diff --git a/ScaFFold/utils/losses.py b/ScaFFold/utils/losses.py
index 0a1636d..5d929c2 100644
--- a/ScaFFold/utils/losses.py
+++ b/ScaFFold/utils/losses.py
@@ -128,7 +128,7 @@ def compute_sharded_cross_entropy_loss(
 
     Each rank only sees a local spatial shard, so we cannot use the local
     `reduction="mean"` result directly. Instead we:
-    1. compute the local CE numerator with `reduction="sum"`,
+    1. compute the local CE numerator by summing the per-voxel losses,
     2. build the correct global denominator,
     3. all-reduce numerator and denominator together across the spatial mesh
        in a single collective, and
@@ -146,40 +146,59 @@ def compute_sharded_cross_entropy_loss(
 
     autocast_device = device_type if device_type != "mps" else "cpu"
     with torch.autocast(autocast_device, enabled=False):
-        # Accumulate CE in full precision. Using reduction="sum" gives us the
-        # numerator of the final global mean; if class weights are present,
-        # PyTorch applies the target-class weight to each voxel here. When the
-        # caller already computed log-softmax, NLL over it is identical to
-        # cross-entropy over the raw logits but avoids a second full upcast.
+        # Accumulate CE in full precision. Summing the per-voxel losses gives
+        # us the numerator of the final global mean; if class weights are
+        # present, PyTorch applies the target-class weight to each voxel here.
+        # When the caller already computed log-softmax, NLL over it is
+        # identical to cross-entropy over the raw logits but avoids a second
+        # full upcast.
+        #
+        # reduction="none" plus an explicit .sum(), not reduction="sum": the
+        # fused CUDA reduction accumulates with atomicAdd, so its summation
+        # order follows whichever order the blocks retire in and the loss
+        # value varies run to run. It has no deterministic implementation, and
+        # `more_determinism` passes warn_only=True, so there it would only
+        # warn and stay nondeterministic. The separate .sum() is a
+        # fixed-order reduction and is bitwise reproducible.
         if log_probs is not None:
-            local_ce_sum = F.nll_loss(
+            local_ce = F.nll_loss(
                 log_probs,
                 local_labels,
                 weight=class_weights,
-                reduction="sum",
+                reduction="none",
             )
         else:
-            local_ce_sum = F.cross_entropy(
+            local_ce = F.cross_entropy(
                 local_preds.float(),
                 local_labels,
                 weight=class_weights,
-                reduction="sum",
+                reduction="none",
             )
+        local_ce_sum = local_ce.sum()
 
+        # Neither branch below may read device memory from the host: that
+        # drains the pipeline mid-step, since the host stops submitting until
+        # the read returns and the queue runs dry behind it.
+        # torch.cuda.set_sync_debug_mode("error") catches a regression.
         if class_weights is None:
             # Sum the actual local voxel counts across spatial shards. We use
             # an all-reduced count instead of numel()*num_shards because shard
             # sizes can differ at chunk boundaries.
-            local_normalizer = local_ce_sum.new_tensor(float(local_labels.numel()))
+            #
+            # new_full rather than new_tensor: numel() is shape metadata the
+            # host already has, and full() fills on the device with the value
+            # as a kernel argument, where new_tensor would stage that float
+            # through a pageable host-to-device copy.
+            local_normalizer = local_ce_sum.new_full((), float(local_labels.numel()))
         else:
             # Weighted CE divides by sum(weight[target_i]) over all voxels.
-            # Build that denominator from the local label histogram.
-            local_class_counts = torch.bincount(
-                local_labels.reshape(-1), minlength=class_weights.numel()
-            ).to(dtype=local_ce_sum.dtype)
-            local_normalizer = torch.dot(
-                local_class_counts, class_weights.to(dtype=local_ce_sum.dtype)
-            )
+            # Gather the per-voxel weights and sum them, which is that
+            # definition transcribed. Not torch.bincount: it sizes its output
+            # from the largest label, so it reads that label back to the host
+            # even when minlength already fixes the width.
+            local_normalizer = class_weights.to(dtype=local_ce_sum.dtype)[
+                local_labels
+            ].sum()
 
     # Reduce the CE numerator and its denominator across the spatial shards in
     # one collective (they share the same mesh) rather than two, halving the
diff --git a/tests/test_perf_hotpath.py b/tests/test_perf_hotpath.py
index eac6a5f..57765c6 100644
--- a/tests/test_perf_hotpath.py
+++ b/tests/test_perf_hotpath.py
@@ -20,6 +20,7 @@
 numerically equivalent to the straightforward reference it replaced.
 """
 
+import pytest
 import torch
 import torch.nn.functional as F
 
@@ -67,6 +68,108 @@ def test_ce_log_probs_path_matches_cross_entropy():
         assert torch.allclose(plain, ref, atol=1e-6)
 
 
+@pytest.mark.gpu
+def test_gpu_ce_numerator_is_bitwise_reproducible():
+    # Pins that the CE numerator is summed outside the loss kernel:
+    # reduction="sum" on CUDA accumulates with atomicAdd, so its value depends
+    # on block retire order and changes between otherwise identical calls.
+    # Both entry points (precomputed log_probs via NLL, and raw logits via CE)
+    # go through the same reduction, so both are checked, bitwise rather than
+    # allclose.
+    #
+    # The volume is load-bearing, not incidental: the atomics only collide
+    # once the shape puts many blocks in flight, so shrinking this test for
+    # speed would quietly turn it into one that passes either way. 128**3 with
+    # 7 classes is scale 7 with the shipped n_categories.
+    torch.manual_seed(3)
+    device = torch.device("cuda")
+    b, c, n = 1, 7, 128
+    preds = torch.randn(b, c, n, n, n, device=device)
+    labels = torch.randint(0, c, (b, n, n, n), device=device)
+    weights = torch.rand(c, device=device) + 0.5
+    log_probs = F.log_softmax(preds.float(), dim=1)
+
+    for w in (None, weights):
+        for kwargs in ({"log_probs": log_probs}, {}):
+            values = {
+                compute_sharded_cross_entropy_loss(
+                    preds, labels, None, (1,), "cuda", w, **kwargs
+                ).item()
+                for _ in range(50)
+            }
+            assert len(values) == 1, f"{len(values)} distinct CE values: {values}"
+
+
+@pytest.mark.gpu
+def test_gpu_ce_does_not_synchronize():
+    # Pins that the CE term issues no host-device sync. It runs once per
+    # training step, so a sync in it drains the whole launch queue mid-step:
+    # the host stops submitting until the read comes back, and the GPU runs
+    # out of queued work behind it. A sync is invisible in the loss value, so
+    # only a test catches one. set_sync_debug_mode is documented as a
+    # prototype that does not catch every synchronizing op, so this is a
+    # floor, not a proof.
+    #
+    # The shape matters only in that it covers both normalizer branches and
+    # both entry points; the debug mode does the checking, so it stays small.
+    torch.manual_seed(5)
+    device = torch.device("cuda")
+    b, c, n = 1, 7, 64
+    preds = torch.randn(b, c, n, n, n, device=device)
+    labels = torch.randint(0, c, (b, n, n, n), device=device)
+    weights = torch.rand(c, device=device) + 0.5
+    log_probs = F.log_softmax(preds.float(), dim=1)
+
+    # .item() is itself a sync: keep the results on device and inspect them
+    # after the debug mode is back off.
+    outs = []
+    torch.cuda.synchronize()
+    torch.cuda.set_sync_debug_mode("error")
+    try:
+        for w in (weights, None):
+            for kwargs in ({"log_probs": log_probs}, {}):
+                outs.append(
+                    compute_sharded_cross_entropy_loss(
+                        preds, labels, None, (1,), "cuda", w, **kwargs
+                    )
+                )
+    finally:
+        torch.cuda.set_sync_debug_mode("default")
+
+    torch.cuda.synchronize()
+    assert all(torch.isfinite(o) for o in outs)
+
+
+@pytest.mark.gpu
+def test_gpu_ce_survives_strict_deterministic_algorithms():
+    # Pins that the CE path is legal under strict determinism.
+    # more_determinism sets use_deterministic_algorithms(warn_only=True), so a
+    # nondeterministic kernel only warns there and the run stays
+    # irreproducible -- a regression in this path would be invisible under the
+    # config that is supposed to catch it. Strict mode raises instead, since
+    # the fused loss reduction has no deterministic implementation. The shape
+    # only has to be a valid volume, so it stays small.
+    torch.manual_seed(4)
+    device = torch.device("cuda")
+    b, c = 1, 5
+    preds = torch.randn(b, c, 16, 16, 16, device=device)
+    labels = torch.randint(0, c, (b, 16, 16, 16), device=device)
+    weights = torch.rand(c, device=device) + 0.5
+    log_probs = F.log_softmax(preds.float(), dim=1)
+
+    was_deterministic = torch.are_deterministic_algorithms_enabled()
+    was_warn_only = torch.is_deterministic_algorithms_warn_only_enabled()
+    torch.use_deterministic_algorithms(True)
+    try:
+        for w in (None, weights):
+            for kwargs in ({"log_probs": log_probs}, {}):
+                compute_sharded_cross_entropy_loss(
+                    preds, labels, None, (1,), "cuda", w, **kwargs
+                )
+    finally:
+        torch.use_deterministic_algorithms(was_deterministic, warn_only=was_warn_only)
+
+
 def test_ce_uses_single_spatial_collective(monkeypatch):
     # The CE numerator and its normalizer are reduced together in one
     # SpatialAllReduce, not two. Count applications; the packed path issues