Actually they do.

When I flew a UA-marketed Cape Air segment connecting to UA, flashing my 1K card exempted my bag fee.


Cape Air currently markets and operates under its own designated code (“9K”), Continental Airlines’ code (“CO”), JetBlue’s code (“B6”), American Airlines’ code (AA”), and United Airlines’code (“UA), and participates in a variety of interline, code share and other marketing agreements with airline partners. For flights marketed and sold under the name and code of another airline, the baggage allowances and fees of the other airline apply throughout the itinerary, including the Cape Air portion.


That said, if this is 9K marketed, 9K allows a free bag:

For travel exclusively on Cape Air marketed flights. (Flights sold under the 9K airline designator.) All bags below must meet the size and weight restrictions noted in the section entitled Bag Allowance. The below charges apply each way and are cumulative.

Free – One (1) checked bag
Free – One (1) gate checked bag
Free – One (1) personal item


Since the OP is Gold, the AC portion is covered by the *G bag fee allowance, so if this is 9K connecting to AC, there should also be no fee for 1 bag.


So the nuance is play the UA status card if the flight is UA coded. If it's not, play the 9K free bag allowance, and if they say something about the AC connection, then play the UA*G card.