diff --git a/src/devkit/sceneStatsStore.ts b/src/devkit/sceneStatsStore.ts
new file mode 100644
index 0000000..ae9290f
--- /dev/null
+++ b/src/devkit/sceneStatsStore.ts
@@ -0,0 +1,80 @@
+/**
+ * DEVKIT — global mutable snapshot of scene + XR session diagnostics.
+ *
+ * Written every frame by SceneStatsProbe (inside Canvas), read by DevPanel
+ * (HTML overlay) and XRDevHud (in-world). Plain object to avoid re-renders.
+ */
+import * as THREE from 'three';
+
+export interface SceneStats {
+  fps: number;
+  meshCount: number;
+  triCount: number;
+  modelBboxSize: [number, number, number];
+  modelGroupPos: [number, number, number];
+  modelGroupRotDeg: [number, number, number];
+  modelGroupScale: number;
+  xrMode: string | null;
+  xrPlanesCount: number;
+  xrHitTestActive: boolean;
+  xrInputCount: number;
+  lastUpdate: number;
+}
+
+export const sceneStats: SceneStats = {
+  fps: 0,
+  meshCount: 0,
+  triCount: 0,
+  modelBboxSize: [0, 0, 0],
+  modelGroupPos: [0, 0, 0],
+  modelGroupRotDeg: [0, 0, 0],
+  modelGroupScale: 1,
+  xrMode: null,
+  xrPlanesCount: 0,
+  xrHitTestActive: false,
+  xrInputCount: 0,
+  lastUpdate: 0,
+};
+
+export function snapshotStats(): SceneStats {
+  return { ...sceneStats };
+}
+
+const _v = new THREE.Vector3();
+const _q = new THREE.Quaternion();
+const _e = new THREE.Euler();
+const _box = new THREE.Box3();
+
+export function updateModelStats(group: THREE.Object3D | null) {
+  if (!group) return;
+  group.updateWorldMatrix(true, true);
+  group.getWorldPosition(_v);
+  sceneStats.modelGroupPos = [_v.x, _v.y, _v.z];
+  group.getWorldQuaternion(_q);
+  _e.setFromQuaternion(_q);
+  sceneStats.modelGroupRotDeg = [
+    THREE.MathUtils.radToDeg(_e.x),
+    THREE.MathUtils.radToDeg(_e.y),
+    THREE.MathUtils.radToDeg(_e.z),
+  ];
+  group.getWorldScale(_v);
+  sceneStats.modelGroupScale = _v.x;
+
+  let meshes = 0;
+  let tris = 0;
+  group.traverse((o) => {
+    if (o instanceof THREE.Mesh && o.geometry) {
+      meshes++;
+      const idx = o.geometry.index;
+      const pos = o.geometry.attributes.position;
+      if (idx) tris += idx.count / 3;
+      else if (pos) tris += pos.count / 3;
+    }
+  });
+  sceneStats.meshCount = meshes;
+  sceneStats.triCount = Math.round(tris);
+
+  _box.setFromObject(group);
+  _box.getSize(_v);
+  sceneStats.modelBboxSize = [_v.x, _v.y, _v.z];
+}