Currently, it is not supported to have more than one level of chained structs in IDLs. There does not seem to be a particular reason for such limitation other than avoiding to have to check for circular dependencies.

This arbitrary limitation is providing developers from using IDLs for some trivial use cases as the one from the example below:

struct:
    Base:
        description: "..."
        fields:
            a:
                description: "a"
                type: int
    
    Specialization:
        description: "..."
        chained_structs:
            Base: Base
        fields:
            b:
                description: "b"
                type: int

     SpecializationOfSpecialization:
        description: "..."
        chained_structs:
            Specialization: Specialization

With the above code, the IDL generator returns the following error: Struct 'SpecializationOfSpecialization' is not allowed to nest struct 'Specialization' since it has chained structs and/or types.