diff --git a/scripts/check-inbox.sh b/scripts/check-inbox.sh
index 67642e7c..b0759002 100755
--- a/scripts/check-inbox.sh
+++ b/scripts/check-inbox.sh
@@ -75,7 +75,8 @@ if [ -n "$SESSION_ID" ]; then
   PIDFILE="$SKILL_DIR/run/watch.$SESSION_ID.pid"
   if [ -f "$PIDFILE" ]; then
     WATCH_PID=$(cat "$PIDFILE" 2>/dev/null || true)
-    if [ -n "$WATCH_PID" ] && kill -0 "$WATCH_PID" 2>/dev/null; then
+    # EPERM-aware liveness (_agmsg_pid_alive): a sandbox-unsignalable watcher is still alive.
+    if [ -n "$WATCH_PID" ] && _agmsg_pid_alive "$WATCH_PID"; then
       exit 0
     fi
   fi
diff --git a/scripts/delivery.sh b/scripts/delivery.sh
index 9ae0e94d..d9f05ff3 100755
--- a/scripts/delivery.sh
+++ b/scripts/delivery.sh
@@ -284,7 +284,9 @@ emit_monitor_directive() {
   if [ -f "$pidfile" ]; then
     local existing
     existing=$(cat "$pidfile" 2>/dev/null || true)
-    if [ -n "$existing" ] && kill -0 "$existing" 2>/dev/null; then
+    # EPERM-aware liveness (_agmsg_pid_alive): a sandbox-unsignalable watcher is
+    # still alive, so we must not re-emit and spawn a duplicate.
+    if [ -n "$existing" ] && _agmsg_pid_alive "$existing"; then
       cat <<EOF
 
 A watch.sh is already streaming into this session (pid $existing). No
diff --git a/scripts/drivers/types/grok-build/_delivery.sh b/scripts/drivers/types/grok-build/_delivery.sh
index 3d9f2106..3203a4ad 100644
--- a/scripts/drivers/types/grok-build/_delivery.sh
+++ b/scripts/drivers/types/grok-build/_delivery.sh
@@ -182,7 +182,8 @@ _agmsg_grok_emit_monitor_directive() {
   if [ -f "$pidfile" ]; then
     local existing
     existing=$(cat "$pidfile" 2>/dev/null || true)
-    if [ -n "$existing" ] && kill -0 "$existing" 2>/dev/null; then
+    # EPERM-aware liveness (_agmsg_pid_alive), mirroring delivery.sh emit dedup.
+    if [ -n "$existing" ] && _agmsg_pid_alive "$existing"; then
       cat <<EOF
 
 A watch.sh is already streaming into this session (pid $existing). No
diff --git a/scripts/lib/instance-id.sh b/scripts/lib/instance-id.sh
index 55f2d37e..206011c6 100644
--- a/scripts/lib/instance-id.sh
+++ b/scripts/lib/instance-id.sh
@@ -45,7 +45,15 @@ _agmsg_pid_alive() {
       return $?
       ;;
   esac
-  kill -0 "$pid" 2>/dev/null
+  # A non-zero `kill -0` is ESRCH (dead) or EPERM (alive but unsignalable under
+  # the sandbox). Only ESRCH is dead. `export LC_ALL=C` (not a bare prefix, which
+  # misses the builtin on bash 3.2) forces English error text for the match.
+  local err
+  err="$(export LC_ALL=C; kill -0 "$pid" 2>&1)" && return 0
+  case "$err" in
+    *[Nn]'o such process'*) return 1 ;;
+    *) return 0 ;;
+  esac
 }
 
 # Compose from an explicit pid. Bare sid when pid is empty/non-numeric.
diff --git a/scripts/lib/resolve-project.sh b/scripts/lib/resolve-project.sh
index b539f709..0d1268a8 100644
--- a/scripts/lib/resolve-project.sh
+++ b/scripts/lib/resolve-project.sh
@@ -335,12 +335,15 @@ agmsg_read_project_marker() {
 agmsg_marker_gc_stale() {
   local dir; dir="$(_agmsg_run_dir)"
   [ -d "$dir" ] || return 0
+  # Skip if _agmsg_pid_alive (EPERM-aware; instance-id.sh) isn't loaded, so a
+  # "command not found" can't fall through to `|| rm -f` and delete a live marker.
+  declare -F _agmsg_pid_alive >/dev/null 2>&1 || return 0
   local f pid
   for f in "$dir"/proj.*.project; do
     [ -f "$f" ] || continue
     pid=${f##*/proj.}; pid=${pid%.project}
     case "$pid" in ''|*[!0-9]*) continue ;; esac
-    kill -0 "$pid" 2>/dev/null || rm -f "$f"
+    _agmsg_pid_alive "$pid" || rm -f "$f"
   done
 }
 
diff --git a/scripts/session-end.sh b/scripts/session-end.sh
index bc863630..e8ba2a26 100755
--- a/scripts/session-end.sh
+++ b/scripts/session-end.sh
@@ -55,7 +55,8 @@ INSTANCE_ID="$(agmsg_instance_id "$SESSION_ID" "$TYPE")"
 PIDFILE="$RUN_DIR/watch.$INSTANCE_ID.pid"
 if [ -f "$PIDFILE" ]; then
   pid=$(cat "$PIDFILE" 2>/dev/null || true)
-  if [ -n "$pid" ] && kill -0 "$pid" 2>/dev/null; then
+  # EPERM-aware liveness (_agmsg_pid_alive): don't skip cleaning a live watcher.
+  if [ -n "$pid" ] && _agmsg_pid_alive "$pid"; then
     # Defensive: only kill if the pid's command line still looks like our
     # watch.sh. Pids can be recycled — a stale pidfile could point at an
     # unrelated process that took the same pid.
diff --git a/scripts/session-start.sh b/scripts/session-start.sh
index 60c5aa81..44ebdb7d 100755
--- a/scripts/session-start.sh
+++ b/scripts/session-start.sh
@@ -145,7 +145,7 @@ for f in "$RUN_DIR"/cc-instance.*; do
   [ -f "$f" ] || continue
   pid=${f##*.}
   case "$pid" in ''|*[!0-9]*) continue ;; esac
-  if kill -0 "$pid" 2>/dev/null; then
+  if _agmsg_pid_alive "$pid"; then
     s=$(cat "$f" 2>/dev/null || true)
     [ -n "$s" ] && live_sids="$live_sids|$s"
   fi
@@ -157,14 +157,14 @@ for f in "$RUN_DIR"/cc-instance.*; do
   [ -f "$f" ] || continue
   pid=${f##*.}
   case "$pid" in ''|*[!0-9]*) continue ;; esac
-  kill -0 "$pid" 2>/dev/null && continue
+  _agmsg_pid_alive "$pid" && continue
   dead_sid=$(cat "$f" 2>/dev/null || true)
   if [ -n "$dead_sid" ] \
       && ! printf '%s\n' "$live_sids" | tr '|' '\n' | grep -Fxq "$dead_sid"; then
     orphan_pidfile="$RUN_DIR/watch.$dead_sid.pid"
     if [ -f "$orphan_pidfile" ]; then
       orphan_pid=$(cat "$orphan_pidfile" 2>/dev/null || true)
-      if [ -n "$orphan_pid" ] && kill -0 "$orphan_pid" 2>/dev/null; then
+      if [ -n "$orphan_pid" ] && _agmsg_pid_alive "$orphan_pid"; then
         # Defensive: only kill if the pid's command line actually matches
         # our watch.sh. Defends against pid recycling — a stale pidfile
         # could point at an unrelated process that took the same pid.
@@ -191,7 +191,7 @@ for f in "$RUN_DIR"/watch.*.pid; do
     rm -f "$f"
     continue
   fi
-  kill -0 "$pid" 2>/dev/null || rm -f "$f"
+  _agmsg_pid_alive "$pid" || rm -f "$f"
 done
 
 # Garbage-collect actas exclusivity locks whose owner session_id no longer
@@ -240,7 +240,7 @@ if [ -n "$CC_PID" ]; then
       prev_pidfile="$RUN_DIR/watch.$prev.pid"
       if [ -f "$prev_pidfile" ]; then
         prev_pid=$(cat "$prev_pidfile" 2>/dev/null || true)
-        if [ -n "$prev_pid" ] && kill -0 "$prev_pid" 2>/dev/null; then
+        if [ -n "$prev_pid" ] && _agmsg_pid_alive "$prev_pid"; then
           kill "$prev_pid" 2>/dev/null || true
         fi
       fi
@@ -258,7 +258,7 @@ fi
 WATCHER_PIDFILE="$RUN_DIR/watch.$INSTANCE_ID.pid"
 if [ -f "$WATCHER_PIDFILE" ]; then
   existing=$(cat "$WATCHER_PIDFILE" 2>/dev/null || true)
-  if [ -n "$existing" ] && kill -0 "$existing" 2>/dev/null; then
+  if [ -n "$existing" ] && _agmsg_pid_alive "$existing"; then
     cat <<EOF
 AGMSG monitor mode: a watch.sh is already streaming for this session (pid $existing).
 No action needed — the existing watcher is the active one.
diff --git a/scripts/watch.sh b/scripts/watch.sh
index 9bc3542b..03011a80 100755
--- a/scripts/watch.sh
+++ b/scripts/watch.sh
@@ -111,18 +111,20 @@ mkdir -p "$RUN_DIR" 2>/dev/null || true
 # against pid recycling — only touch processes whose cmdline still matches
 # our watch.sh. See #66.
 #
-# When ps is unavailable (e.g. Claude Code sandbox), fall back to kill -0
-# which confirms the pid is alive but cannot validate the cmdline.
+# When ps is unavailable (e.g. Claude Code sandbox), fall back to _agmsg_pid_alive
+# which confirms the pid is alive but cannot validate the cmdline. It is EPERM-aware
+# so a live-but-unsignalable sibling watcher isn't misread as dead and left running.
 if [ -f "$PIDFILE" ]; then
   prev_pid=$(cat "$PIDFILE" 2>/dev/null || true)
-  if [ -n "$prev_pid" ] && [ "$prev_pid" != "$$" ] && kill -0 "$prev_pid" 2>/dev/null; then
+  if [ -n "$prev_pid" ] && [ "$prev_pid" != "$$" ] && _agmsg_pid_alive "$prev_pid"; then
     prev_cmd=$(compat_get_cmdline "$prev_pid" 2>/dev/null || true)
     if [ -n "$prev_cmd" ]; then
       case "$prev_cmd" in
         *"$SKILL_DIR/scripts/watch.sh"*) kill "$prev_pid" 2>/dev/null || true ;;
       esac
     else
-      # ps unavailable (sandboxed) — skip cmdline validation, rely on kill -0
+      # ps unavailable (sandboxed) — skip cmdline validation, rely on the
+      # _agmsg_pid_alive check above
       kill "$prev_pid" 2>/dev/null || true
     fi
   fi
diff --git a/tests/test_instance_id.bats b/tests/test_instance_id.bats
index 0c54d538..8b976e92 100644
--- a/tests/test_instance_id.bats
+++ b/tests/test_instance_id.bats
@@ -130,6 +130,52 @@ teardown() { teardown_test_env; }
   ! agmsg_instance_alive ""
 }
 
+# --- _agmsg_pid_alive: EPERM vs ESRCH (Claude Code sandbox) ---
+#
+# Under the sandbox `kill -0` on a live pid returns EPERM, not ESRCH. Only ESRCH
+# is dead; EPERM must read as alive or the watcher self-exits. `kill` is stubbed
+# to script each errno string (real EPERM is hard to force).
+
+@test "pid_alive: a real live pid (self) is alive" {
+  skip_on_windows "POSIX kill path; Windows uses tasklist (#134)"
+  _agmsg_pid_alive $$
+}
+
+@test "pid_alive: a real dead pid is not alive (ESRCH end-to-end)" {
+  skip_on_windows "POSIX kill path; Windows uses tasklist (#134)"
+  ! _agmsg_pid_alive 2147483647
+}
+
+@test "pid_alive: a signalable pid (kill -0 exit 0) is alive" {
+  skip_on_windows "POSIX kill path; Windows uses tasklist (#134)"
+  kill() { return 0; }
+  _agmsg_pid_alive 12345
+}
+
+@test "pid_alive: ESRCH 'No such process' reads as dead" {
+  skip_on_windows "POSIX kill path; Windows uses tasklist (#134)"
+  kill() { echo "bash: kill: (999) - No such process" >&2; return 1; }
+  ! _agmsg_pid_alive 999
+}
+
+@test "pid_alive: lowercase 'no such process' (zsh wording) also reads as dead" {
+  skip_on_windows "POSIX kill path; Windows uses tasklist (#134)"
+  kill() { echo "kill: (999) - no such process" >&2; return 1; }
+  ! _agmsg_pid_alive 999
+}
+
+@test "pid_alive: EPERM 'Operation not permitted' reads as alive (sandbox)" {
+  skip_on_windows "POSIX kill path; Windows uses tasklist (#134)"
+  kill() { echo "bash: kill: (1) - Operation not permitted" >&2; return 1; }
+  _agmsg_pid_alive 1
+}
+
+@test "pid_alive: an unrecognized kill failure defaults to alive (fail-safe)" {
+  skip_on_windows "POSIX kill path; Windows uses tasklist (#134)"
+  kill() { echo "kill: some novel platform error" >&2; return 1; }
+  _agmsg_pid_alive 42
+}
+
 # --- agmsg_normalize_instance_id ---
 
 @test "normalize: a composite token passes through unchanged (idempotent)" {
diff --git a/tests/test_resolve_project.bats b/tests/test_resolve_project.bats
index f8f41c7d..e8fa7045 100644
--- a/tests/test_resolve_project.bats
+++ b/tests/test_resolve_project.bats
@@ -24,6 +24,10 @@ setup() {
 
   # shellcheck disable=SC1090
   source "$SKILL_DIR/scripts/lib/resolve-project.sh"
+  # Real gc_stale callers load _agmsg_pid_alive via actas-lock.sh; mirror that so
+  # the GC exercises the real liveness path, not its missing-helper guard.
+  # shellcheck disable=SC1090
+  source "$SKILL_DIR/scripts/lib/instance-id.sh"
 }
 
 teardown() {
@@ -202,6 +206,40 @@ JSON
   [ -f "$(agmsg_project_marker_path "$$")" ]
 }
 
+# EPERM-aware GC: under the sandbox `kill -0` on a live pid returns EPERM. Reading
+# that as dead would delete a live session's marker; only ESRCH drops it. `kill`
+# is stubbed to script each errno string (real EPERM is hard to force).
+
+@test "marker-gc: keeps a marker whose pid is EPERM-live (sandbox)" {
+  skip_on_windows "POSIX kill path; Windows uses tasklist (#134)"
+  agmsg_write_project_marker 4242 "$ROOT"
+  kill() { echo "bash: kill: (4242) - Operation not permitted" >&2; return 1; }
+  agmsg_marker_gc_stale
+  [ -f "$(agmsg_project_marker_path 4242)" ]
+}
+
+@test "marker-gc: drops a marker whose pid is ESRCH-dead" {
+  skip_on_windows "POSIX kill path; Windows uses tasklist (#134)"
+  agmsg_write_project_marker 4242 "$ROOT"
+  kill() { echo "bash: kill: (4242) - No such process" >&2; return 1; }
+  agmsg_marker_gc_stale
+  [ ! -f "$(agmsg_project_marker_path 4242)" ]
+}
+
+@test "marker-gc: skips (keeps marker) when _agmsg_pid_alive is unavailable" {
+  # Guard: without the helper, GC must skip rather than `|| rm -f` a live marker.
+  # Isolated shell sources ONLY resolve-project.sh, so the helper is truly absent.
+  agmsg_write_project_marker 4242 "$ROOT"
+  run bash -c '
+    export SKILL_DIR="'"$SKILL_DIR"'"
+    source "$SKILL_DIR/scripts/lib/resolve-project.sh"
+    declare -F _agmsg_pid_alive >/dev/null && { echo "helper unexpectedly present"; exit 2; }
+    agmsg_marker_gc_stale
+  '
+  [ "$status" -eq 0 ]
+  [ -f "$(agmsg_project_marker_path 4242)" ]
+}
+
 # --- pid-recycling guard ---
 
 @test "pid-is-agent: a live non-agent process is not trusted" {