@@ -326,13 +326,44 @@ expect block "echo \"\$(echo \"\$(printf '\"')\")\" ; sed -i s/a/b/ pkg/x.ts"
 # introducer behind it is found and the body stays prose.
 expect allow "$(printf 'echo "$(echo " #")" <<EOF\nsed -i s/a/b/ pkg/x.ts\nEOF\n')"
 
+echo "== a \${…} inside \"…\" is its own quoting context too (#24901) =="
+CWD="$MAIN"
+# The measured false block: the `"` that opens the default was read as CLOSING the outer string,
+# so `b}` became a write target. Pure reads — nothing is written. Each one exercises a different
+# pass: tokenisation (the first two), segmentation (the `;`), nesting both ways (the third).
+expect allow 'echo "${x:-"a > b"}"'
+expect allow 'echo "${x#"a > b"}"'
+expect allow 'echo "${x:-"$(printf "a > b")"}"'
+expect allow 'echo "${x:-"a; sed -i s/a/b/ pkg/x.ts"}"'
+expect allow "echo \"\${x:-'a > b'}\""          # control: no nested `"`, allowed before and after
+# The heredoc scan agrees: the ` #` inside the expansion is no comment, so the introducer
+# behind it is found and the body stays prose.
+expect allow "$(printf 'echo "${x:-" #"}" <<EOF\nsed -i s/a/b/ pkg/x.ts\nEOF\n')"
+# Recall is untouched: a write behind, beside or inside the script of a quoted expansion. A
+# '…' run holding no character the shells disagree on is skipped, so the write behind it stays.
+expect block 'sed -i "s/a/${x:-"b"}/" pkg/x.ts'
+expect block 'sed -i "s/a/${x}/" pkg/x.ts'
+expect block 'echo "${HOME}" > pkg/x.ts'
+expect block 'echo "${x:-"a"}" ; sed -i s/a/b/ pkg/x.ts'
+expect block "echo \"\${x:-'a'}\" ; sed -i s/a/b/ pkg/x.ts"
+# A `$(…)` inside the expansion is read by the same function, quotes and all: the `"` inside
+# its single quotes closes nothing, so the separator behind the word still splits.
+expect block "echo \"\${x:-\$(printf '\"')}\" ; sed -i s/a/b/ pkg/x.ts"
+
 echo "== shapes this guard deliberately does NOT claim (documented fail-open) =="
 CWD="$MAIN"
 expect allow "bash -c \"sed -i s/a/b/ $MAIN/pkg/x.ts\""
 expect allow 'echo pkg/x.ts | xargs sed -i s/a/b/'
 expect allow 'node -e "fs.writeFileSync(\"pkg/x.ts\", \"y\")"'
 expect allow 'python3 -c "open(\"pkg/x.ts\",\"w\").write(\"y\")"'
 expect allow 'echo "$(sed -i s/a/b/ pkg/x.ts)"'      # a QUOTED $(…) body is one inert word
+expect allow 'echo "${x:-"$(sed -i s/a/b/ pkg/x.ts)"}"'   # …and so is a QUOTED ${…} body
+expect allow 'echo hi > "${x:-"a > b"}"'              # a target holding an expansion
+# A '…' run inside a quoted ${…} that holds `}` (or `"` `\` `$` and a backtick): bash 5.2 reads
+# it as quoted and POSIX shells as literal, so they end the expansion in different places and
+# everything after it is inert. Under bash the first is a pure read, under dash a real write.
+expect allow "echo \"\${x:-'}\"; sed -i s/a/b/ pkg/x.ts'}\""
+expect allow "echo \"\${x:-'}'}\" ; sed -i s/a/b/ pkg/x.ts"
 expect allow 'sed -i s/a/b/ $SOMEDIR/x.ts'
 expect allow 'sed -i s/a/b/ pkg/*.ts'
 
 
@@ -44,6 +44,9 @@
 #     node -e, awk > (the target is not a shell token, so there is nothing to read);
 #   - any target containing an expansion or glob ($VAR, `…`, $(…), *, ?, ~) — unresolvable
 #     without running it;
+#   - everything after a `'` inside a QUOTED "${…}" unless the quoted run holds none of
+#     `"` `\` `$` `}` or a backtick: bash and POSIX shells disagree on where such an expansion
+#     ends (subst_end());
 #   - relative targets when the payload carries no cwd.
 # Each of those is a hole by choice, not by accident: OS_ALLOW_MAIN_EDITS=1 already exists
 # for anyone who means to write there, and the target of this guard is the reflexive
@@ -52,8 +55,8 @@
 # Writing ABOUT the ban must never trip the ban (#4890's lesson). Four layers:
 #   1. quote-aware segmentation + tokenisation — a `>` or a `sed -i` inside '…' or "…" is
 #      literal text, so `grep -n "sed -i" .claude/` and `echo "never sed -i in main"` pass;
-#      a `$(…)` or backtick inside "…" is a quoting context of its own, so a `"` in its body
-#      nests instead of closing the outer string (subst_end(), shared by every pass);
+#      a `$(…)`, a backtick or a `${…}` inside "…" is a quoting context of its own, so a `"` in
+#      its body nests instead of closing the outer string (subst_end(), shared by every pass);
 #   2. heredoc bodies are stripped before analysis — the LINES of a `cat > /tmp/notes <<EOF`
 #      body are documentation, not commands, and segmentation alone (which splits on
 #      newlines) would happily read them as such;
@@ -121,35 +124,66 @@ fi
 
 [ -n "$cmd" ] || exit 0
 
-# --- a command substitution inside "…" is its own quoting context ---------------------
-# Inside "…", `$(` or a backtick opens a NEW quoting context: a `"` in the body starts a string
-# of its own and does not close the outer one. Each scanner below used to read that `"` as the
-# closer, so the rest of the body became bare shell, and a `>` in a quoted curl body was taken
-# for a redirect whose "target" was the next fragment — a false BLOCK (#24812). All three
-# quote-aware scanners hand such a body to this ONE function, so they agree on where it ends.
+# --- an expansion inside "…" is its own quoting context --------------------------------
+# Inside "…", `$(`, a backtick or `${` opens a NEW quoting context: a `"` in the body starts a
+# string of its own and does not close the outer one. Each scanner below used to read that `"`
+# as the closer, so the rest of the body became bare shell, and a `>` in a quoted curl body was
+# taken for a redirect whose "target" was the next fragment — a false BLOCK (#24812); the same
+# reading of `echo "${x:-"a > b"}"` named `b}` as a target (#24901). All three quote-aware
+# scanners hand such a body to this ONE function, so they agree on where it ends.
+# guard-shared-stash.sh, guard-process-kill.sh and guard-tree-enum.sh segment commands the same
+# way and carry a byte-identical copy; their self-tests fail when a copy differs from this one.
 #
-# subst_end S I: S[I] is the `$` of a `$(`, or a backtick, met inside "…". Sets SUBST_END to
-# the index of the character that closes the substitution, reading the body with its own
-# quotes, escapes and nested `(…)` / `$(…)` / backticks; when the input ends first, to the
-# last index — the rest is one inert run (bash refuses such a command anyway). The body is
-# kept verbatim as part of the word, so a target holding it still fails open on its `$`.
-# Boundary: a `(` or `)` bash would not count (a `case` pattern, a comment) moves the end —
-# too early, and the rest is read as it was before this function existed; too late, and the
-# text swallowed with the body is inert, i.e. fails open.
+# subst_end S I: when S[I] opens such a context (the `$` of a `$(` or a `${`, or a backtick),
+# sets SUBST_END to the index of the character that closes it and returns 0; otherwise returns 1
+# and changes nothing. The body is read with its own quotes, escapes and nesting; when the input
+# ends first, SUBST_END is the last index — the rest is one inert run (bash refuses such a
+# command anyway). The body is kept verbatim as part of the word, so a target holding it still
+# fails open on its `$`.
+#
+# A `${…}` body reads like "…" with one more closer: `"`, `$(`, `${` and a backtick nest in it,
+# a backslash escapes, and `}` closes it. A `'` there is where shells part ways, measured: bash
+# 5.2 reads `'…'` as a quoted run while it looks for the `}` (`"${x:-'}'}"` prints `'}'`), and
+# `bash --posix` and dash read the `'` as a literal. The two readings agree only when the run
+# holds none of `"` `\` `$` `}` or a backtick, so such a run is skipped, and any other `'` makes
+# the rest one inert run: an extent that depends on the shell is not one this guard can read.
+#
+# Boundary: a `(` or `)` bash would not count (a `case` pattern, a comment, an unquoted `${…}`
+# holding one) moves the end — too early, and the rest is read as it was before this function
+# existed; too late, and the text swallowed with the body is inert, i.e. fails open.
 SUBST_END=0
 subst_end() {
-  local s="$1" i="$2" n=${#1} st ch top
-  if [ "${s:i:1}" = '`' ]; then st='`'; else st=')' ; i=$((i + 1)); fi
+  local s="$1" i="$2" n=${#1} st ch top run
+  case "${s:i:2}" in
+    '$(') st=')' ; i=$((i + 1)) ;;
+    '${') st='}' ; i=$((i + 1)) ;;
+    '`'*) st='`' ;;
+    *) return 1 ;;
+  esac
   for ((i = i + 1; i < n; i++)); do
     ch="${s:i:1}" ; top="${st:${#st}-1:1}"
     if [ "$top" = "'" ]; then [ "$ch" = "'" ] && st="${st%?}"; continue; fi
     if [ "$ch" = '\' ]; then i=$((i + 1)); continue; fi
-    if [ "$top" = '"' ]; then
+    if [ "$top" = '"' ] || [ "$top" = '}' ]; then
       case "$ch" in
-        '"') st="${st%?}" ;;
+        '"') if [ "$top" = '"' ]; then st="${st%?}"; else st+='"'; fi ;;
         '`') st+='`' ;;
-        '$') if [ "${s:i+1:1}" = '(' ]; then st+=')' ; i=$((i + 1)); fi ;;
+        '$')
+          case "${s:i+1:1}" in
+            '(') st+=')' ; i=$((i + 1)) ;;
+            '{') st+='}' ; i=$((i + 1)) ;;
+          esac ;;
+        '}') [ "$top" = '}' ] && st="${st%?}" ;;
+        "'")
+          if [ "$top" = '}' ]; then
+            run="${s:i+1}"
+            case "$run" in *"'"*) ;; *) SUBST_END=$((n - 1)); return 0 ;; esac
+            run="${run%%"'"*}"
+            case "$run" in *[\"\\\`\$\}]*) SUBST_END=$((n - 1)); return 0 ;; esac
+            i=$((i + ${#run} + 1))
+          fi ;;
       esac
+      if [ -z "$st" ]; then SUBST_END=$i; return 0; fi
       continue
     fi
     case "$ch" in
@@ -158,9 +192,10 @@ subst_end() {
       ')') [ "$top" = ')' ] && st="${st%?}" ;;
       '`') if [ "$top" = '`' ]; then st="${st%?}"; else st+='`'; fi ;;
     esac
-    if [ -z "$st" ]; then SUBST_END=$i; return; fi
+    if [ -z "$st" ]; then SUBST_END=$i; return 0; fi
   done
   SUBST_END=$((n - 1))
+  return 0
 }
 
 # --- heredoc bodies are text, not commands ---------------------------------------------
@@ -193,8 +228,8 @@ strip_line_comment() {
     ch="${s:i:1}"
     if [ -n "$q" ]; then
       if [ "$q" = '"' ] && [ "$ch" = '\' ] && [ $((i + 1)) -lt "$n" ]; then i=$((i + 1)); continue; fi
-      if [ "$q" = '"' ] && { [ "$ch" = '`' ] || [ "${s:i:2}" = '$(' ]; }; then
-        subst_end "$s" "$i" ; i=$SUBST_END ; continue
+      if [ "$q" = '"' ] && { [ "$ch" = '$' ] || [ "$ch" = '`' ]; } && subst_end "$s" "$i"; then
+        i=$SUBST_END ; continue
       fi
       [ "$ch" = "$q" ] && q=""
       continue
@@ -265,8 +300,8 @@ split_segments() {
             seg+="$ch" ; i=$((i + 1)) ; seg+="${s:i:1}" ; continue ;;
         esac
       fi
-      if [ "$q" = '"' ] && { [ "$ch" = '`' ] || [ "${s:i:2}" = '$(' ]; }; then
-        subst_end "$s" "$i" ; seg+="${s:i:SUBST_END-i+1}" ; i=$SUBST_END ; continue
+      if [ "$q" = '"' ] && { [ "$ch" = '$' ] || [ "$ch" = '`' ]; } && subst_end "$s" "$i"; then
+        seg+="${s:i:SUBST_END-i+1}" ; i=$SUBST_END ; continue
       fi
       seg+="$ch"
       [ "$ch" = "$q" ] && q=""
@@ -325,8 +360,8 @@ tokenize() {
             i=$((i + 1)) ; tok+="${s:i:1}" ; have=1 ; continue ;;
         esac
       fi
-      if [ "$q" = '"' ] && { [ "$ch" = '`' ] || [ "${s:i:2}" = '$(' ]; }; then
-        subst_end "$s" "$i" ; tok+="${s:i:SUBST_END-i+1}" ; i=$SUBST_END ; have=1 ; continue
+      if [ "$q" = '"' ] && { [ "$ch" = '$' ] || [ "$ch" = '`' ]; } && subst_end "$s" "$i"; then
+        tok+="${s:i:SUBST_END-i+1}" ; i=$SUBST_END ; have=1 ; continue
       fi
       if [ "$ch" = "$q" ]; then q=""; else tok+="$ch"; fi
       have=1
 
@@ -176,6 +176,41 @@ echo "== an escaped \\\" INSIDE a double-quoted word does NOT close it (#10406 h
 expect allow 'grep -rn "he said \"pkill -f vitest\" once" .claude/'
 expect block 'echo "he said \"x\"" && pkill -f vitest'
 
+echo "== a \$(…), backtick or \${…} inside \"…\" is its own quoting context (#24901) =="
+# Reading the `"` inside the body as CLOSING the outer string split the body at its `;`, so a
+# pure read became a name-matched kill. All four were BLOCKED before subst_end() existed here.
+expect allow 'echo "a $(echo "x; pkill -f node")"'
+expect allow 'echo "a `echo "x; pkill -f node"`"'
+expect allow 'echo "${x:-"a; pkill -f node"}"'
+expect allow 'echo "${x:-"; killall node"}"'
+# Recall: a kill behind a quoted expansion is still seen, and a pattern held in one is still a
+# pattern — the expansion is part of the word, not a way around the rule.
+expect block 'echo "${x:-"a"}" ; pkill -f node'
+expect block 'kill $(pgrep -f "${pat:-"node"}")'
+expect block 'pkill -f "${pat:-"vitest; x"}"'
+# The fail-OPEN mirror: the `"` inside the single quotes left the outer string hanging, so the
+# separator behind it went inert. ALLOWED before subst_end() existed here.
+expect block "echo \"\$(printf '\"')\" ; pkill -f vitest"
+# Deliberately not claimed, as before: a kill INSIDE a quoted substitution is one inert word.
+expect allow 'echo "$(pkill -f node)"'
+
+echo "== the shared quoting rules are copies, byte for byte (#24901) =="
+# split_segments() is guard-shared-stash.sh's and subst_end() is guard-main-checkout-bash.sh's.
+# A copy that drifts is a quoting rule the guards no longer share, and nothing else notices:
+# each hook's own cases keep passing on its own dialect.
+fn_text() { sed -n "/^$1() {\$/,/^}\$/p" "$2"; }
+same_fn() { # same_fn <function> <file holding the copy this hook must equal>
+  local mine theirs
+  mine="$(fn_text "$1" "$hook")"; theirs="$(fn_text "$1" "$2")"
+  if [ -n "$mine" ] && [ "${mine%%$'\n'*}" = "$1() {" ] && [ "$mine" = "$theirs" ]; then
+    pass=$((pass + 1)); printf '  ok   same   %s() as %s\n' "$1" "${2##*/}"
+  else
+    fail=$((fail + 1)); printf '  FAIL %s() is missing here or differs from %s\n' "$1" "${2##*/}"
+  fi
+}
+same_fn split_segments "$here/guard-shared-stash.sh"
+same_fn subst_end "$here/guard-main-checkout-bash.sh"
+
 echo "== escape hatch =="
 expect allow 'pkill -f vitest' OS_ALLOW_PROCESS_KILL=1
 expect allow 'ps aux | grep node | xargs kill' OS_ALLOW_PROCESS_KILL=1
 
@@ -142,10 +142,63 @@ fi
 #     (#11738 on the stash guard, #11131 on guard-main-checkout-bash.sh);
 #   - INSIDE "…" the rule inverts: `\"` is a literal quote that leaves the region OPEN, and
 #     reading it as closing splits where bash would not, turning the tail of a pure READ
-#     into a segment judged on its own head word — a false BLOCK (#11804 / #10406).
+#     into a segment judged on its own head word — a false BLOCK (#11804 / #10406);
+#   - INSIDE "…" a `$(`, a backtick or a `${` opens a quoting context of its own, so a `"` in
+#     its body nests instead of closing the outer string. Reading it as the closer split the
+#     body: `echo "a $(echo "x; pkill -f node")"` and `echo "${x:-"a; killall node"}"` were
+#     BLOCKED, measured on this repo's copy before the fix (#24901).
 #
 # Both characters are kept verbatim: this pass only SPLITS, and the classifiers below
-# re-read the words afterwards.
+# re-read the words afterwards. The self-test fails when this function differs from
+# guard-shared-stash.sh's, and when subst_end() differs from guard-main-checkout-bash.sh's,
+# whose comment above the function is its documentation.
+SUBST_END=0
+subst_end() {
+  local s="$1" i="$2" n=${#1} st ch top run
+  case "${s:i:2}" in
+    '$(') st=')' ; i=$((i + 1)) ;;
+    '${') st='}' ; i=$((i + 1)) ;;
+    '`'*) st='`' ;;
+    *) return 1 ;;
+  esac
+  for ((i = i + 1; i < n; i++)); do
+    ch="${s:i:1}" ; top="${st:${#st}-1:1}"
+    if [ "$top" = "'" ]; then [ "$ch" = "'" ] && st="${st%?}"; continue; fi
+    if [ "$ch" = '\' ]; then i=$((i + 1)); continue; fi
+    if [ "$top" = '"' ] || [ "$top" = '}' ]; then
+      case "$ch" in
+        '"') if [ "$top" = '"' ]; then st="${st%?}"; else st+='"'; fi ;;
+        '`') st+='`' ;;
+        '$')
+          case "${s:i+1:1}" in
+            '(') st+=')' ; i=$((i + 1)) ;;
+            '{') st+='}' ; i=$((i + 1)) ;;
+          esac ;;
+        '}') [ "$top" = '}' ] && st="${st%?}" ;;
+        "'")
+          if [ "$top" = '}' ]; then
+            run="${s:i+1}"
+            case "$run" in *"'"*) ;; *) SUBST_END=$((n - 1)); return 0 ;; esac
+            run="${run%%"'"*}"
+            case "$run" in *[\"\\\`\$\}]*) SUBST_END=$((n - 1)); return 0 ;; esac
+            i=$((i + ${#run} + 1))
+          fi ;;
+      esac
+      if [ -z "$st" ]; then SUBST_END=$i; return 0; fi
+      continue
+    fi
+    case "$ch" in
+      "'" | '"') st+="$ch" ;;
+      '(') st+=')' ;;
+      ')') [ "$top" = ')' ] && st="${st%?}" ;;
+      '`') if [ "$top" = '`' ]; then st="${st%?}"; else st+='`'; fi ;;
+    esac
+    if [ -z "$st" ]; then SUBST_END=$i; return 0; fi
+  done
+  SUBST_END=$((n - 1))
+  return 0
+}
+
 segments=()
 split_segments() {
   local s="$1" seg="" q="" ch i n=${#1}
@@ -158,6 +211,9 @@ split_segments() {
             seg+="$ch" ; i=$((i + 1)) ; seg+="${s:i:1}" ; continue ;;
         esac
       fi
+      if [ "$q" = '"' ] && { [ "$ch" = '$' ] || [ "$ch" = '`' ]; } && subst_end "$s" "$i"; then
+        seg+="${s:i:SUBST_END-i+1}" ; i=$SUBST_END ; continue
+      fi
       seg+="$ch"
       [ "$ch" = "$q" ] && q=""
       continue
 
@@ -158,6 +158,43 @@ expect block 'echo "a \\" ; git stash pop'
 # quotes it would swallow the closing `'` and let the stash behind it through.
 expect block "echo 'a \\' ; git stash pop"
 
+echo "== a \$(…), backtick or \${…} inside \"…\" is its own quoting context (#24901) =="
+# Reading the `"` inside the body as CLOSING the outer string split the body at its `;`, so a
+# pure read became a stash. The first three were BLOCKED before subst_end() existed here; the
+# fourth (both kinds nested) was allowed then only because two misread quotes cancelled out,
+# and the single-level `echo "x; git stash pop"` was allowed throughout.
+expect allow 'echo "a $(echo "x; git stash pop")"'
+expect allow 'echo "a `echo "x; git stash pop"`"'
+expect allow 'echo "${x:-"a; git stash pop"}"'
+expect allow 'echo "${x:-"$(echo "a; git stash pop")"}"'
+expect allow 'echo "x; git stash pop"'
+# Recall: a stash behind or beside a quoted expansion is still seen.
+expect block 'echo "${x:-"a"}" ; git stash pop'
+expect block 'X="${Y:-"z"}" && git stash push -m wip'
+expect block 'git stash push -m "${msg:-"wip; x"}"'
+# The fail-OPEN mirror: the `"` inside the single quotes left the outer string hanging, so the
+# separator behind it went inert and the real stash rode through as text. Both were ALLOWED
+# before subst_end() existed here.
+expect block "echo \"\$(printf '\"')\" ; git stash pop"
+expect block "echo \"\${x:-\$(printf '\"')}\" ; git stash pop"
+# Deliberately not claimed, as before: a stash INSIDE a quoted substitution is one inert word.
+expect allow 'echo "$(git stash pop)"'
+
+echo "== the shared quoting rule is guard-main-checkout-bash.sh's, byte for byte (#24901) =="
+# subst_end() lives in four hooks. A copy that drifts is a quoting rule the guards no longer
+# share, and nothing else notices: each hook's own cases keep passing on its own dialect.
+fn_text() { sed -n "/^$1() {\$/,/^}\$/p" "$2"; }
+same_fn() { # same_fn <function> <file holding the copy this hook must equal>
+  local mine theirs
+  mine="$(fn_text "$1" "$hook")"; theirs="$(fn_text "$1" "$2")"
+  if [ -n "$mine" ] && [ "${mine%%$'\n'*}" = "$1() {" ] && [ "$mine" = "$theirs" ]; then
+    pass=$((pass + 1)); printf '  ok   same   %s() as %s\n' "$1" "${2##*/}"
+  else
+    fail=$((fail + 1)); printf '  FAIL %s() is missing here or differs from %s\n' "$1" "${2##*/}"
+  fi
+}
+same_fn subst_end "$here/guard-main-checkout-bash.sh"
+
 echo "== escape hatch =="
 expect allow 'git stash pop' OS_ALLOW_STASH=1