$res->dom (Mojo::Message::Body's own DOM-parsing shortcut) — parses body as HTML on first access. A getter (not parsed eagerly at fetch time) since most call sites only ever read .body as plain text and never touch .dom at all.

$res->json (Mojo::Message::Body's own JSON-decoding shortcut) — parses body as JSON on first access. Real Mojo::Message::Body::json returns undef on a parse failure rather than throwing (legacy code routinely checks if (defined $res->json) or wraps the whole call in eval {} expecting exactly that); matched here by returning undefined instead of letting JSON.parse throw.

$res->is_error (Mojo::Message's own status-class check) — true for any 4xx/5xx HTTP status.

$res->headers — not a general Mojo::Headers shim, just the one accessor (->location, read when manually following a 301/302 redirect with max_redirects(0)) any real call site in the legacy plugin corpus actually reads off a response's headers.