diff --git a/.agents/skills/firstmate-coding-guidelines/SKILL.md b/.agents/skills/firstmate-coding-guidelines/SKILL.md
index 83ab1d2a..ef47bca5 100644
--- a/.agents/skills/firstmate-coding-guidelines/SKILL.md
+++ b/.agents/skills/firstmate-coding-guidelines/SKILL.md
@@ -74,6 +74,9 @@ Firstmate adds this skill's load instruction to firstmate-repo briefs by hand in
 - Never add an agent name as a commit co-author.
 - `bin/*.sh` and `bin/backends/*.sh` must pass `shellcheck`.
 - Run `shellcheck bin/*.sh bin/backends/*.sh tests/*.sh` before treating a script change as done.
+- Scripts must parse under macOS `/bin/bash` (bash 3.2), not just modern bash on Linux CI.
+- Never assign a heredoc through command substitution (`VAR=$(cat <<EOF ... EOF)`): bash 3.2 tracks quote state through the heredoc body while scanning for the closing `)`, so a lone apostrophe in the body breaks parsing of the whole script (issue #166), and CI's modern bash cannot catch it via `bash -n`.
+- Assign multi-line bodies with `read -r -d '' VAR <<EOF ... EOF || true` instead; `tests/fm-brief.test.sh` structurally refuses the fragile construct across `bin/`.
 - Colocate tests with the existing pattern in `tests/`, name them `<subject>.test.sh`, and extend an existing script rather than inventing a new runner.
 - A backend-verification doc (`docs/*-backend.md`) records empirical facts, not assumptions.
 - Include the date, version, exact commands run, and exact output.
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 6e066fa8..ab49e6f6 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -88,7 +88,7 @@ tests/fm-grok-harness.test.sh             # grok adapter spawn hook, token guard
 tests/fm-fleet-sync.test.sh               # project clone refresh: safe detached recovery, STUCK drift reports, benign skips, and bootstrap relay
 tests/fm-x-mode.test.sh                   # X-mode poll, inbox context round-trip, reply threading, dismiss, completion follow-up counters/caps, dry-run preview, and .env-presence activation tests
 tests/fm-tangle-guard.test.sh             # primary-checkout tangle detection, read-only remediation suppression, and spawn/brief isolation tests
-tests/fm-brief.test.sh                    # fm-brief.sh bash -n parse regression guard (issue #166) and clean no-mistakes/direct-PR/local-only brief generation tests
+tests/fm-brief.test.sh                    # fm-brief.sh bash -n parse regression guard (issue #166, also run under /bin/bash), a structural guard refusing $(cat <<...) across bin/*.sh and bin/backends/*.sh, and clean no-mistakes/direct-PR/local-only brief generation tests
 tests/fm-spawn-batch.test.sh              # batch dispatch and FM_HOME project-path scoping tests
 tests/fm-spawn-dispatch-profile.test.sh   # concrete dispatch profile flags: active-profile backstop, harness/model/effort meta, launch templates, batch forwarding, and secondmate exemption
 tests/fm-update.test.sh                   # fast-forward-only self-update, reread, nudge, dedup, and skip-safety tests
diff --git a/bin/fm-bootstrap.sh b/bin/fm-bootstrap.sh
index c28814dd..c9b57ef8 100755
--- a/bin/fm-bootstrap.sh
+++ b/bin/fm-bootstrap.sh
@@ -295,25 +295,23 @@ x_mode_setup() {
 
   mkdir -p "$STATE" "$CONFIG" 2>/dev/null || { fmx_arm_failed; return 0; }
 
-  shim_body=$(cat <<EOF
+  read -r -d '' shim_body <<EOF || true
 #!/usr/bin/env bash
 # Auto-generated by fm-bootstrap.sh - X mode connector poll shim.
 # The watcher runs this each check cycle; output becomes a check: wake.
 export FM_HOME=$(printf '%q' "$FM_HOME")
 exec $(printf '%q' "$FM_ROOT/bin/fm-x-poll.sh")
 EOF
-)
   write_if_changed "$shim" "$shim_body" || { fmx_arm_failed; return 0; }
   chmod +x "$shim" 2>/dev/null || { fmx_arm_failed; return 0; }
 
-  cadence_body=$(cat <<'EOF'
+  read -r -d '' cadence_body <<'EOF' || true
 # Auto-generated by fm-bootstrap.sh - X mode watcher cadence.
 # Source this before arming the watcher (see AGENTS.md "X mode") so fm-watch.sh
 # polls the X check every 30s. Non-X instances have no such file and keep the
 # default 300s cadence.
 export FM_CHECK_INTERVAL=30
 EOF
-)
   write_if_changed "$cadence" "$cadence_body" || { fmx_arm_failed; return 0; }
 
   echo "FMX: X mode on - relay poll armed via state/x-watch.check.sh; 30s watcher cadence in config/x-mode.env"
diff --git a/bin/fm-brief.sh b/bin/fm-brief.sh
index e0f2ec78..b1dcf9f0 100755
--- a/bin/fm-brief.sh
+++ b/bin/fm-brief.sh
@@ -171,23 +171,26 @@ read -r MODE _ <<EOF
 $("$FM_ROOT/bin/fm-project-mode.sh" "$REPO")
 EOF
 
+# The DOD heredocs are assigned with `read -r -d ''` instead of `$(cat <<EOF ...)`:
+# bash 3.2 (macOS /bin/bash) tracks quote state through a heredoc body nested in
+# $( ), so a lone apostrophe in the brief text breaks parsing of the whole script
+# (issue #166). read's exit status at EOF is nonzero, hence the `|| true`.
 case "$MODE" in
   direct-PR)
     SETUP2=""
     RULE1='1. Never push to the default branch (push only your `fm/'"$ID"'` branch). Never merge a PR.'
-    DOD=$(cat <<EOF
+    read -r -d '' DOD <<EOF || true
 # Definition of done
 This project ships **direct-PR**: you raise the PR yourself, without the no-mistakes pipeline.
 The task is complete only when committed on your branch.
 When it is implemented and committed, push your branch and open a PR with \`gh-axi\`, then append \`done: PR {url}\` to the status file and stop.
 Do NOT run /no-mistakes. The captain reviews and merges the PR; firstmate relays it.
 EOF
-)
     ;;
   local-only)
     SETUP2=""
     RULE1="1. Never push to any remote and never open a PR. Work only on your \`fm/$ID\` branch; firstmate handles the merge into local \`main\`."
-    DOD=$(cat <<EOF
+    read -r -d '' DOD <<EOF || true
 # Definition of done
 This project ships **local-only**: no remote, no PR, no pipeline.
 The task is complete only when committed on your branch \`fm/$ID\`. Do NOT push, do NOT open a PR, do NOT merge.
@@ -195,13 +198,12 @@ Keep your branch a clean fast-forward onto the current default branch - if \`mai
 When it is implemented and committed, append \`done: ready in branch fm/$ID\` to the status file and stop.
 Firstmate then reviews your branch diff, the captain approves, and firstmate merges it into local \`main\`.
 EOF
-)
     ;;
   *)  # no-mistakes (default)
     SETUP2="
 2. Run \`no-mistakes doctor\`; if it reports the repo is not initialized here, run \`no-mistakes init\`."
     RULE1='1. Never push to the default branch. Never merge a PR.'
-    DOD=$(cat <<EOF
+    read -r -d '' DOD <<EOF || true
 # Definition of done
 The task is complete only when committed on your branch.
 When you believe it is complete, append \`done: {summary}\` to the status file and stop.
@@ -218,7 +220,6 @@ Two firstmate-specific rules layer on top of that guidance:
 
 After /no-mistakes reports CI green, append \`done: PR {url} checks green\` and stop. You are finished.
 EOF
-)
     ;;
 esac
 
diff --git a/tests/fm-brief.test.sh b/tests/fm-brief.test.sh
index be1df2be..765bb322 100755
--- a/tests/fm-brief.test.sh
+++ b/tests/fm-brief.test.sh
@@ -2,13 +2,17 @@
 # Behavior tests for bin/fm-brief.sh.
 #
 # Regression coverage for the heredoc-in-command-substitution parse bug (issue
-# #166): each ship-mode branch builds its Definition-of-done text with
-# `VAR=$(cat <<EOF ... EOF)`. Bash's lexer tracks quote state through the
-# heredoc body while it scans for the matching `)` of the command
-# substitution, so a single unescaped apostrophe anywhere in that body breaks
-# parsing of the *entire rest of the script* - `bash -n` fails, not just the
-# generated brief. A plain `cat > file <<EOF ... EOF` (not wrapped in `$(...)`)
-# is unaffected, so the secondmate charter block does not need this guard.
+# #166): the ship-mode branches used to build their Definition-of-done text
+# with `VAR=$(cat <<EOF ... EOF)`. Bash 3.2 (macOS /bin/bash) tracks quote
+# state through the heredoc body while it scans for the matching `)` of the
+# command substitution, so a single unescaped apostrophe anywhere in that body
+# breaks parsing of the *entire rest of the script* - `bash -n` fails, not
+# just the generated brief. Modern bash (4+) parses it fine, so Linux CI never
+# sees the failure. The branches now assign via `read -r -d '' VAR <<EOF`,
+# which is immune; the structural test below keeps the fragile construct from
+# coming back, since `bash -n` alone only catches it on a bash-3.2 machine.
+# A plain `cat > file <<EOF ... EOF` (not wrapped in `$(...)`) is unaffected,
+# so the secondmate charter block does not need this guard.
 set -u
 
 # shellcheck source=tests/lib.sh
@@ -17,13 +21,29 @@ set -u
 TMP_ROOT=$(fm_test_tmproot fm-brief)
 
 # The script itself must always parse. This is the direct regression test for
-# issue #166: a stray apostrophe in any of the three DOD heredoc bodies
-# (no-mistakes/direct-PR/local-only) breaks `bash -n` on the whole file.
+# issue #166; on macOS it runs under the affected /bin/bash 3.2 itself.
 test_script_parses() {
   bash -n "$ROOT/bin/fm-brief.sh" 2>&1 || fail "bin/fm-brief.sh fails bash -n (heredoc/quote regression)"
+  if [ -x /bin/bash ]; then
+    /bin/bash -n "$ROOT/bin/fm-brief.sh" 2>&1 || fail "bin/fm-brief.sh fails /bin/bash -n (bash 3.2 heredoc/quote regression)"
+  fi
   pass "fm-brief.sh: bash -n succeeds"
 }
 
+# Structural guard for issue #166: heredocs nested inside \$(cat <<...) are a
+# bash-3.2 parse trap that Linux CI's modern bash cannot detect via `bash -n`,
+# so refuse the construct itself anywhere in bin/ (including bin/backends/).
+test_no_heredoc_in_command_substitution() {
+  local script
+  for script in "$ROOT"/bin/*.sh "$ROOT"/bin/backends/*.sh; do
+    # shellcheck disable=SC2016  # literal grep pattern; expansion is not wanted
+    if grep -n '^[^#]*\$(cat <<' "$script"; then
+      fail "${script#"$ROOT"/} uses \$(cat <<...) - breaks bash 3.2 parsing if the body ever gains an apostrophe; assign with read -r -d '' instead"
+    fi
+  done
+  pass "bin/*.sh, bin/backends/*.sh: no heredoc-in-command-substitution construct"
+}
+
 # Registry with one project per delivery mode, so each ship-mode DOD branch is
 # exercised. A project absent from the registry defaults to no-mistakes.
 write_registry() {
@@ -77,5 +97,6 @@ test_no_mistakes_dod_wording() {
 }
 
 test_script_parses
+test_no_heredoc_in_command_substitution
 test_ship_modes_generate_clean_briefs
 test_no_mistakes_dod_wording