The go command will use the right version but only if the person writing the library has bothered to tag the version to use. Many people don't. Part of the point of Go 1 is to stabilize the language. We're committing to slowing down the rate of churn and to not changing the APIs in backwards incompatible ways for the lifetime of Go 1 (1.1, 1.2, and so on), so now there should be much less problem with just using tip/HEAD/whatever in the repositories without tags.