Isn’t that just a consequence of Go’s backwards compatibility guarantee?

Adding new arguments to the Mux interface would break existing code, given the method signature can’t be changed, these magic strings seem like a reasonable compromise. It not like HTTP verbs are going to change anytime soon, and it’s trivial to validate them during register, or via static analysis. So I’m not sure what value the use of constants would bring, especially if it either broke backwards compatibility, or forced the creation of a new, but slightly different mux API that would have to live in parallel with the old API forever.