diff --git a/src/devkit/SceneStatsProbe.tsx b/src/devkit/SceneStatsProbe.tsx
new file mode 100644
index 0000000..14fc30c
--- /dev/null
+++ b/src/devkit/SceneStatsProbe.tsx
@@ -0,0 +1,48 @@
+/**
+ * DEVKIT — samples scene + XR session every frame, writes to sceneStatsStore.
+ * Mounted inside <Canvas>. No visual output.
+ */
+import { useEffect, useRef } from 'react';
+import { useFrame, useThree } from '@react-three/fiber';
+import { useXR } from '@react-three/xr';
+import { sceneStats, updateModelStats } from './sceneStatsStore';
+
+export function SceneStatsProbe() {
+  const { scene } = useThree();
+  const session = useXR((s) => s.session);
+  const frameCount = useRef(0);
+  const lastFpsTime = useRef(performance.now());
+
+  useEffect(() => {
+    sceneStats.xrMode = session?.mode ?? null;
+  }, [session]);
+
+  useFrame((_state, _dt, frame: XRFrame | undefined) => {
+    frameCount.current++;
+    const now = performance.now();
+    if (now - lastFpsTime.current >= 500) {
+      sceneStats.fps = Math.round((frameCount.current * 1000) / (now - lastFpsTime.current));
+      frameCount.current = 0;
+      lastFpsTime.current = now;
+    }
+
+    // Find the grabbable model group by traversing for first userData marker, fallback: scan
+    let modelGroup: import('three').Object3D | null = null;
+    scene.traverse((o) => {
+      if (!modelGroup && o.userData?.__xrModelRoot) modelGroup = o;
+    });
+    updateModelStats(modelGroup);
+
+    if (frame) {
+      try {
+        // @ts-expect-error detectedPlanes is experimental
+        const planes = frame.detectedPlanes;
+        sceneStats.xrPlanesCount = planes ? (planes.size ?? planes.length ?? 0) : 0;
+      } catch { sceneStats.xrPlanesCount = 0; }
+      sceneStats.xrInputCount = frame.session?.inputSources?.length ?? 0;
+    }
+    sceneStats.lastUpdate = now;
+  });
+
+  return null;
+}