Server.shellInvocation

A bare tmux there is whichever one is on the pane's PATH, which is not necessarily this one — and a client whose protocol version differs from the server's is refused with server exited unexpectedly rather than anything that names the cause. Composing a command with this instead removes both the PATH lookup and the guess about which server $TMUX refers to:

Quoted for a POSIX shell, so a path containing spaces survives being typed into one.