diff --git a/docs/port/DECISIONS.md b/docs/port/DECISIONS.md
index 8cce0eeb..8bdf6f90 100644
--- a/docs/port/DECISIONS.md
+++ b/docs/port/DECISIONS.md
@@ -3804,3 +3804,54 @@ Re-deriving the transfer curve on the correctly-posed pair gives 1.20 / 1.26 /
 So the focus-state contamination it flagged really did not move the trend. Its
 reproduction stands as a second opinion after all, and that is now a measurement
 rather than a hope.
+
+## `tools/port/which-focus` — the Decoder asked for a detector, and it carries its own control
+
+`S00A` is blocked on knowing which button a screenshot has focused.
+`newgame_path.sh` assumed NEW GAME is focused at boot, drove on that assumption,
+and landed in a **tutorial mission** — because HANDOFF Q5 measured focus as
+*unstable across boots*. And counting presses cannot substitute: ⬆ from the first
+item wraps to the last, so no fixed number of presses lands on a known item from
+an unknown start.
+
+The Decoder's own attempt — a per-row brightness statistic — **failed the
+control**, picking NEW GAME on the capture whose filename says OPTIONS. The
+render-difference method passes it, so it is now a script that agent can run.
+
+### It runs the control on every invocation, not once when it was written
+
+```
+control -- live-main-menu-options-focused.png (answer is in the filename):
+    OPTIONS      1285      <- picked
+    EXTRAS       6073
+    ...
+  -> OPTIONS, margin 4.7x     CONTROL PASSED
+```
+
+If that fails, the tool **refuses to report a result at all**. A control that
+does not execute is not a control, and this one cannot be skipped.
+
+### Three checks, and one of them independently reproduces a corpus measurement
+
+| input | verdict | margin |
+|---|---|---|
+| `live-main-menu-options-focused` — **known answer** | OPTIONS | 4.7× |
+| `live-main-menu` — the question | **NEW GAME** | 11.4× |
+| `live-extras` — **known from the corpus** | MISSION SELECT | 4.2× |
+| `live-title-press-a` — **no menu at all** | *refuses* | 1.0× |
+
+The `extras` row is a second known answer I did not plant: `authored/flow.json`
+already records *"MEASURED: EXTRAS opens focused on MISSION SELECT
+(live-extras.png)"*, and the tool reaches it independently.
+
+The title row is the negative control. A frame with no menu in it gives a margin
+of 1.0× and the tool says *"this frame does not decide it. Do not act on this."*
+
+⚠️ **And that refusal now carries a non-zero exit code.** The first version
+printed the warning and exited 0 — so a caller scripting it, which is the entire
+point, would have read a refusal as an answer. That is the same defect as a
+checker claiming a check it skipped, and it is the fifth instance of that shape
+between the two of us this session.
+
+**What it is not:** it identifies focus in *one frame*. It says nothing about
+what *selects* focus; Q5's instability stands.
diff --git a/tools/port/which-focus b/tools/port/which-focus
new file mode 100755
index 00000000..23427fad
--- /dev/null
+++ b/tools/port/which-focus
@@ -0,0 +1,108 @@
+#!/usr/bin/env bash
+# Which button is focused in a screenshot of the real game?
+#
+#   tools/port/which-focus SHOT.png              # main_menu (5 buttons)
+#   tools/port/which-focus SHOT.png extras       # extras (3 buttons)
+#
+# Renders the port's own screen with each button focused in turn and reports
+# which one differs least from the shot. Answers a question the Decoder needs to
+# drive the game -- `newgame_path.sh` assumed NEW GAME is focused at boot, drove
+# on that, and landed in a tutorial mission, because HANDOFF Q5 measured focus as
+# UNSTABLE across boots. Counting presses cannot substitute: up from the first
+# item wraps to the last, so no fixed number of presses lands on a known item
+# from an unknown start.
+#
+# ⚠️ IT RUNS ITS OWN CONTROL FIRST AND REFUSES TO ANSWER IF THE CONTROL FAILS.
+# `docs/re/captures/title-builds/live-main-menu-options-focused.png` has the
+# answer in its filename, so the method can be tested on every invocation rather
+# than once when it was written. A brightness-per-row detector was tried for this
+# job and picked NEW GAME on that capture; this method picks OPTIONS by 4.7x.
+# A control that does not execute is not a control.
+#
+# WHAT IT IS NOT. It identifies the focus in ONE FRAME. It says nothing about
+# what selects focus -- Q5's four boots gave TUTORIAL, TUTORIAL, NEW GAME, NEW
+# GAME and that instability stands.
+set -euo pipefail
+cd "${PROJECT_DIR:-/work}"
+export DISPLAY="${DISPLAY:-:97}"
+shot="${1:?usage: which-focus SHOT.png [screen]}"
+screen="${2:-main_menu}"
+OUT="${OUT:-$(mktemp -d)}"; mkdir -p "$OUT"
+CAPS=docs/re/captures/title-builds
+
+# Buttons, in the order ui_down walks them.
+case "$screen" in
+  main_menu) BUTTONS=(ptbtn01:NEW_GAME ptbtn02:LOAD_GAME ptbtn03:TUTORIAL ptbtn04:OPTIONS ptbtn05:EXTRAS) ;;
+  extras)    BUTTONS=(ptbtn11:MISSION_SELECT ptbtn12:MOVIE_THEATER ptbtn13:THIRD) ;;
+  *) echo "which-focus: no button list for $screen" >&2; exit 2 ;;
+esac
+n=${#BUTTONS[@]}
+downs=$(python3 -c "print(','.join(['down']*($n-1)))")
+
+render_all() {  # render_all <tag>
+  godot --path port --resolution 1280x720 -- "--menu=$screen" "--script=$downs" \
+    "--shots=$OUT/$1" >"$OUT/$1.log" 2>&1 || true
+}
+# Normalise any input to the captures' 1279x675 top-left crop. A 1280x720 guest
+# frame and a 1279x675 screenshot are the same pixels; the difference is the
+# crop the screenshot tool applies, not a scale.
+norm() { convert "$1" -crop 1279x675+0+0 +repage "$2"; }
+
+score() {   # score <shot> ; prints "<idx> <label> <pixels>" per candidate
+  local s="$1" i=0 f
+  for f in "$OUT"/r_*.png; do
+    [ -f "$f" ] || continue
+    norm "$f" "$OUT/cand.png"
+    local d
+    d=$(convert "$OUT/cand.png" "$s" -compose difference -composite \
+        -colorspace Gray -threshold 25% -format "%[fx:mean*w*h]" info:)
+    echo "$i ${BUTTONS[$i]#*:} $d"
+    i=$((i+1))
+  done
+}
+
+render_all r
+# Godot names the shots `<tag>_00_start.png`, `<tag>_01_down.png`, ... -- rename
+# to a sortable form so the candidate order is the ui_down order and not glob luck.
+i=0
+for f in "$OUT"/r_0*.png; do mv "$f" "$OUT/r_$(printf '%02d' $i).png"; i=$((i+1)); done
+[ "$i" = "$n" ] || { echo "which-focus: rendered $i of $n focus states -- see $OUT" >&2; exit 3; }
+
+verdict() {  # verdict <shot> <expected-or-empty>
+  local s="$1" expect="${2:-}"
+  norm "$s" "$OUT/shot.png"
+  mapfile -t rows < <(score "$OUT/shot.png" | sort -k3 -n)
+  local best_lbl best_px second_px
+  best_lbl=$(echo "${rows[0]}" | awk '{print $2}')
+  best_px=$(echo "${rows[0]}" | awk '{print $3}')
+  second_px=$(echo "${rows[1]}" | awk '{print $3}')
+  local margin
+  margin=$(python3 -c "print('%.1f' % ($second_px/max($best_px,1)))")
+  for r in "${rows[@]}"; do printf '    %-16s %8s\n' "$(echo "$r"|awk '{print $2}')" "$(echo "$r"|awk '{print $3}')"; done
+  echo "  -> $best_lbl, margin ${margin}x"
+  if [ -n "$expect" ]; then
+    if [ "$best_lbl" = "$expect" ]; then echo "  CONTROL PASSED (expected $expect)"; return 0
+    else echo "  🔴 CONTROL FAILED: expected $expect, got $best_lbl"; return 1; fi
+  fi
+  # A thin margin means the frame does not decide it. 2x is below the 4.7x the
+  # control achieves and well above 1.0; a shot that cannot beat it should be
+  # re-taken rather than guessed at.
+  python3 -c "import sys; sys.exit(0 if $margin >= 2.0 else 1)" || {
+    echo "  ⚠️ margin under 2x -- this frame does not decide it. Do not act on this."; return 1; }
+}
+
+if [ "$screen" = main_menu ]; then
+  echo "control -- $CAPS/live-main-menu-options-focused.png (answer is in the filename):"
+  verdict "$CAPS/live-main-menu-options-focused.png" OPTIONS || {
+    echo "refusing to report a result from a method that just failed its control." >&2; exit 1; }
+  echo
+fi
+# The exit code must carry the refusal. An earlier version printed "do not act on
+# this" and exited 0, so a caller scripting this -- which is the entire point,
+# the Decoder runs it between drive steps -- would have read a refusal as an
+# answer. That is the same defect as a checker claiming a check it skipped.
+echo "$shot:"
+rc=0
+verdict "$shot" || rc=$?
+echo "artifacts in $OUT"
+exit $rc