The brand under which an errorHandler middleware function carries its resolved IErrorResponder.

Created with Symbol.for, not Symbol(), so two copies of this package in one process resolve the same key — the same failure mode UPGRADE_INTENT guards against. The brand is the kernel's only route to the resolved formatter: the drain 503, the malformed-request 400, and the request-lifecycle hooks all run BEFORE the pipeline, where errorHandler's ctx.state publication cannot reach them, and the kernel may not import @setu-ts/exceptions (AI_GUIDELINES §2.2). The kernel reads the brand off the compiled pipeline at startup and seeds the responder into the state those sites hand to respondWithError.

The property is written non-enumerable and is an implementation detail of the seam — not part of the MiddlewareFunction contract.