diff --git a/bin/fm-backlog-audit.sh b/bin/fm-backlog-audit.sh
new file mode 100755
index 00000000000..50ce41a5b1c
--- /dev/null
+++ b/bin/fm-backlog-audit.sh
@@ -0,0 +1,128 @@
+#!/usr/bin/env bash
+# Read-only consistency audit for firstmate backlog/state drift.
+#
+# Checks data/backlog.md against state/*.meta for common supervision drift:
+# duplicate In flight/Done entries, orphan meta files, In flight items without
+# meta, PR-ready/merged work still parked In flight, and Watchlist items that
+# already have local adoption signals.
+# Usage: fm-backlog-audit.sh
+set -eu
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+FM_ROOT="${FM_ROOT_OVERRIDE:-$(cd "$SCRIPT_DIR/.." && pwd)}"
+FM_HOME="${FM_HOME:-${FM_ROOT_OVERRIDE:-$FM_ROOT}}"
+DATA="${FM_DATA_OVERRIDE:-$FM_HOME/data}"
+STATE="${FM_STATE_OVERRIDE:-$FM_HOME/state}"
+BACKLOG="$DATA/backlog.md"
+
+declare -A IN_FLIGHT_LINES=()
+declare -A DONE_LINES=()
+declare -A WATCHLIST_LINES=()
+FINDINGS=()
+
+add_finding() {
+  FINDINGS+=("$1")
+}
+
+parse_backlog() {
+  awk '
+    function task_id(line, rest, id) {
+      rest = line
+      sub(/^[[:space:]]*-[[:space:]]+/, "", rest)
+      sub(/^\[[ xX]\][[:space:]]+/, "", rest)
+      if (rest ~ /^\*\*/) {
+        sub(/^\*\*/, "", rest)
+        id = rest
+        sub(/\*\*.*/, "", id)
+        return id
+      }
+      id = rest
+      sub(/[[:space:]].*/, "", id)
+      return id
+    }
+    /^##[[:space:]]+/ {
+      section = $0
+      sub(/^##[[:space:]]+/, "", section)
+      next
+    }
+    /^[[:space:]]*-[[:space:]]+/ {
+      id = task_id($0)
+      if (id != "") {
+        print section "\t" id "\t" $0
+      }
+    }
+  ' "$BACKLOG"
+}
+
+looks_pr_ready_or_merged() {
+  local line=$1
+  case "$line" in
+    *"PR ready"*|*"checks green"*|*"merged"*|*"MERGED"*)
+      return 0
+      ;;
+  esac
+  return 1
+}
+
+if [ ! -f "$BACKLOG" ]; then
+  echo "backlog-audit: missing backlog at $BACKLOG" >&2
+  exit 1
+fi
+
+while IFS=$'\t' read -r section id line; do
+  case "$section" in
+    "In flight")
+      IN_FLIGHT_LINES["$id"]=$line
+      ;;
+    "Done")
+      DONE_LINES["$id"]=$line
+      ;;
+    "Watchlist")
+      WATCHLIST_LINES["$id"]=$line
+      ;;
+  esac
+done < <(parse_backlog)
+
+for id in "${!IN_FLIGHT_LINES[@]}"; do
+  if [ -n "${DONE_LINES[$id]+set}" ]; then
+    add_finding "duplicate-done: $id listed in both In flight and Done"
+  fi
+  if [ ! -f "$STATE/$id.meta" ]; then
+    add_finding "inflight-without-meta: $id is In flight but has no state meta"
+  fi
+  if looks_pr_ready_or_merged "${IN_FLIGHT_LINES[$id]}"; then
+    add_finding "inflight-pr-ready: $id is still In flight but looks PR-ready or merged"
+  fi
+done
+
+if [ -d "$STATE" ]; then
+  shopt -s nullglob
+  for meta in "$STATE"/*.meta; do
+    id=$(basename "$meta" .meta)
+    if [ -z "${IN_FLIGHT_LINES[$id]+set}" ]; then
+      add_finding "meta-without-inflight: $id has state meta but is not in In flight"
+    fi
+  done
+  shopt -u nullglob
+fi
+
+for id in "${!WATCHLIST_LINES[@]}"; do
+  if [ -n "${IN_FLIGHT_LINES[$id]+set}" ]; then
+    add_finding "watchlist-adopted: $id is still on Watchlist but already listed In flight"
+  elif [ -f "$STATE/$id.meta" ]; then
+    add_finding "watchlist-adopted: $id is still on Watchlist but already has local state meta"
+  fi
+done
+
+if [ "${#FINDINGS[@]}" -eq 0 ]; then
+  echo "No backlog/state drift found."
+  echo "No changes made."
+  exit 0
+fi
+
+echo "Backlog/state drift found:"
+for finding in "${FINDINGS[@]}"; do
+  printf -- '- %s\n' "$finding"
+done
+echo "No changes made."
+exit 1
diff --git a/docs/scripts.md b/docs/scripts.md
index 44c8f4aea5e..107ce1dbe3b 100644
--- a/docs/scripts.md
+++ b/docs/scripts.md
@@ -9,6 +9,7 @@ Each file also starts with a short header comment.
 | `fm-fleet-sync.sh`       | Fetch clones, fast-forward safe default-branch states, self-heal clean detached ancestor drift, report unsafe drift as `STUCK:`, and safely prune branches whose remote is gone |
 | `fm-update.sh`           | Self-update the running firstmate repo and registered secondmate homes with fast-forward-only pulls from origin     |
 | `fm-backlog-handoff.sh`  | Move already-judged in-scope queued backlog items from the main home into a seeded secondmate home                 |
+| `fm-backlog-audit.sh`    | Read-only audit for backlog/state drift between `data/backlog.md`, `state/*.meta`, and local adoption signals      |
 | `fm-brief.sh`            | Scaffold a ship brief with a worktree-isolation assertion, a report-only scout brief with `--scout`, or a secondmate charter with `--secondmate` |
 | `fm-ensure-agents-md.sh` | Ensure project `AGENTS.md` is the real memory file and `CLAUDE.md` symlinks to it                                   |
 | `fm-guard.sh`            | Warn when the primary checkout is tangled, when queued wakes are pending, or when watcher liveness is not proved by a fresh beacon plus a live matching lock |
diff --git a/tests/fm-backlog-audit.test.sh b/tests/fm-backlog-audit.test.sh
new file mode 100644
index 00000000000..df0d610266a
--- /dev/null
+++ b/tests/fm-backlog-audit.test.sh
@@ -0,0 +1,118 @@
+#!/usr/bin/env bash
+# Tests for bin/fm-backlog-audit.sh's read-only backlog/state drift checks.
+set -u
+
+# shellcheck source=tests/lib.sh
+. "$(dirname "${BASH_SOURCE[0]}")/lib.sh"
+
+AUDIT="$ROOT/bin/fm-backlog-audit.sh"
+TMP_ROOT=$(fm_test_tmproot fm-backlog-audit-tests)
+
+make_home() {
+  local name=$1 home
+  home="$TMP_ROOT/$name"
+  mkdir -p "$home/data" "$home/state"
+  printf '%s\n' "$home"
+}
+
+write_backlog() {
+  local home=$1
+  cat > "$home/data/backlog.md"
+}
+
+write_meta() {
+  local home=$1 id=$2
+  fm_write_meta "$home/state/$id.meta" \
+    "window=fm-$id" \
+    "worktree=$home/worktrees/$id" \
+    "project=$home/projects/demo" \
+    "kind=ship" \
+    "mode=direct-PR"
+}
+
+run_audit() {
+  local home=$1 out=$2 rc
+  set +e
+  FM_HOME="$home" "$AUDIT" > "$out" 2>&1
+  rc=$?
+  printf '%s\n' "$rc"
+}
+
+test_clean_backlog_passes() {
+  local home out rc
+  home=$(make_home clean)
+  write_backlog "$home" <<'MD'
+## In flight
+- [ ] alpha-task - Active work (repo: demo, since 2026-06-29)
+
+## Queued
+- [ ] beta-task - Later work (repo: demo)
+
+## Done
+- [x] old-task - Shipped work - https://github.com/example/repo/pull/1 (merged 2026-06-28)
+
+## Watchlist
+- [ ] external-task - Follow upstream PR
+MD
+  write_meta "$home" alpha-task
+  out="$home/out.txt"
+  rc=$(run_audit "$home" "$out")
+  expect_code 0 "$rc" "clean backlog audit"
+  assert_contains "$(cat "$out")" "No backlog/state drift found." "clean audit reports no drift"
+  pass "clean backlog/state audit passes"
+}
+
+test_detects_required_drift_cases() {
+  local home out rc
+  home=$(make_home drift)
+  write_backlog "$home" <<'MD'
+## In flight
+- [ ] dup-task - Active and also done (repo: demo, since 2026-06-29)
+- [ ] no-meta-task - Missing meta file (repo: demo, since 2026-06-29)
+- [ ] pr-ready-task - PR ready https://github.com/example/repo/pull/2 (repo: demo, since 2026-06-29)
+- **bold-task** - Bold in-flight item form (repo: demo, since 2026-06-29)
+
+## Done
+- [x] dup-task - Active and also done - https://github.com/example/repo/pull/1 (merged 2026-06-28)
+
+## Watchlist
+- [ ] watched-task - Track until adopted locally
+MD
+  write_meta "$home" dup-task
+  write_meta "$home" meta-only-task
+  write_meta "$home" watched-task
+  write_meta "$home" pr-ready-task
+  write_meta "$home" bold-task
+  out="$home/out.txt"
+  rc=$(run_audit "$home" "$out")
+  expect_code 1 "$rc" "drift backlog audit"
+  assert_contains "$(cat "$out")" "duplicate-done: dup-task listed in both In flight and Done" "duplicate done drift reported"
+  assert_contains "$(cat "$out")" "meta-without-inflight: meta-only-task has state meta but is not in In flight" "meta-only drift reported"
+  assert_contains "$(cat "$out")" "inflight-without-meta: no-meta-task is In flight but has no state meta" "missing meta drift reported"
+  assert_contains "$(cat "$out")" "inflight-pr-ready: pr-ready-task is still In flight but looks PR-ready or merged" "PR-ready drift reported"
+  assert_contains "$(cat "$out")" "watchlist-adopted: watched-task is still on Watchlist but already has local state meta" "watchlist adoption drift reported"
+  assert_not_contains "$(cat "$out")" "inflight-without-meta: bold-task" "bold in-flight form is parsed"
+  pass "required backlog/state drift cases are reported"
+}
+
+test_audit_is_read_only() {
+  local home out before after rc
+  home=$(make_home readonly)
+  write_backlog "$home" <<'MD'
+## In flight
+- [ ] no-meta-task - Missing meta file (repo: demo, since 2026-06-29)
+
+## Done
+MD
+  out="$home/out.txt"
+  before=$(find "$home" -type f -print0 | sort -z | xargs -0 sha256sum)
+  rc=$(run_audit "$home" "$out")
+  expect_code 1 "$rc" "read-only audit with drift"
+  after=$(find "$home" -type f ! -name out.txt -print0 | sort -z | xargs -0 sha256sum)
+  [ "$before" = "$after" ] || fail "audit modified home files"
+  pass "backlog audit is read-only"
+}
+
+test_clean_backlog_passes
+test_detects_required_drift_cases
+test_audit_is_read_only