diff --git a/.github/workflows/verify.yml b/.github/workflows/verify.yml
index 191732b..77e6cba 100644
--- a/.github/workflows/verify.yml
+++ b/.github/workflows/verify.yml
@@ -784,8 +784,14 @@ jobs:
       # a digest already falls back to sha256sum, and that fallback is now exercised rather than
       # assumed. If a future change really does need a GNU tool, install that ONE package and
       # say which behaviour required it -- do not restore the blanket userland swap.
+      # python3 is here for one named behaviour: check 63 runs tests/transcript-census.sh
+      # --self-test, whose engine is stdlib Python. Without it the check skips, and this lane's
+      # own skip audit -- correctly -- refuses a skip it never declared. That exact sequence
+      # destroyed the v1.61.0 tag on 2026-09-01 (the fourth destruction): the census fix
+      # provisioned python3 in tests/container-matrix.sh's three images and missed this fourth
+      # container, the one wired into release gating. Same defect, fourth copy of the fix.
       - name: Tools a slim image does not ship (BusyBox kept, deliberately)
-        run: apk add --no-cache bash git jq curl nodejs npm
+        run: apk add --no-cache bash git jq curl nodejs npm python3
 
       # check 29 ("shellcheck clean") skips without a validator on PATH, and a check that skips
       # on a lane specifically kept around to catch BusyBox-vs-GNU differences is a green
diff --git a/claude/hooks/format.sh b/claude/hooks/format.sh
index 812301b..22707b3 100755
--- a/claude/hooks/format.sh
+++ b/claude/hooks/format.sh
@@ -104,6 +104,28 @@ is_project_trusted() {
   else h=$(sha256sum "$v" 2>/dev/null | cut -d' ' -f1); fi
   [ -n "$h" ] && grep -qxF "$h  $v" "$ts" 2>/dev/null
 }
+# package.json only *configures* prettier via a top-level "prettier" key. A grep for the
+# substring `"prettier":` cannot tell that key apart from the identically-shaped entry
+# `"prettier": "^3.4.2"` sitting in dependencies/devDependencies -- a version pin, not a
+# config -- so it matched there too, won the package.json slot ahead of a sibling .prettierrc
+# in the same directory (package.json is first in the name list, matching prettier's own
+# cosmiconfig search order), and got handed to `prettier --config package.json`. Cosmiconfig
+# then finds no real "prettier" field in it and silently falls back to prettier's built-in
+# defaults instead of erroring -- so the file gets reformatted to the wrong style, and the
+# .prettierrc this loop would otherwise have reached next is never tried. Parse the JSON and
+# check for a genuine top-level key instead of pattern-matching the source text.
+pkg_has_top_level_prettier() { # $1: package.json path
+  nb=$(find_bin node) || nb=$(command -v node) || return 1
+  "$nb" -e '
+    const fs = require("fs");
+    try {
+      const pkg = JSON.parse(fs.readFileSync(process.argv[1], "utf8"));
+      process.exit(Object.prototype.hasOwnProperty.call(pkg, "prettier") ? 0 : 1);
+    } catch (e) {
+      process.exit(1);
+    }
+  ' "$1" 2>/dev/null
+}
 find_prettier_cfg() {
   d="$dir"
   while [ "$d" != "/" ] && [ -n "$d" ]; do
@@ -114,7 +136,11 @@ find_prettier_cfg() {
       p="$d/$name"
       [ -e "$p" ] || continue
       if [ "$name" = package.json ]; then
-        grep -q '"prettier"[[:space:]]*:' "$p" 2>/dev/null || continue
+        # No node, or a package.json that fails to parse: fall through via `continue` so the
+        # loop keeps walking toward .prettierrc in this same directory. Degrading to the
+        # dedicated config file is the safe direction; silently formatting with built-in
+        # defaults because we couldn't check is not.
+        pkg_has_top_level_prettier "$p" || continue
       fi
       printf '%s\n' "$p"
       return 0
diff --git a/tests/repro/formatter-config-devdep.sh b/tests/repro/formatter-config-devdep.sh
new file mode 100755
index 0000000..dbb5795
--- /dev/null
+++ b/tests/repro/formatter-config-devdep.sh
@@ -0,0 +1,133 @@
+#!/usr/bin/env bash
+# tests/repro/formatter-config-devdep.sh
+#
+# format.sh's find_prettier_cfg() walks up from the edited file's directory and, for each
+# directory, tries config filenames in order with package.json FIRST -- matching prettier's own
+# cosmiconfig search order. To decide whether a given package.json actually configures prettier,
+# it used to grep it for `"prettier"[[:space:]]*:`. That regex matches the source text of
+# `"prettier": "^3.4.2"` sitting inside "devDependencies" just as happily as a real top-level
+# "prettier" key -- a version pin is not a config, but the grep cannot tell them apart. So a
+# perfectly ordinary package.json with prettier merely listed as a devDependency won the
+# package.json slot ahead of a sibling .prettierrc in the very same directory, and the hook ran
+# `prettier --config package.json --write`. Prettier finds no real "prettier" field in that
+# file, silently falls back to its own built-in defaults (this is not an error prettier raises;
+# cosmiconfig just treats "found the file, no matching field" as "nothing configured"), and
+# reformats the file to those defaults -- the repo's real .prettierrc is never reached, because
+# find_prettier_cfg already returned on the first name it tried.
+#
+# This builds one throwaway repo with a package.json whose only "prettier" mention is a
+# devDependency version range, and a sibling .prettierrc with {"singleQuote": true}. It writes a
+# double-quoted .ts file and fires format.sh at it exactly the way Claude Code's PostToolUse
+# hook would. Then, to prove this is not a no-op, it re-runs the identical case against the
+# newest committed blob of format.sh that predates the parse-instead-of-grep fix -- derived by
+# walking this file's history newest-first, the same technique tests/repro/formatter-config.sh
+# uses for its own no-op proof, so the baseline cannot rot once the fix is committed, rebased,
+# or reverted.
+#
+# Exit 0 -- fixed: .prettierrc's singleQuote wins, and the pre-fix blob demonstrably picked
+#           package.json instead (proving the test measures something real).
+# Exit 1 -- the devDependency version range still wins, or the harness itself is broken.
+set -uo pipefail
+
+SCRIPT_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+REPO_ROOT=$(cd "$SCRIPT_DIR/../.." && pwd)
+HOOK="$REPO_ROOT/claude/hooks/format.sh"
+
+FAIL=0
+note(){ printf '%s\n' "$1"; }
+ok(){   printf 'ok    %s\n' "$1"; }
+bad(){  printf 'FAIL  %s\n' "$1"; FAIL=1; }
+
+[ -x "$HOOK" ] || { echo "FAIL  $HOOK missing or not executable"; exit 1; }
+command -v npm >/dev/null 2>&1 || { echo "skip  npm not installed on this host; cannot install a real prettier to exercise config resolution"; exit 0; }
+command -v git >/dev/null 2>&1 || { echo "skip  git not installed; cannot fetch the pre-fix blob for the no-op proof"; exit 0; }
+
+# Own sandbox variable, never $HOME itself -- guard-destructive.sh blocks rm -rf "$HOME"-shaped
+# commands even with HOME reassigned, and everything below cleans up its own tree, not HOME.
+FMT_SANDBOX=$(mktemp -d "${TMPDIR:-/tmp}/vstack-formatter-devdep.XXXXXX")
+FMT_SANDBOX=$(cd "$FMT_SANDBOX" && pwd)  # normalize away a doubled slash from a trailing-slash TMPDIR
+cleanup(){ rm -rf "$FMT_SANDBOX"; }
+trap cleanup EXIT
+
+VENDOR="$FMT_SANDBOX/vendor"
+mkdir -p "$VENDOR"
+
+note "-- installing prettier@3.9.6 once, shared across repo copies --"
+if ! npm install prettier@3.9.6 --no-save --prefix "$VENDOR" >"$FMT_SANDBOX/npm-install.log" 2>&1; then
+  echo "skip  npm install prettier failed (offline?); see $FMT_SANDBOX/npm-install.log before cleanup races it"
+  cat "$FMT_SANDBOX/npm-install.log"
+  exit 0
+fi
+[ -x "$VENDOR/node_modules/.bin/prettier" ] || { echo "skip  prettier install did not produce an executable; cannot exercise config resolution"; exit 0; }
+
+make_repo() { # $1: repo dir
+  d="$1"
+  mkdir -p "$d"
+  ln -s "$VENDOR/node_modules" "$d/node_modules"
+  # The only "prettier" mention in this package.json is a devDependency version range -- never
+  # a config. .prettierrc is the real config, and it disagrees with prettier's built-in default
+  # (singleQuote:false) so the two outcomes are distinguishable by inspecting quote style alone.
+  cat > "$d/package.json" <<'EOF'
+{
+  "name": "devdep-repro",
+  "private": true,
+  "devDependencies": {
+    "prettier": "^3.4.2"
+  }
+}
+EOF
+  cat > "$d/.prettierrc" <<'EOF'
+{"singleQuote": true, "printWidth": 90}
+EOF
+  printf 'const x = "hello";\n' > "$d/target.ts"
+}
+
+run_hook() { # $1: hook script, $2: repo dir
+  f="$2/target.ts"
+  echo '{"tool_input":{"file_path":"'"$f"'"}}' | env HOME="$FMT_SANDBOX" CLAUDE_PROJECT_DIR="$2" "$1" >/dev/null 2>&1
+}
+
+# --- the claimed defect: fixed hook must prefer .prettierrc over the devDependency line ------
+REPO_FIXED="$FMT_SANDBOX/fixed-repo"
+make_repo "$REPO_FIXED"
+run_hook "$HOOK" "$REPO_FIXED"
+if grep -q "'hello'" "$REPO_FIXED/target.ts"; then
+  ok ".prettierrc won: devDependency version range no longer mistaken for a prettier config"
+elif grep -q '"hello"' "$REPO_FIXED/target.ts"; then
+  bad "HOLE OPEN: package.json's devDependency line still won the package.json slot -- file was formatted (or left unformatted) with built-in defaults, not .prettierrc"
+else
+  bad "target.ts contents unrecognized after running the fixed hook: $(cat "$REPO_FIXED/target.ts" 2>/dev/null)"
+fi
+
+# --- no-op proof: same case against the newest committed blob predating the parse fix --------
+PREFIX_HOOK="$FMT_SANDBOX/format-prefix.sh"
+# The guard function's own name is the discriminator, anchored on the function rather than a
+# pinned SHA -- a SHA is a second place to remember, and the one that goes stale first.
+GUARD_MARK='pkg_has_top_level_prettier'
+PREFIX_SHA=''
+for _c in $(git -C "$REPO_ROOT" log --format=%H -- claude/hooks/format.sh); do
+  if ! git -C "$REPO_ROOT" show "$_c:claude/hooks/format.sh" 2>/dev/null | grep -q "$GUARD_MARK"; then
+    PREFIX_SHA="$_c"
+    break
+  fi
+done
+if [ -z "$PREFIX_SHA" ]; then
+  bad "no commit of claude/hooks/format.sh predates the $GUARD_MARK guard -- the no-op proof has no baseline to revert to, so this test cannot show it measures anything"
+  exit 1
+fi
+if ! git -C "$REPO_ROOT" show "$PREFIX_SHA:claude/hooks/format.sh" > "$PREFIX_HOOK" 2>/dev/null; then
+  bad "could not read $PREFIX_SHA:claude/hooks/format.sh to build the no-op proof"
+  exit 1
+fi
+chmod +x "$PREFIX_HOOK"
+REPO_REVERT="$FMT_SANDBOX/revert-repo"
+make_repo "$REPO_REVERT"
+run_hook "$PREFIX_HOOK" "$REPO_REVERT"
+if grep -q '"hello"' "$REPO_REVERT/target.ts" && ! grep -q "'hello'" "$REPO_REVERT/target.ts"; then
+  ok "no-op proof: format.sh at $PREFIX_SHA does mistake the devDependency line for a config -- this test is not vacuous"
+else
+  bad "no-op proof failed: reverting to $PREFIX_SHA (the newest format.sh without $GUARD_MARK) did NOT reproduce the false match -- this test cannot detect the bug it claims to"
+fi
+
+[ "$FAIL" -eq 0 ] && echo "FORMATTER PACKAGE.JSON FALSE MATCH: CLOSED" || echo "FORMATTER PACKAGE.JSON FALSE MATCH: OPEN"
+exit "$FAIL"