Go errors cannot be string-typed, since they need to implement the error interface. The reason testing error types sometimes won't work is that the error types themselves may be private to the package where they are defined or that the error is just a generic error created by errors.New().

In this case the Error has an easy-to-check public type (*MaxBytesError) and the documentation clearly indicates that. But that has not always been the case. The original sin is that the API returned a generic error and the only way to test that error was to use a string comparison.

This is an important context to have when you need to make balanced decisions about Hyrum's law. As some commentators already mentioned, you should be wary of taking the extreme version of the law, which suggest that every single observable behavior of the API becomes part of the API itself and needs to be preserved. If you follow this extreme version, every error or exception message in every language must be left be left unchanged forever. But most client code doesn't just go around happily comparing exception messages to strings if there is another method to detect the exception.