> They are within their rights to ask you to get rid of it.

Only the logo and name. The functionality side should be perfectly fine, there are oodles of precedent for reversing a workable program from someone's commercial API: https://en.wikipedia.org/wiki/Google_LLC_v._Oracle_America,_....

True, but a cease & desist will likely include the demand that they cease all operations, and they might be willing to fight that in court, possibly seek damages, etc however likely or unlikely it is that it would hold up. They can pay for a few hours of a lawyer's time to intimidate the OP.

But prolly can still go after if you resemble it closely enough in terms of looks? I get the API point though, thanks.