Wire marker. Symbol.for keys survive neither JSON.stringify nor structuredClone, so the serialized form carries an explicit field in the brand's place.

It is load-bearing rather than decorative: without it the structural check in isFailure() would have to accept "has a string code and a string message", and every Node system error satisfies that — an ENOENT would report itself as a Failure and error.data would silently read undefined from there on.