diff --git a/backend/key-probe.js b/backend/key-probe.js
new file mode 100644
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--- /dev/null
+++ b/backend/key-probe.js
@@ -0,0 +1,72 @@
+// key-probe.js: inspect keyboard events INSIDE a game's guest webview.
+//
+// Why: the open question on the keyboard fix is whether tool-issued
+// sendInputEvent delivers correct key codes once the guest is focused, or
+// whether it arrives with an empty `code` / keyCode:0 (which games ignore) and
+// we must switch to CDP Input.dispatchKeyEvent. This probe answers it directly
+// instead of inferring from whether the sprite moved.
+//
+// Usage:
+//   1. Right-click the game webview -> Inspect -> Console (this is the GUEST
+//      console, http://127.0.0.1:<port>/..., NOT the host app devtools).
+//   2. Paste this whole file and hit Enter. You'll see "[key-probe] armed".
+//   3. Fire one tool-issued press_key, e.g. { key: "ArrowRight", hold_ms: 800 }.
+//   4. Read the logged lines, or call __keyProbe.dump() for a table.
+//
+// What to look for per event:
+//   - isTrusted: should be true (native OS-level event). false = a JS
+//     dispatchEvent, not our native path.
+//   - code: should be e.g. "ArrowRight". EMPTY string is the failure signal.
+//   - keyCode/which: should be e.g. 39. 0 is the failure signal.
+//   - holdMs (on keyup): wall-clock ms the key was held. ~0 means the hold
+//     collapsed to an instant tap (down+up same tick); ~800 means hold worked.
+// Decision: clean code + nonzero keyCode -> stay on sendInputEvent. Empty
+// code / keyCode 0 -> move the keyboard path to CDP Input.dispatchKeyEvent.
+
+(function () {
+  if (window.__keyProbe) {
+    try { window.__keyProbe.disarm(); } catch (_) {}
+  }
+  var events = [];
+  var downAt = Object.create(null); // key -> timestamp, to measure hold duration
+
+  function row(e) {
+    var now = (performance && performance.now) ? performance.now() : Date.now();
+    var holdMs = null;
+    if (e.type === 'keydown') {
+      downAt[e.code || e.key] = now;
+    } else if (e.type === 'keyup') {
+      var k = e.code || e.key;
+      if (downAt[k] != null) { holdMs = Math.round(now - downAt[k]); delete downAt[k]; }
+    }
+    var r = {
+      type: e.type,
+      key: e.key,
+      code: e.code,            // EMPTY = failure signal
+      keyCode: e.keyCode,      // 0 = failure signal
+      which: e.which,
+      isTrusted: e.isTrusted,  // true = native; false = synthetic JS
+      repeat: e.repeat,
+      target: (e.target && (e.target.tagName || e.target.nodeName)) || '(none)',
+      holdMs: holdMs,          // only set on keyup
+    };
+    events.push(r);
+    var warn = (r.code === '' || r.code == null) ? '  <-- EMPTY code'
+             : (r.keyCode === 0) ? '  <-- keyCode 0' : '';
+    console.log('[key-probe]', r.type, JSON.stringify(r) + warn);
+  }
+
+  // Capture phase + on window so we see the event even if the game stops
+  // propagation on its own listener.
+  var types = ['keydown', 'keyup', 'keypress'];
+  types.forEach(function (t) { window.addEventListener(t, row, true); });
+
+  window.__keyProbe = {
+    events: events,
+    dump: function () { try { console.table(events); } catch (_) { console.log(events); } return events; },
+    clear: function () { events.length = 0; for (var k in downAt) delete downAt[k]; console.log('[key-probe] cleared'); },
+    disarm: function () { types.forEach(function (t) { window.removeEventListener(t, row, true); }); console.log('[key-probe] disarmed'); },
+  };
+
+  console.log('[key-probe] armed on', location.href, '- fire a press_key, then __keyProbe.dump(). document.activeElement =', document.activeElement && (document.activeElement.tagName || document.activeElement.nodeName));
+})();