This appears to be a definitive answer:

https://www.ecfr.gov/current/title-14/section-399.87

§ 399.87 Baggage allowances and fees.

For passengers whose ultimate ticketed origin or destination is a U.S. point, U.S. and foreign carriers must apply the baggage allowances and fees that apply at the beginning of a passenger's itinerary throughout his or her entire itinerary. In the case of code-share flights that form part of an itinerary whose ultimate ticketed origin or destination is a U.S. point, U.S. and foreign carriers must apply the baggage allowances and fees of the marketing carrier throughout the itinerary to the extent that they differ from those of any operating carrier.

[Doc. No. DOT-OST-2010-0140, [url=https://www.federalregister.gov/citation/76-FR-23167]76 FR 23167, Apr. 25, 2011]

Here is an airline industry practices website and an attached document:
https://www.iata.org/en/programs/ops...age/standards/

It looks like this pretty much answers the question with bikes/baggage policy on code-share flights. You might want to save this for reference.