additionalPayload

Optional user-defined payload.

Must be a nested map structure, where values are Constrained (primitives, nested maps, or null).

Serialization uses a custom, format-agnostic encoding to avoid pitfalls of formats that may omit default scalar values (e.g. 0, false, "") on the wire (as in ProtoBuf-style encodings). Each value is encoded as a "typed envelope" that always includes a non-default discriminator, so a missing value can be reconstructed as the correct default, and null can be represented without relying on the underlying format's null support.