diff --git a/mksh/mkshrc b/mksh/mkshrc
index 87859a7..63df11f 100644
--- a/mksh/mkshrc
+++ b/mksh/mkshrc
@@ -215,3 +215,11 @@ PS4='[$EPOCHREALTIME] '; PS1='#'; (( USER_ID )) && PS1='→'; PS1=$'\001\r''${|
 
 	\\builtin return $e
 } '"${hostcol}${bld}$PS1 ${rst}"
+
+# tmux auto-attach: one shared session per host. No -exec, so if tmux is
+# missing or exits/crashes for any reason, this shell just continues normally.
+tmuxbin=$(\\builtin whence -p tmux 2>/dev/null)
+if [[ $- = *i* ]] && [[ -t 0 ]] && [[ -z $TMUX ]] && [[ -n $tmuxbin ]]; then
+  "$tmuxbin" new-session -A -s "${HOSTNAME:-main}"
+fi
+unset tmuxbin