diff --git a/bin/fm-harness.sh b/bin/fm-harness.sh
index 3267e922..25084c38 100755
--- a/bin/fm-harness.sh
+++ b/bin/fm-harness.sh
@@ -23,6 +23,8 @@
 set -u
 
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+# shellcheck source=bin/fm-ps-portable.sh
+. "$SCRIPT_DIR/fm-ps-portable.sh"
 FM_ROOT="${FM_ROOT_OVERRIDE:-$(cd "$SCRIPT_DIR/.." && pwd)}"
 FM_HOME="${FM_HOME:-${FM_ROOT_OVERRIDE:-$FM_ROOT}}"
 CONFIG="${FM_CONFIG_OVERRIDE:-$FM_HOME/config}"
@@ -38,7 +40,7 @@ detect_own() {
   # Layer 2: walk the parent chain and match the command name.
   local pid=$$ comm args
   for _ in 1 2 3 4 5 6 7 8; do
-    comm=$(ps -o comm= -p "$pid" 2>/dev/null) || break
+    comm=$(fm_ps_field comm "$pid") || break
     case "$(basename "$comm")" in
       *claude*) echo claude; return ;;
       *codex*) echo codex; return ;;
@@ -47,7 +49,7 @@ detect_own() {
       pi) echo pi; return ;;
       node*|python*)
         # Bare interpreter: match the harness name in its script path.
-        args=$(ps -o args= -p "$pid" 2>/dev/null)
+        args=$(fm_ps_field args "$pid")
         case "$args" in
           *claude*) echo claude; return ;;
           *codex*) echo codex; return ;;
@@ -56,7 +58,7 @@ detect_own() {
           *" pi "*|*/pi) echo pi; return ;;
         esac ;;
     esac
-    pid=$(ps -o ppid= -p "$pid" 2>/dev/null | tr -d ' ')
+    pid=$(fm_ps_field ppid "$pid")
     if [ -z "$pid" ] || [ "$pid" -le 1 ]; then
       break
     fi
diff --git a/bin/fm-lock.sh b/bin/fm-lock.sh
index 33e4b0d2..569d006c 100755
--- a/bin/fm-lock.sh
+++ b/bin/fm-lock.sh
@@ -8,6 +8,8 @@
 set -u
 
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+# shellcheck source=bin/fm-ps-portable.sh
+. "$SCRIPT_DIR/fm-ps-portable.sh"
 FM_ROOT="${FM_ROOT_OVERRIDE:-$(cd "$SCRIPT_DIR/.." && pwd)}"
 FM_HOME="${FM_HOME:-${FM_ROOT_OVERRIDE:-$FM_ROOT}}"
 STATE="${FM_STATE_OVERRIDE:-$FM_HOME/state}"
@@ -20,8 +22,8 @@ HARNESS_RE='claude|codex|opencode|grok|^pi$'
 harness_pid() {
   local pid=$$ comm args
   for _ in 1 2 3 4 5 6 7 8; do
-    comm=$(ps -o comm= -p "$pid" 2>/dev/null) || return 1
-    args=$(ps -o args= -p "$pid" 2>/dev/null)
+    comm=$(fm_ps_field comm "$pid") || return 1
+    args=$(fm_ps_field args "$pid")
     if printf '%s' "$(basename "$comm")" | grep -qE "$HARNESS_RE"; then
       echo "$pid"; return 0
     fi
@@ -29,7 +31,7 @@ harness_pid() {
     case "$comm" in
       *node*|*python*) printf '%s' "$args" | grep -qE "$HARNESS_RE" && { echo "$pid"; return 0; } ;;
     esac
-    pid=$(ps -o ppid= -p "$pid" 2>/dev/null | tr -d ' ')
+    pid=$(fm_ps_field ppid "$pid")
     [ -n "$pid" ] && [ "$pid" -gt 1 ] || return 1
   done
   return 1
@@ -38,8 +40,8 @@ harness_pid() {
 holder_alive() {  # true if $1 is a live process that looks like a harness
   local pid=$1 comm
   kill -0 "$pid" 2>/dev/null || return 1
-  comm=$(ps -o comm= -p "$pid" 2>/dev/null) || return 1
-  printf '%s' "$(basename "$comm") $(ps -o args= -p "$pid" 2>/dev/null)" | grep -qE "$HARNESS_RE"
+  comm=$(fm_ps_field comm "$pid") || return 1
+  printf '%s' "$(basename "$comm") $(fm_ps_field args "$pid")" | grep -qE "$HARNESS_RE"
 }
 
 if [ "${1:-}" = "status" ]; then
diff --git a/bin/fm-ps-portable.sh b/bin/fm-ps-portable.sh
new file mode 100644
index 00000000..21045d6e
--- /dev/null
+++ b/bin/fm-ps-portable.sh
@@ -0,0 +1,59 @@
+#!/usr/bin/env bash
+# fm-ps-portable.sh - portable process introspection for firstmate.
+#
+# Emulates the `ps -o <field>= -p <pid>` queries firstmate uses to walk the
+# harness process ancestry (fm-lock.sh, fm-harness.sh) and to fingerprint a
+# pid for reuse-detection (fm-wake-lib.sh). MSYS2 / Git Bash on Windows ships
+# a `ps` with no `-o` flag - it errors "ps: unknown option -- o" - so those
+# walks break there as written.
+#
+# On macOS/Linux the native `ps -o` path is used unchanged (exact, fast). Only
+# where `ps -o` is unavailable do we fall back to parsing `ps -f`, which MSYS
+# supports and which prints the full command line WITH arguments:
+#     UID  PID  PPID  TTY  STIME  COMMAND...
+# This mirrors the existing platform-branch convention in fm-wake-lib.sh
+# (`stat -f` on Darwin vs `stat -c` elsewhere).
+#
+# Sourced, never executed. Sets no global shell options.
+
+# Capability probe, run once and cached (idempotent across multiple `source`s).
+if [ -z "${FM_PS_NATIVE:-}" ]; then
+  if ps -o comm= -p "$$" >/dev/null 2>&1; then
+    FM_PS_NATIVE=1
+  else
+    FM_PS_NATIVE=0
+  fi
+fi
+
+# fm_ps_field <comm|args|ppid|identity> <pid>
+#   comm     - basename of argv[0] (handles / and \ so Windows node.exe resolves)
+#   args     - full command line
+#   ppid     - parent pid (whitespace stripped)
+#   identity - a stable "start-time command" fingerprint for pid-reuse detection
+# Prints the field to stdout; returns non-zero when the pid is not found.
+fm_ps_field() {
+  local field=$1 pid=$2
+  if [ "${FM_PS_NATIVE:-1}" = 1 ]; then
+    case "$field" in
+      comm)     ps -o comm= -p "$pid" 2>/dev/null ;;
+      args)     ps -o args= -p "$pid" 2>/dev/null ;;
+      ppid)     ps -o ppid= -p "$pid" 2>/dev/null | tr -d ' ' ;;
+      identity) ps -p "$pid" -o lstart= -o command= 2>/dev/null | sed 's/^[[:space:]]*//' ;;
+    esac
+    return
+  fi
+  # MSYS fallback: parse one `ps -f` row. Columns: UID PID PPID TTY STIME COMMAND...
+  local row ppid stime cmd first
+  row=$(ps -f 2>/dev/null | awk -v want="$pid" 'NR>1 && $2==want {
+    p=$3; s=$5; c=""; for (i=6; i<=NF; i++) c = c (i>6 ? " " : "") $i;
+    print p "\t" s "\t" c; exit }')
+  [ -n "$row" ] || return 1
+  ppid=${row%%$'\t'*}; row=${row#*$'\t'}
+  stime=${row%%$'\t'*}; cmd=${row#*$'\t'}
+  case "$field" in
+    ppid)     printf '%s' "$ppid" ;;
+    args)     printf '%s' "$cmd" ;;
+    comm)     first=${cmd%%[[:space:]]*}; printf '%s' "${first##*[\\/]}" ;;
+    identity) printf '%s %s' "$stime" "$cmd" ;;
+  esac
+}
diff --git a/bin/fm-wake-lib.sh b/bin/fm-wake-lib.sh
index af2112a6..1e9bfcf2 100755
--- a/bin/fm-wake-lib.sh
+++ b/bin/fm-wake-lib.sh
@@ -2,6 +2,8 @@
 # Shared durable wake queue and portable lock helpers.
 
 FM_WAKE_LIB_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+# shellcheck source=bin/fm-ps-portable.sh
+. "$FM_WAKE_LIB_DIR/fm-ps-portable.sh"
 FM_WAKE_DEFAULT_ROOT="$(cd "$FM_WAKE_LIB_DIR/.." && pwd)"
 FM_ROOT="${FM_ROOT_OVERRIDE:-${FM_ROOT:-$FM_WAKE_DEFAULT_ROOT}}"
 FM_HOME="${FM_HOME:-${FM_ROOT_OVERRIDE:-$FM_ROOT}}"
@@ -28,7 +30,7 @@ fm_pid_identity() {
   case "$pid" in
     ''|*[!0-9]*) return 1 ;;
   esac
-  out=$(ps -p "$pid" -o lstart= -o command= 2>/dev/null) || return 1
+  out=$(fm_ps_field identity "$pid") || return 1
   [ -n "$out" ] || return 1
   printf '%s\n' "$out" | sed 's/^[[:space:]]*//'
 }
diff --git a/tests/fm-ps-portable.test.sh b/tests/fm-ps-portable.test.sh
new file mode 100644
index 00000000..4066ded6
--- /dev/null
+++ b/tests/fm-ps-portable.test.sh
@@ -0,0 +1,82 @@
+#!/usr/bin/env bash
+# Behavior tests for bin/fm-ps-portable.sh - the portable process-introspection
+# shim that keeps the harness-ancestry walk (fm-lock.sh, fm-harness.sh) and the
+# pid fingerprint (fm-wake-lib.sh) working on platforms whose `ps` lacks the
+# `-o` flag (MSYS2 / Git Bash on Windows), with byte-identical behavior on
+# macOS/Linux via the native `ps -o` path.
+#
+# The assertions are platform-neutral: each must hold on BOTH the native
+# `ps -o` path and the MSYS `ps -f` fallback. Nothing here mocks ps; the shim
+# is exercised against this live test process ($$).
+set -u
+
+# shellcheck source=tests/lib.sh
+. "$(dirname "${BASH_SOURCE[0]}")/lib.sh"
+
+# shellcheck source=bin/fm-ps-portable.sh
+. "$ROOT/bin/fm-ps-portable.sh"
+
+test_native_flag_is_boolean() {
+  case "${FM_PS_NATIVE:-unset}" in
+    0|1) pass "FM_PS_NATIVE resolved to a boolean ($FM_PS_NATIVE)" ;;
+    *) fail "FM_PS_NATIVE should be 0 or 1, got '${FM_PS_NATIVE:-unset}'" ;;
+  esac
+}
+
+test_comm_of_self_is_bash() {
+  local comm
+  comm=$(fm_ps_field comm "$$") || fail "fm_ps_field comm returned non-zero for self"
+  # The test process is bash on every platform; comm must name it.
+  assert_contains "$comm" "bash" "comm of the test process should mention bash"
+  pass "fm_ps_field comm resolves the current process ($comm)"
+}
+
+test_ppid_of_self_is_numeric() {
+  local ppid
+  ppid=$(fm_ps_field ppid "$$")
+  case "$ppid" in
+    ''|*[!0-9]*) fail "fm_ps_field ppid should be numeric, got '$ppid'" ;;
+    *) pass "fm_ps_field ppid is numeric ($ppid)" ;;
+  esac
+}
+
+test_args_of_self_mentions_bash() {
+  local args
+  args=$(fm_ps_field args "$$")
+  assert_contains "$args" "bash" "args of the test process should mention bash"
+  pass "fm_ps_field args returns the command line"
+}
+
+test_identity_of_self_nonempty() {
+  local id
+  id=$(fm_ps_field identity "$$") || fail "fm_ps_field identity returned non-zero for self"
+  [ -n "$id" ] || fail "fm_ps_field identity should be non-empty"
+  pass "fm_ps_field identity returns a fingerprint"
+}
+
+test_bogus_pid_yields_empty() {
+  # A nonexistent pid must yield empty output on both paths (native prints
+  # nothing; the fallback returns non-zero). Assert on output, not exit code,
+  # so the check is platform-neutral.
+  local out
+  out=$(fm_ps_field ppid 999999 2>/dev/null || true)
+  [ -z "$out" ] || fail "fm_ps_field ppid for a bogus pid should be empty, got '$out'"
+  pass "fm_ps_field yields empty for a nonexistent pid"
+}
+
+test_source_is_idempotent() {
+  local before=$FM_PS_NATIVE
+  # shellcheck source=bin/fm-ps-portable.sh
+  . "$ROOT/bin/fm-ps-portable.sh"
+  expect_code 0 "$?" "re-sourcing the shim"
+  [ "$FM_PS_NATIVE" = "$before" ] || fail "FM_PS_NATIVE changed on re-source ($before -> $FM_PS_NATIVE)"
+  pass "sourcing the shim twice is idempotent"
+}
+
+test_native_flag_is_boolean
+test_comm_of_self_is_bash
+test_ppid_of_self_is_numeric
+test_args_of_self_mentions_bash
+test_identity_of_self_nonempty
+test_bogus_pid_yields_empty
+test_source_is_idempotent