I use Tmux a lot in general development, and find it useful to separate my Tmux sessions on a per-project basis.

The following config saves me having to start Tmux when I switch to a project directory.

In ~/.config/direnv/direnvrc:

session_name(){
  export TMUX_SESSION_NAME="${*:?session_name needs a name as argument}"
}

In ~/.config/fish/config.fish:

function autotmux --on-variable TMUX_SESSION_NAME
        if test -n "$TMUX_SESSION_NAME" #only if set
    if test -z $TMUX #not if in TMUX
      if tmux has-session -t $TMUX_SESSION_NAME
        exec tmux new-session -t "$TMUX_SESSION_NAME"
      else
        exec tmux new-session -s "$TMUX_SESSION_NAME"
      end
    end
  end
end

Used like this (in <project>/.envrc):

session_name Project

From then on, z project (because of the excellent fasd) jumps me to that project directory, and starts up a Tmux session called "Project." If there's already such a session, it merely attaches to that session (noticeable because the client name will be a number), which is my prompt to ^D and find the terminal I'm using for that project - easy, because the terminal title will be the same as the Tmux session name.

See also: Tmux