X_SCHEMA_DEFAULT_UNSHAREABLE

a schema default cannot be copied per parse

.default(value) where value holds a function, a symbol or anything else structuredClone refuses. Every parse that omits the field must receive its OWN copy — one shared object meant a handler's mutation became the next request's starting value — so a default that cannot be copied is refused where it is WRITTEN, at the first import of the authoring file, rather than shared and bled

pass a JSON-shaped default (plain object, array, Date, Map, Set), or drop .default() and answer the absent value in the handler