This breaks rsync & many other things that rely on SSH as transport. Try:
alias ssh="ssh -o RequestTTY=force -o RemoteCommand='tmux new -A -s foobar'" alias ssh="ssh -o RequestTTY=force -o RemoteCommand='tmux new -A -s foobar'"Though I agree, probably not the best solution to start all sessions with tmux. There's other issues it can cause.