diff --git a/bin/fm-wake-lib.sh b/bin/fm-wake-lib.sh
index c0e27b1e21c..eb1859d68d4 100755
--- a/bin/fm-wake-lib.sh
+++ b/bin/fm-wake-lib.sh
@@ -24,10 +24,30 @@ fm_pid_alive() {
 }
 
 fm_pid_identity() {
-  local pid=$1 out
+  local pid=$1 out proc_root stat_line starttime cmdline_hex
+  local -a stat_fields
   case "$pid" in
     ''|*[!0-9]*) return 1 ;;
   esac
+  proc_root=${FM_PROC_ROOT_OVERRIDE:-/proc}
+  # Prefer /proc on Linux: stat field 22 (starttime, clock ticks since boot) is
+  # immune to the wall-clock steps that re-render the ps lstart fallback's date
+  # (observed as WSL2 btime drift) and would evict a live watcher; combining the
+  # full NUL-separated cmdline keeps PID reuse a mismatch even on a tick collision.
+  if [ "$(uname)" = Linux ] && [ -r "$proc_root/$pid/stat" ] && [ -r "$proc_root/$pid/cmdline" ]; then
+    stat_line=$(cat "$proc_root/$pid/stat" 2>/dev/null) || return 1
+    # After the final comm delimiter, array index 19 is proc stat field 22.
+    read -r -a stat_fields <<< "${stat_line##*)}"
+    [ "${#stat_fields[@]}" -ge 20 ] || return 1
+    starttime=${stat_fields[19]}
+    case "$starttime" in
+      ''|*[!0-9]*) return 1 ;;
+    esac
+    cmdline_hex=$(od -An -v -tx1 "$proc_root/$pid/cmdline" 2>/dev/null | tr -d '[:space:]') || return 1
+    [ -n "$cmdline_hex" ] || return 1
+    printf 'linux-starttime=%s cmdline-hex=%s\n' "$starttime" "$cmdline_hex"
+    return 0
+  fi
   # Pin LC_ALL=C so lstart's date format is locale-invariant: the identity is
   # written under one locale but re-read under the machine's ambient locale, which
   # would otherwise mismatch on a non-C locale (e.g. ko_KR) and reject a live watcher.
diff --git a/docs/configuration.md b/docs/configuration.md
index bbe5ca4f355..a23ebb03b93 100644
--- a/docs/configuration.md
+++ b/docs/configuration.md
@@ -348,6 +348,7 @@ FM_STATE_OVERRIDE=       # alternate state dir, mainly for tests
 FM_DATA_OVERRIDE=        # alternate data dir, mainly for tests
 FM_PROJECTS_OVERRIDE=    # alternate projects dir, mainly for tests
 FM_CONFIG_OVERRIDE=      # alternate config dir, mainly for tests
+FM_PROC_ROOT_OVERRIDE=   # alternate /proc root for the Linux process-identity read in fm-wake-lib.sh, mainly for tests
 FM_BACKEND=             # optional runtime backend override for new spawns; tmux/herdr/zellij/orca/cmux support ship/scout spawns, codex-app is not accepted
 HERDR_SESSION=default  # herdr-only: named session for normal backend ops; not enough for destructive cleanup (docs/herdr-backend.md)
 FM_BACKEND_HERDR_COMPOSER_LINES=20  # herdr-only: tail lines scanned by composer-state guard/fallback paths; idle-baseline submit confirmation uses agent-state
diff --git a/tests/fm-pr-check-security.test.sh b/tests/fm-pr-check-security.test.sh
index 9d4154e505c..705f765dd9a 100755
--- a/tests/fm-pr-check-security.test.sh
+++ b/tests/fm-pr-check-security.test.sh
@@ -147,7 +147,7 @@ write_watcher_lock() {
   local state=$1 home=$2 pid=$3 identity
   rm -rf "$state/.watch.lock"
   mkdir "$state/.watch.lock"
-  identity=$(LC_ALL=C ps -p "$pid" -o lstart= -o command= 2>/dev/null | sed 's/^[[:space:]]*//')
+  identity=$(FM_STATE_OVERRIDE="$state" bash -c '. "$1"; fm_pid_identity "$2"' _ "$ROOT/bin/fm-wake-lib.sh" "$pid")
   [ -n "$identity" ] || fail "could not capture fake older-watcher identity"
   printf '%s\n' "$pid" > "$state/.watch.lock/pid"
   printf '%s\n' "$home" > "$state/.watch.lock/fm-home"
diff --git a/tests/fm-watcher-lock.test.sh b/tests/fm-watcher-lock.test.sh
index 8a077f84335..4de8e4b0ff6 100755
--- a/tests/fm-watcher-lock.test.sh
+++ b/tests/fm-watcher-lock.test.sh
@@ -887,20 +887,20 @@ test_stopped_watcher_is_live_but_stale_then_exit_is_classified() {
 }
 
 test_pid_identity_is_locale_invariant() {
-  # The watcher records its process identity under one locale; arm/guard/turn-end
-  # re-read it under the machine's ambient locale. ps's lstart date format follows
-  # LC_TIME, so an unpinned read on a non-C locale (e.g. ko_KR) would differ only
-  # in the date portion and reject a genuinely live watcher. The fix pins LC_ALL=C
-  # inside fm_pid_identity, so its output must be byte-identical regardless of the
-  # caller's exported LC_ALL/LC_TIME. That invariant holds on any host because the
-  # pin is internal, so this stays deterministic on CI even where an alternate
+  # The portable fallback records its process identity under one locale, then
+  # arm/guard/turn-end re-read it under the machine's ambient locale. ps's lstart
+  # date format follows LC_TIME, so an unpinned read on a non-C locale (e.g. ko_KR)
+  # would reject a genuinely live watcher. The fallback pins LC_ALL=C inside
+  # fm_pid_identity, so its output must be byte-identical regardless of the caller's
+  # exported LC_ALL/LC_TIME. This stays deterministic on CI even where an alternate
   # locale like ko_KR.UTF-8 is not installed (the equality then holds trivially).
-  local live baseline via_lc_all via_lc_time
+  local live no_proc baseline via_lc_all via_lc_time
   sleep 300 &
   live=$!
-  baseline=$(LC_ALL=C bash -c '. "$1"; fm_pid_identity "$2"' _ "$LIB" "$live" 2>/dev/null)
-  via_lc_all=$(LC_ALL=ko_KR.UTF-8 bash -c '. "$1"; fm_pid_identity "$2"' _ "$LIB" "$live" 2>/dev/null)
-  via_lc_time=$(LC_TIME=ko_KR.UTF-8 bash -c 'unset LC_ALL; . "$1"; fm_pid_identity "$2"' _ "$LIB" "$live" 2>/dev/null)
+  no_proc="$TMP_ROOT/no-proc"
+  baseline=$(FM_PROC_ROOT_OVERRIDE="$no_proc" LC_ALL=C bash -c '. "$1"; fm_pid_identity "$2"' _ "$LIB" "$live" 2>/dev/null)
+  via_lc_all=$(FM_PROC_ROOT_OVERRIDE="$no_proc" LC_ALL=ko_KR.UTF-8 bash -c '. "$1"; fm_pid_identity "$2"' _ "$LIB" "$live" 2>/dev/null)
+  via_lc_time=$(FM_PROC_ROOT_OVERRIDE="$no_proc" LC_TIME=ko_KR.UTF-8 bash -c 'unset LC_ALL; . "$1"; fm_pid_identity "$2"' _ "$LIB" "$live" 2>/dev/null)
   kill "$live" 2>/dev/null || true
   wait "$live" 2>/dev/null || true
   [ -n "$baseline" ] || fail "fm_pid_identity produced no baseline identity under LC_ALL=C"
@@ -909,8 +909,49 @@ test_pid_identity_is_locale_invariant() {
   pass "fm_pid_identity is locale-invariant across LC_ALL/LC_TIME"
 }
 
+write_fake_proc_identity() {
+  local proc_root=$1 pid=$2 starttime=$3
+  mkdir -p "$proc_root/$pid"
+  printf '%s\n' "$pid (watcher ) with spaces) S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 $starttime 20 21 22" > "$proc_root/$pid/stat"
+  printf 'bash\0/path with spaces/fm-watch.sh\0--flag\0' > "$proc_root/$pid/cmdline"
+}
+
+test_linux_pid_identity_ignores_wall_clock_and_detects_pid_reuse() {
+  local dir state proc_root pid before after_time_jump after_pid_reuse
+  [ "$(uname)" = Linux ] || {
+    pass "Linux process identity clock-step regression skipped on non-Linux host"
+    return
+  }
+  dir=$(make_case linux-pid-identity)
+  state="$dir/state"
+  proc_root="$dir/proc"
+  pid=4242
+  mkdir -p "$proc_root"
+  printf 'btime 1784094040\n' > "$proc_root/stat"
+  write_fake_proc_identity "$proc_root" "$pid" 987654
+
+  before=$(FM_PROC_ROOT_OVERRIDE="$proc_root" FM_STATE_OVERRIDE="$state" bash -c '. "$1"; fm_pid_identity "$2"' _ "$LIB" "$pid") \
+    || fail "could not read initial fake Linux process identity"
+  printf 'btime 1784094016\n' > "$proc_root/stat"
+  after_time_jump=$(FM_PROC_ROOT_OVERRIDE="$proc_root" FM_STATE_OVERRIDE="$state" bash -c '. "$1"; fm_pid_identity "$2"' _ "$LIB" "$pid") \
+    || fail "could not re-read fake Linux process identity after btime change"
+
+  [ "$after_time_jump" = "$before" ] \
+    || fail "Linux process identity changed with btime (before '$before', after '$after_time_jump')"
+  [ "$before" = 'linux-starttime=987654 cmdline-hex=62617368002f706174682077697468207370616365732f666d2d77617463682e7368002d2d666c616700' ] \
+    || fail "Linux process identity did not combine parsed starttime field 22 with the full cmdline ('$before')"
+  pass "Linux process identity ignores simulated btime changes"
+
+  write_fake_proc_identity "$proc_root" "$pid" 987655
+  after_pid_reuse=$(FM_PROC_ROOT_OVERRIDE="$proc_root" FM_STATE_OVERRIDE="$state" bash -c '. "$1"; fm_pid_identity "$2"' _ "$LIB" "$pid") \
+    || fail "could not read reused fake Linux pid identity"
+  [ "$after_pid_reuse" != "$before" ] || fail "Linux process identity missed changed starttime for reused pid"
+  pass "Linux process identity detects pid reuse"
+}
+
 test_singleton_start
 test_pid_identity_is_locale_invariant
+test_linux_pid_identity_ignores_wall_clock_and_detects_pid_reuse
 test_stale_watch_lock_reclaimed
 test_live_stale_watch_lock_is_actionable
 test_guard_warnings