaxLength(50)]
    public string Name{ get; set; }
}

The basic idea how it can be done is to use OperationFilter. As second parameter it consumes OpenApiOperation which has RequestType property. You can find your type by this RequestType and find all properties which has [MaxLength] attribute via reflection. Then iterate through Parameters of operation and set MaxLength property for all parameters which has this property.

new OpenApiFeature{
   OperationFilter = (verb, op) => {
       string requestTypeName = op.RequestType;

       //get type by its string name (you can use any other approach, for example holding types in dictionary)
      Assembly asm = typeof(Project.ServiceModel).Assembly;
      Type type = asm.GetType(requestTypeName);

     //find properties with [MaxLength] Attribute
     var properties = ....

     //set max length in Open API parameters
     foreach(var prop in properties)
     {
        var attrValue = ... //get attribute value from property
        op.Parameters.FirstOrDefault(p => p.Name == prop.Name)?.MaxLength = attrValue;
     }
   } 
};

@xplicit thanks for this. I got it working on the path/operations. However I need to modify the parameters on the Schema Objects too. I'll try and put a PR together to make the SchemaType available on the OpenApiSchema like RequestType is on OpenApiOperation.