SemVer is just a pinky-swear not to break people's code. In the real world, people's code breaks anyway, and then you get an argument about what's API, and expected behavior, and so on, and so forth.

What I'm proposing is simply to replace the pinky promise with tests. From some of the other comments, I think this point may have been missed: it isn't every test in your test suite, it's the ones marked "API", only.

This is a strict improvement over social-contract SemVer in two ways: one is that the package manager won't let the maintainers break the API tests without a major version bump. The other is that, if you, as a user, are unsure if some behavior is part of the stable API, you can write and submit a test to that package. If that test is accepted, great: that behavior now cannot change without a major version bump, because, again, the package manager will not bundle the package if that test breaks. Furthermore, even on a major version bump, it is instantly clear if that test is still valid, or not, you can just check before upgrading. If they don't accept the PR, you know that it isn't considered part of the API, so you add the test to your own test suite, so that at least you know quickly what broke if they change it.