[Certhas]

It seems like in this case you want to version API and program separately. API changes aren't the only thing people care about or that should be communicated in a version number. So semver would not just be "not perfect" but be actively misleading.

[cwilkes]

Then have a semver for the two different parts.

Ie “version 3.3.1 of Awesome Server supporting the 2.x api as well as 3.1.x”