diff --git a/trellis2/modules/sparse/basic.py b/trellis2/modules/sparse/basic.py
index 880973b8..03873f41 100644
--- a/trellis2/modules/sparse/basic.py
+++ b/trellis2/modules/sparse/basic.py
@@ -267,7 +267,7 @@ def __getitem__(self, idx):
     def reduce(self, op: str, dim: Optional[Union[int, Tuple[int,...]]] = None, keepdim: bool = False) -> torch.Tensor:
         if isinstance(dim, int):
             dim = (dim,)
-        
+
         if op =='mean':
             red = self.feats.mean(dim=dim, keepdim=keepdim)
         elif op =='sum':
@@ -276,12 +276,65 @@ def reduce(self, op: str, dim: Optional[Union[int, Tuple[int,...]]] = None, keep
             red = self.feats.prod(dim=dim, keepdim=keepdim)
         else:
             raise ValueError(f"Unsupported reduce operation: {op}")
-        
+
         if dim is None or 0 in dim:
             return red
-        
-        red = torch.segment_reduce(red, reduce=op, lengths=self.seqlen)
-        return red
+
+        # Defensive cache validation: in the cascade Shape-SLat sampler's
+        # CFG-rescale path (classifier_free_guidance_mixin.std_pos = ...),
+        # x_0_pos is derived from x_t via an elementwise chain that
+        # propagates _spatial_cache by reference (see replace() /
+        # __merge_sparse_cache below). If the inherited cached `seqlen`
+        # was computed at a different scale than x_0_pos.feats, the
+        # invariant sum(seqlen) == feats.shape[0] no longer holds.
+        # CUDA's segment_reduce kernel silently consumes the mismatch;
+        # the CPU path (MPS fallback) raises RuntimeError. Recompute
+        # lengths from coords (authoritative) and evict the stale
+        # cache entries so subsequent reads stay correct.
+        lengths = self.seqlen
+        n_data = red.shape[0]
+        if int(lengths.sum().item()) != n_data:
+            fresh = None
+            coords = getattr(self, 'coords', None)
+            if coords is not None and coords.shape[0] == n_data:
+                batch_size = int(coords[:, 0].max().item()) + 1 if coords.shape[0] > 0 else 1
+                fresh = torch.bincount(coords[:, 0].long(), minlength=batch_size).to(
+                    dtype=torch.long, device=red.device
+                )
+            if fresh is None or int(fresh.sum().item()) != n_data:
+                fresh = torch.tensor(
+                    [l.stop - l.start for l in self.layout],
+                    dtype=torch.long, device=red.device,
+                )
+            if hasattr(self, '_spatial_cache') and hasattr(self, '_scale'):
+                try:
+                    scale_key = str(self._scale)
+                    slot = self._spatial_cache.get(scale_key, {})
+                    for k in ('seqlen', 'cum_seqlen', 'batch_boardcast_map', 'layout'):
+                        slot.pop(k, None)
+                except Exception:
+                    pass
+            elif hasattr(self, '_cache'):
+                try:
+                    for k in ('seqlen', 'cum_seqlen', 'batch_boardcast_map'):
+                        self._cache.pop(k, None)
+                except Exception:
+                    pass
+            lengths = fresh
+            if int(lengths.sum().item()) != n_data:
+                raise RuntimeError(
+                    f"VarLenTensor.reduce: cannot reconcile seqlen "
+                    f"sum({int(lengths.sum().item())}) with data.size(0)={n_data}. "
+                    f"layout has {len(self.layout)} segments."
+                )
+
+        # torch.segment_reduce has no Metal kernel; PyTorch's auto fallback
+        # is racy on cascade-sized workloads. Run explicitly on CPU and
+        # copy back. No-op on CUDA / CPU.
+        if red.device.type == 'mps':
+            reduced = torch.segment_reduce(red.cpu(), reduce=op, lengths=lengths.cpu())
+            return reduced.to(red.device)
+        return torch.segment_reduce(red, reduce=op, lengths=lengths)
     
     def mean(self, dim: Optional[Union[int, Tuple[int,...]]] = None, keepdim: bool = False) -> torch.Tensor:
         return self.reduce(op='mean', dim=dim, keepdim=keepdim)