npm's client-facing error body, as a codec.

The agnostic serve layer decides the HTTP status of a refusal; the body shape is npm's, and it lives here as one NpmError type with an autodocodec codec. That codec is the single source of truth: the serve path encodes the wire denial from it, and the capability manifest renders the same codec to the documented schema (in its own tier, so openapi3 never reaches the proxy). npm clients read the human-facing reason from a JSON {"error": …} object, matching npm's own denial bodies.

There is no separate renderer handle. A route's abstract ResponseContract pairs each status with its body codec, the handler receives only constructors for values admitted by that contract, and the emitted body is therefore the documented body by construction.