diff --git a/aliases/_longrun.alias b/aliases/_longrun.alias
index 19eeea8..c53117a 100644
--- a/aliases/_longrun.alias
+++ b/aliases/_longrun.alias
@@ -12,21 +12,78 @@ alias awake='caffeinate -dimsu'
 #   `tmux kill-session -t <session>`.
 #   e.g.  longrun phase3 codex --dangerously-bypass-approvals-and-sandbox "Read ... and execute it"
 longrun() {
+  # --- usage ---
   if [ "$#" -lt 2 ]; then
     echo "usage: longrun <session> <command> [args...]" >&2
     echo "  e.g. longrun phase3 codex --full-auto \"<the overnight prompt>\"" >&2
+    echo "  env: LONGRUN_LOG_DIR overrides the log directory (default: \$HOME)" >&2
     return 2
   fi
-  command -v tmux >/dev/null 2>&1 || { echo "longrun: tmux is not installed" >&2; return 1; }
+
+  # --- required dependency: tmux ---
+  if ! command -v tmux >/dev/null 2>&1; then
+    echo "longrun: 'tmux' is required but not installed." >&2
+    echo "  install: macOS 'brew install tmux'  |  Debian/Ubuntu 'sudo apt-get install -y tmux'" >&2
+    return 1
+  fi
+
   local sess="$1"; shift
-  local ts log keep cmd inner
+
+  # --- session name must be free ---
+  if tmux has-session -t "$sess" 2>/dev/null; then
+    echo "longrun: a tmux session named '$sess' already exists." >&2
+    echo "  attach: ta $sess  |  stop: tmux kill-session -t $sess  |  or choose another name" >&2
+    return 1
+  fi
+
+  # --- the command must be runnable ---
+  if ! command -v "$1" >/dev/null 2>&1; then
+    echo "longrun: command not found on PATH: '$1'" >&2
+    echo "  make sure the agent CLI is installed and your PATH is set for non-login shells." >&2
+    return 127
+  fi
+
+  # --- log file must be writable ---
+  local ts log dir
+  dir="${LONGRUN_LOG_DIR:-$HOME}"
   ts="$(date +%Y%m%d-%H%M%S)"
-  log="$HOME/longrun-${sess}-${ts}.log"
-  keep=""
-  command -v caffeinate >/dev/null 2>&1 && keep="caffeinate -dimsu "
+  log="$dir/longrun-${sess}-${ts}.log"
+  if ! mkdir -p "$dir" 2>/dev/null || ! ( : > "$log" ) 2>/dev/null; then
+    echo "longrun: cannot write log file: $log" >&2
+    echo "  set LONGRUN_LOG_DIR to a writable directory and retry." >&2
+    return 1
+  fi
+
+  # --- optional dependency: caffeinate (keep-awake) ---
+  local keep=""
+  if command -v caffeinate >/dev/null 2>&1; then
+    keep="caffeinate -dimsu "
+  elif [ "$(uname -s 2>/dev/null)" = "Darwin" ]; then
+    echo "longrun: WARNING — 'caffeinate' not found on macOS; the machine may sleep mid-run." >&2
+  else
+    echo "longrun: note — no 'caffeinate' (non-macOS). To prevent sleep, wrap with 'systemd-inhibit'." >&2
+  fi
+
+  # --- launch (detached), capturing any tmux startup error ---
+  local cmd inner errf
   cmd="$(printf '%q ' "$@")"
   inner="${keep}${cmd}2>&1 | tee \"$log\""
-  tmux new-session -d -s "$sess" "$inner"
+  errf="$(mktemp 2>/dev/null || echo "/tmp/longrun-err.$$")"
+  if ! tmux new-session -d -s "$sess" "$inner" 2>"$errf"; then
+    echo "longrun: failed to start tmux session '$sess':" >&2
+    [ -s "$errf" ] && sed 's/^/  /' "$errf" >&2
+    rm -f "$errf" "$log"
+    return 1
+  fi
+  rm -f "$errf"
+
+  # --- confirm it is actually running (quick-exit detection) ---
+  if ! tmux has-session -t "$sess" 2>/dev/null; then
+    echo "longrun: WARNING — session '$sess' is not running; the command may have exited immediately." >&2
+    echo "  check the log: $log" >&2
+    return 1
+  fi
+
   echo "[longrun] session=$sess"
   echo "[longrun] log=$log"
   echo "[longrun] attach: ta $sess   |   tail: tail -f \"$log\"   |   stop: tmux kill-session -t $sess"