The tests are there to give us confidence about _our_ code. They can help in those situations, but they don't protect us from others pulling the rug from under us.

Unless you're upgrading production before lower environments, you'll still find out exactly what you need to fix, won't you? And unless you're waiting until the last possible moment to upgrade core infrastructure I don't see how anyone is getting any rug pulled out from anywhere.

Again, seeing what will be broken is the baseline expectation. My comment is not about that. It’s about breaking changes, having to resolve them, facing incompatible versions (language, libs, tooling).

You telling me you broke my code in advance doesn’t make it OK.