Yeah, that's the only thing I don't like about this proposal. Just seems so fragile.

The go vet command checks format strings. I expect that the command will be updated to check for correct use of %w.

What's the point of making it a format string if you are hardcoding where the % identifier goes anyway?!

The %w (or %v) identifier must be at the end for the magic to work, but you can still meaningfully format things in front of the wrapped error, e.g. fmt.Errorf("Write(%s): %w", filename, err)

Why does it need to be at the end?

forward compatibility?