refactor: stateless MoE routing with grouped dispatch

- replace per-expert mask scan with sort+bincount grouped dispatch
- carry router stats in forward output instead of module state
- keep MoE diagnostics working under DDP/FSDP wrappers
- remove unused _load_balancing_loss helper

astrai/model/components/decoder_block.py

@@ -6,13 +6,14 @@ from torch import Tensor
 
 from astrai.inference.core.cache import KVCache
 from astrai.model.components.attention import AttnFactory
-from astrai.model.components.mlp import FFNFactory
+from astrai.model.components.mlp import FFNFactory, RouterStats
 from astrai.model.components.norm import RMSNorm
 
 
 class DecoderOutput(TypedDict):
     hidden_states: Tensor
     aux_loss: Optional[Tensor]
+    router_stats: Optional[RouterStats]
 
 
 class DecoderBlock(nn.Module):
@@ -66,4 +67,8 @@ class DecoderBlock(nn.Module):
         mlp_output = self.mlp(normalized)
         x = mlp_output["hidden_states"] + x
 
-        return {"hidden_states": x, "aux_loss": mlp_output["aux_loss"]}
+        return {
+            "hidden_states": x,
+            "aux_loss": mlp_output["aux_loss"],
+            "router_stats": mlp_output.get("router_stats"),
+        }

astrai/model/components/mlp.py

@@ -1,4 +1,4 @@
-from typing import List, Optional, TypedDict
+from typing import Optional, TypedDict
 
 import torch
 import torch.nn as nn
@@ -13,14 +13,26 @@ class FFNFactory(BaseFactory[nn.Module]):
     pass
 
 
+class RouterStats(TypedDict):
+    """Per-layer MoE routing statistics for training diagnostics.
+
+    Both tensors are detached monitoring data produced during forward.
+    """
+
+    probs: Tensor
+    topk_indices: Tensor
+
+
 class FFNOutput(TypedDict):
     hidden_states: Tensor
     aux_loss: Optional[Tensor]
+    router_stats: Optional[RouterStats]
 
 
 class RoutedOutput(TypedDict):
     hidden_states: Tensor
     aux_loss: Optional[Tensor]
+    router_stats: Optional[RouterStats]
 
 
 @FFNFactory.register("mlp")
@@ -34,7 +46,7 @@ class MLP(nn.Module):
     def forward(self, x: Tensor) -> FFNOutput:
         gated = self.up(x) * F.silu(self.gate(x))
         out = self.down(gated)
-        return {"hidden_states": out, "aux_loss": None}
+        return {"hidden_states": out, "aux_loss": None, "router_stats": None}
 
 
 @FFNFactory.register("moe")
@@ -70,7 +82,6 @@ class DeepSeekMoE(nn.Module):
         )
 
         self.router = Linear(dim, n_routed_experts, bias=False)
-        self._router_probs: Optional[Tensor] = None
         moe_scale = 1 / max(n_shared_experts, 1) + 1 / n_activated_experts
         down_init_std = 0.02 / (2 * n_layers * moe_scale) ** 0.5
 
@@ -96,7 +107,11 @@ class DeepSeekMoE(nn.Module):
         routed_output = self._routed_forward(x_flat, include_aux_loss)
 
         out = (shared_out + routed_output["hidden_states"]).view(bsz, seq_len, dim)
-        return {"hidden_states": out, "aux_loss": routed_output["aux_loss"]}
+        return {
+            "hidden_states": out,
+            "aux_loss": routed_output["aux_loss"],
+            "router_stats": routed_output["router_stats"],
+        }
 
     def _shared_forward(self, x: Tensor) -> Tensor:
         if self.n_shared_experts == 0:
@@ -109,44 +124,54 @@ class DeepSeekMoE(nn.Module):
     def _routed_forward(self, x: Tensor, include_aux_loss: bool) -> RoutedOutput:
         N, D = x.shape
         K = self.n_activated_experts
+        E = self.n_routed_experts
 
         router_logits = self.router(x)
         router_probs = torch.softmax(router_logits.float(), dim=-1).to(x.dtype)
-        self._router_probs = router_probs.detach()
 
-        topk_weights, topk_indices = torch.topk(router_probs, K, dim=-1)
+        topk_weights, topk_indices = torch.topk(router_probs, K, dim=-1, sorted=False)
         if self.norm_topk_prob:
             topk_weights = topk_weights / topk_weights.sum(dim=-1, keepdim=True)
 
         aux_loss = None
+        router_stats = None
         if include_aux_loss:
-            expert_load = F.one_hot(
-                topk_indices, num_classes=self.n_routed_experts
-            ).float()
+            expert_load = F.one_hot(topk_indices, num_classes=E).float()
             expert_load = expert_load.mean(dim=(0, 1))
             router_prob = router_probs.float().mean(dim=0)
-            aux_loss = self.n_routed_experts * (expert_load * router_prob).sum()
+            aux_loss = E * (expert_load * router_prob).sum()
+            router_stats = {
+                "probs": router_probs.detach(),
+                "topk_indices": topk_indices,
+            }
+
+        # Grouped dispatch: sort (token, slot) pairs by expert so each expert
+        # consumes one contiguous slice instead of a per-expert mask scan.
+        flat_experts = topk_indices.reshape(-1)
+        sorted_experts, order = torch.sort(flat_experts)
+        flat_tokens = x.repeat_interleave(K, dim=0)[order]
+        flat_weights = topk_weights.reshape(-1, 1)[order]
+        boundaries = torch.cumsum(
+            torch.bincount(sorted_experts, minlength=E), dim=0
+        ).tolist()
 
         output = torch.zeros(N, D, device=x.device, dtype=x.dtype)
-        for expert_idx in range(self.n_routed_experts):
-            expert_mask = topk_indices == expert_idx
-            token_idx, k_idx = expert_mask.nonzero(as_tuple=True)
-            if token_idx.numel() == 0:
+        start = 0
+        for expert_idx, end in enumerate(boundaries):
+            if end == start:
                 continue
-            expert = self.routed_experts[expert_idx]
-            expert_input = x[token_idx]
-            expert_output = expert(expert_input)["hidden_states"]
+            expert_output = self.routed_experts[expert_idx](flat_tokens[start:end])[
+                "hidden_states"
+            ]
+            output.index_add_(
+                0,
+                order[start:end] // K,
+                expert_output * flat_weights[start:end],
+            )
+            start = end
 
-            weights = topk_weights[token_idx, k_idx].unsqueeze(-1)
-            output.index_add_(0, token_idx, expert_output * weights)
-
-        return {"hidden_states": output, "aux_loss": aux_loss}
-
-    @staticmethod
-    def collect_router_probs(module: nn.Module) -> List[Tensor]:
-        """Recursively collect router_probs from all DeepSeekMoE submodules."""
-        probs: List[Tensor] = []
-        for m in module.modules():
-            if isinstance(m, DeepSeekMoE) and m._router_probs is not None:
-                probs.append(m._router_probs)
-        return probs
+        return {
+            "hidden_states": output,
+            "aux_loss": aux_loss,
+            "router_stats": router_stats,
+        }

astrai/model/transformer.py

@@ -1,4 +1,4 @@
-from typing import Any, Dict, List, Mapping, Optional
+from typing import Any, Dict, Mapping, Optional
 
 import torch
 import torch.nn as nn
@@ -10,7 +10,6 @@ from astrai.model.automodel import AutoModel, ModelFactory
 from astrai.model.components.decoder_block import DecoderBlock
 from astrai.model.components.embedding import Embedding
 from astrai.model.components.linear import Linear
-from astrai.model.components.mlp import DeepSeekMoE
 from astrai.model.components.norm import RMSNorm
 from astrai.model.components.rope import RotaryEmbedding
 
@@ -115,6 +114,7 @@ class AutoRegressiveLM(AutoModel):
         use_sdpa_causal_mask = attn_mask is None
 
         aux_losses = []
+        router_stats_list = []
         for layer in self.layers:
             layer_output = layer(
                 x,
@@ -124,8 +124,10 @@ class AutoRegressiveLM(AutoModel):
                 use_sdpa_causal_mask,
             )
             x = layer_output["hidden_states"]
-            if layer_output["aux_loss"] is not None:
+            stats = layer_output.get("router_stats")
+            if stats is not None:
                 aux_losses.append(layer_output["aux_loss"])
+                router_stats_list.append(stats)
 
         hidden_states = self.norm(x)
         logits = self.lm_head(hidden_states)
@@ -133,8 +135,5 @@ class AutoRegressiveLM(AutoModel):
         output = {"logits": logits, "hidden_states": hidden_states}
         if aux_losses:
             output["aux_loss"] = torch.stack(aux_losses).mean()
+            output["router_stats"] = router_stats_list
         return output
-
-    def get_moe_router_probs(self) -> List[Tensor]:
-        """Return router_probs from all MoE layers for strategy-side aux loss."""
-        return DeepSeekMoE.collect_router_probs(self)

astrai/trainer/strategy.py

@@ -9,6 +9,7 @@ import torch.nn.functional as F
 from torch import Tensor
 
 from astrai.factory import BaseFactory
+from astrai.model.components.mlp import RouterStats
 from astrai.parallel.executor import broadcast_state_dict
 from astrai.trainer.rollout import RolloutResult
 
@@ -21,6 +22,7 @@ class LossOutput(TypedDict):
 class LogprobsOutput(TypedDict):
     logprobs: Tensor
     aux_loss: Optional[Tensor]
+    router_stats: Optional[List[RouterStats]]
 
 
 def move_to_device(batch: Dict[str, Tensor], device: str) -> Dict[str, Tensor]:
@@ -75,7 +77,11 @@ def get_logprobs(
         logprobs = (token_logprobs * shifted_loss_mask).sum(dim=-1)
     else:
         logprobs = token_logprobs * shifted_loss_mask
-    return {"logprobs": logprobs, "aux_loss": outputs.get("aux_loss")}
+    return {
+        "logprobs": logprobs,
+        "aux_loss": outputs.get("aux_loss"),
+        "router_stats": outputs.get("router_stats"),
+    }
 
 
 def make_doc_boundary_mask(position_ids: Tensor) -> Tensor:
@@ -94,36 +100,14 @@ def make_doc_boundary_mask(position_ids: Tensor) -> Tensor:
     return (same_doc & causal).unsqueeze(1)
 
 
-def _load_balancing_loss(router_probs: Tensor) -> Tensor:
-    """Compute MoE load balancing auxiliary loss from router probabilities.
-
-    Implements the Switch Transformer load balancing loss (eq. 4-6).
-    Encourages tokens to be uniformly distributed across experts.
-
-    Args:
-        router_probs: (N, num_experts) tensor of softmax router probabilities.
-
-    Returns:
-        Scalar aux loss = num_experts * sum(f_i * P_i).
-    """
-    num_experts = router_probs.size(-1)
-    # f_i: fraction of tokens dispatched to expert i (soft mean)
-    f_i = router_probs.mean(dim=0)
-    # P_i: average routing probability for expert i
-    P_i = router_probs.mean(dim=0)
-    return num_experts * torch.sum(f_i * P_i)
-
-
 def _collect_moe_diagnostics(
-    router_probs_list: List[Tensor],
-    top_k: int,
+    router_stats_list: List[RouterStats],
 ) -> Dict[str, float]:
-    """Collect MoE routing diagnostic metrics from router probabilities.
+    """Collect MoE routing diagnostic metrics from per-layer router stats.
 
     Args:
-        router_probs_list: List of (N, num_experts) router probability tensors,
-            one per MoE layer.
-        top_k: Number of top experts selected per token.
+        router_stats_list: One :class:`RouterStats` dict per MoE layer with
+            keys ``probs`` (N, E) and ``topk_indices`` (N, K), both detached.
 
     Returns:
         Dict with keys: router_entropy, dead_expert_fraction,
@@ -135,33 +119,26 @@ def _collect_moe_diagnostics(
     layer_imbalance_means: List[Tensor] = []
     layer_imbalance_maxs: List[Tensor] = []
 
-    for probs in router_probs_list:
-        probs = probs.detach().to(dtype=torch.float32)
-        if probs.ndim == 0 or probs.shape[-1] == 0:
+    for stats in router_stats_list:
+        probs = stats["probs"].float()
+        topk_indices = stats["topk_indices"]
+        num_experts = probs.shape[-1]
+        if num_experts == 0:
             continue
-        probs = probs.reshape(-1, probs.shape[-1])
+        probs = probs.reshape(-1, num_experts)
         if probs.numel() == 0:
             continue
 
-        num_experts = probs.shape[-1]
-        num_tokens = probs.shape[0]
-
         # Router entropy
         entropy = -(probs * torch.log(probs.clamp_min(1e-8))).sum(dim=-1).mean()
 
-        # Top-k expert selection
-        selected_experts = torch.topk(probs, top_k, dim=-1).indices  # [tokens, top_k]
-        expert_mask = F.one_hot(selected_experts, num_experts)  # [tokens, top_k, E]
-        expert_counts = expert_mask.sum(dim=(0, 1)).to(dtype=torch.float32)  # [E]
-
-        # Ideal load: tokens * top_k / num_experts
-        ideal_load = (num_tokens * top_k) / max(num_experts, 1)
-
-        # Load imbalance ratios
-        load_ratios = expert_counts / max(ideal_load, 1.0)
+        # Load from the actual dispatch: one-hot sum of top-k assignments.
+        expert_counts = F.one_hot(topk_indices, num_experts).sum(dim=(0, 1)).float()
+        ideal_load = expert_counts.mean()  # N*K / E
+        load_ratios = expert_counts / max(float(ideal_load), 1.0)
         imbalance_mean = (load_ratios - 1.0).abs().mean()
         imbalance_max = load_ratios.max()
-        dead_fraction = (expert_counts == 0).to(dtype=torch.float32).mean()
+        dead_fraction = (expert_counts == 0).float().mean()
 
         layer_entropies.append(entropy)
         layer_dead_fractions.append(dead_fraction)
@@ -230,6 +207,7 @@ class BaseStrategy(ABC):
         task_loss: Tensor,
         metrics: Dict[str, Tensor],
         aux_loss: Optional[Tensor] = None,
+        router_stats: Optional[List[RouterStats]] = None,
     ) -> LossOutput:
         total_loss = task_loss
         if aux_loss is not None:
@@ -237,7 +215,7 @@ class BaseStrategy(ABC):
             total_loss = total_loss + weighted_aux_loss
             metrics["moe_aux_loss"] = aux_loss
             metrics["moe_aux_loss_weighted"] = weighted_aux_loss
-            self._refresh_moe_diagnostics(aux_loss)
+            self._refresh_moe_diagnostics(aux_loss, router_stats)
         metrics["loss"] = total_loss
         return {
             "loss": total_loss,
@@ -281,21 +259,18 @@ class BaseStrategy(ABC):
         """
         pass
 
-    def _refresh_moe_diagnostics(self, aux_loss: Tensor) -> None:
-        """Collect MoE routing diagnostics from model router probs.
+    def _refresh_moe_diagnostics(
+        self,
+        aux_loss: Tensor,
+        router_stats: Optional[List[RouterStats]] = None,
+    ) -> None:
+        """Collect MoE routing diagnostics from the latest forward pass.
 
         Populates ``self._moe_metrics`` with router entropy, dead expert
         fraction, load imbalance, and aux_loss.  Called from
         :meth:`_loss_output` when an MoE aux loss is present.
         """
-        router_probs_list: List[Tensor] = self.model.get_moe_router_probs()
-        if not router_probs_list:
-            self._moe_metrics = {}
-            return
-        self._moe_metrics = _collect_moe_diagnostics(
-            router_probs_list,
-            self.model.config.n_activated_experts,
-        )
+        self._moe_metrics = _collect_moe_diagnostics(router_stats or [])
         self._moe_metrics["aux_loss"] = float(aux_loss.detach().cpu().item())
 
     def on_optimizer_step(self):
@@ -368,7 +343,12 @@ class SEQStrategy(BaseStrategy):
             label_smoothing=self.label_smoothing,
         )
 
-        return self._loss_output(loss, {"task_loss": loss}, outputs.get("aux_loss"))
+        return self._loss_output(
+            loss,
+            {"task_loss": loss},
+            outputs.get("aux_loss"),
+            outputs.get("router_stats"),
+        )
 
 
 @StrategyFactory.register("sft")
@@ -416,7 +396,12 @@ class SFTStrategy(BaseStrategy):
             label_smoothing=self.label_smoothing,
         )
 
-        return self._loss_output(loss, {"task_loss": loss}, outputs.get("aux_loss"))
+        return self._loss_output(
+            loss,
+            {"task_loss": loss},
+            outputs.get("aux_loss"),
+            outputs.get("router_stats"),
+        )
 
 
 @StrategyFactory.register("dpo")
@@ -491,7 +476,12 @@ class DPOStrategy(BaseStrategy):
         ratio_diff = pi_log_ratio - ref_log_ratio
         dpo_loss = -F.logsigmoid(self.beta * ratio_diff).mean()
 
-        return self._loss_output(dpo_loss, {"dpo_loss": dpo_loss}, aux_loss)
+        return self._loss_output(
+            dpo_loss,
+            {"dpo_loss": dpo_loss},
+            aux_loss,
+            policy_output.get("router_stats"),
+        )
 
     def supports_online(self) -> bool:
         return True
@@ -661,6 +651,7 @@ class GRPOStrategy(BaseStrategy):
             task_loss,
             {"policy_loss": policy_loss, "kl_loss": kl_penalty},
             aux_loss,
+            policy_output.get("router_stats"),
         )
 
     def supports_online(self) -> bool:

tests/module/test_forward_configs.py

@@ -265,10 +265,9 @@ def test_moe_defaults_preserve_normalized_routing():
     assert model.layers[0].mlp.norm_topk_prob is True
 
 
-def test_moe_router_probs_populated_after_forward():
-    """Verify DeepSeekMoE._router_probs is set after forward in training mode."""
+def test_moe_router_stats_in_output_during_training():
+    """Verify forward output carries per-layer router_stats in training mode."""
     from astrai.config.model_config import AutoRegressiveLMConfig
-    from astrai.model.components.mlp import DeepSeekMoE
 
     config = AutoRegressiveLMConfig(
         **TINY_CONFIG,
@@ -283,19 +282,18 @@ def test_moe_router_probs_populated_after_forward():
     input_ids = torch.randint(0, config.vocab_size, (2, 8))
 
     with torch.enable_grad():
-        model(input_ids)
+        outputs = model(input_ids)
 
-    # All MoE layers should have router_probs set
-    moe_layers = [m for m in model.modules() if isinstance(m, DeepSeekMoE)]
-    assert len(moe_layers) > 0
-    for layer in moe_layers:
-        assert layer._router_probs is not None
-        assert layer._router_probs.ndim == 2
-        assert layer._router_probs.shape[-1] == 4  # n_routed_experts
+    stats = outputs["router_stats"]
+    assert isinstance(stats, list)
+    assert len(stats) == config.num_hidden_layers
+    for s in stats:
+        assert s["probs"].shape == (2 * 8, 4)  # (N, n_routed_experts)
+        assert s["topk_indices"].shape == (2 * 8, 2)  # (N, n_activated_experts)
 
 
-def test_get_moe_router_probs_moe_model():
-    """Verify get_moe_router_probs() returns a list of tensors for MoE models."""
+def test_moe_router_stats_absent_in_eval():
+    """Verify no router_stats are emitted outside training."""
     from astrai.config.model_config import AutoRegressiveLMConfig
 
     config = AutoRegressiveLMConfig(
@@ -306,60 +304,27 @@ def test_get_moe_router_probs_moe_model():
         n_activated_experts=2,
     )
     model = AutoRegressiveLM(config)
-    model.train()
+    model.eval()
 
-    with torch.enable_grad():
-        model(torch.randint(0, config.vocab_size, (2, 8)))
+    with torch.no_grad():
+        outputs = model(torch.randint(0, config.vocab_size, (2, 8)))
 
-    probs = model.get_moe_router_probs()
-    assert isinstance(probs, list)
-    assert len(probs) == 2  # num_hidden_layers
-    for p in probs:
-        assert p.ndim == 2
-        assert p.shape[-1] == 4
+    assert "router_stats" not in outputs
 
 
-def test_get_moe_router_probs_non_moe_model():
-    """Verify get_moe_router_probs() returns empty list for non-MoE models."""
+def test_no_router_stats_for_mlp_model():
+    """Verify pure MLP models emit no router_stats and no aux_loss."""
     from astrai.config.model_config import AutoRegressiveLMConfig
 
     config = AutoRegressiveLMConfig(**TINY_CONFIG, ffn_type="mlp")
     model = AutoRegressiveLM(config)
-
-    probs_untrained = model.get_moe_router_probs()
-    assert probs_untrained == []
-
     model.train()
+
     with torch.enable_grad():
-        model(torch.randint(0, config.vocab_size, (2, 8)))
+        outputs = model(torch.randint(0, config.vocab_size, (2, 8)))
 
-    probs = model.get_moe_router_probs()
-    assert probs == []
-
-
-def test_collect_router_probs_static_method():
-    """Verify DeepSeekMoE.collect_router_probs static method."""
-    from astrai.model.components.mlp import DeepSeekMoE
-
-    moe = DeepSeekMoE(
-        dim=8,
-        dim_ffn=16,
-        n_routed_experts=4,
-        n_shared_experts=1,
-        n_activated_experts=2,
-    )
-    moe.train()
-    with torch.enable_grad():
-        moe(torch.randn(2, 8, 8))
-
-    # collect_router_probs should find the MoE layer
-    probs = DeepSeekMoE.collect_router_probs(moe)
-    assert len(probs) == 1
-    assert probs[0].shape[-1] == 4
-
-    # On a plain MLP module, should return empty
-    mlp_module = MLP(8, 16)
-    assert DeepSeekMoE.collect_router_probs(mlp_module) == []
+    assert "router_stats" not in outputs
+    assert "aux_loss" not in outputs
 
 
 def test_moe_aux_loss_only_emitted_during_training():

tests/module/test_model_forward.py

@@ -9,19 +9,15 @@ import pytest
 import torch
 
 from astrai.config.model_config import AutoRegressiveLMConfig
-from astrai.model.components.mlp import DeepSeekMoE
 from astrai.model.transformer import AutoRegressiveLM
 from astrai.trainer.strategy import (
     SEQStrategy,
     SFTStrategy,
     StrategyFactory,
     _collect_moe_diagnostics,
-    _load_balancing_loss,
 )
 from tests.helpers import TINY_CONFIG
 
-# ── helpers ──────────────────────────────────────────────────────────
-
 
 def _make_tiny_moe_config(**overrides) -> AutoRegressiveLMConfig:
     return AutoRegressiveLMConfig(
@@ -43,14 +39,16 @@ def _make_model(config=None) -> AutoRegressiveLM:
     return AutoRegressiveLM(config)
 
 
-# ── _collect_moe_diagnostics unit tests ─────────────────────────────
+def _router_stats(probs, topk_indices):
+    return {"probs": probs, "topk_indices": topk_indices}
 
 
 def test_collect_moe_diagnostics_returns_all_keys():
     """_collect_moe_diagnostics should return the four expected keys."""
     # Simulate two MoE layers with uniform routing probabilities
     probs = torch.ones(128, 4) / 4.0
-    diag = _collect_moe_diagnostics([probs, probs], top_k=2)
+    topk = torch.zeros(128, 2, dtype=torch.long)
+    diag = _collect_moe_diagnostics([_router_stats(probs, topk)] * 2)
 
     assert set(diag.keys()) == {
         "router_entropy",
@@ -64,11 +62,11 @@ def test_collect_moe_diagnostics_returns_all_keys():
 
 def test_collect_moe_diagnostics_empty_list():
     """Empty list returns empty dict."""
-    assert _collect_moe_diagnostics([], top_k=2) == {}
+    assert _collect_moe_diagnostics([]) == {}
 
 
 def test_collect_moe_diagnostics_uniform_routing():
-    """Uniform routing probabilities with top_k=2 → tie-breaking by index.
+    """Uniform routing with top_k=2 → tie-breaking by index.
 
     torch.topk breaks ties by index, so with equal probabilities
     experts 0 and 1 always win over experts 2 and 3:
@@ -77,7 +75,8 @@ def test_collect_moe_diagnostics_uniform_routing():
       - load_imbalance_max = 2.0
     """
     probs = torch.ones(128, 4) / 4.0
-    diag = _collect_moe_diagnostics([probs], top_k=2)
+    topk = torch.tensor([[0, 1]] * 128)
+    diag = _collect_moe_diagnostics([_router_stats(probs, topk)])
 
     assert diag["dead_expert_fraction"] == pytest.approx(0.5, abs=1e-6)
     assert diag["load_imbalance_mean"] == pytest.approx(1.0, abs=1e-6)
@@ -88,38 +87,41 @@ def test_collect_moe_diagnostics_max_entropy():
     """Uniform probabilities should give log(num_experts) entropy."""
     num_experts = 4
     probs = torch.ones(128, num_experts) / num_experts
-    diag = _collect_moe_diagnostics([probs], top_k=2)
+    topk = torch.zeros(128, 2, dtype=torch.long)
+    diag = _collect_moe_diagnostics([_router_stats(probs, topk)])
     expected_entropy = float(torch.log(torch.tensor(num_experts, dtype=torch.float32)))
     assert diag["router_entropy"] == pytest.approx(expected_entropy, abs=1e-5)
 
 
-# ── _load_balancing_loss unit tests ──────────────────────────────────
+def test_moe_metrics_flow_through_wrapped_model(device):
+    """DDP-like wrappers (no .config / get_moe_router_probs) still collect MoE metrics."""
+    import torch.nn as nn
 
+    from astrai.trainer.strategy import SEQStrategy
 
-def test_load_balancing_loss_shape_and_range():
-    """Verify _load_balancing_loss returns a non-negative scalar tensor."""
-    probs = torch.randn(64, 8).softmax(dim=-1)
-    loss = _load_balancing_loss(probs)
-    assert loss.ndim == 0
-    assert loss.item() >= 0
+    class ForwardOnlyWrapper(nn.Module):
+        def __init__(self, model):
+            super().__init__()
+            self.module = model
 
+        def forward(self, *args, **kwargs):
+            return self.module(*args, **kwargs)
 
-def test_load_balancing_loss_uniform_minimum():
-    """Uniform routing gives the lowest possible load balancing loss."""
-    probs = torch.ones(64, 8) / 8.0
-    loss = _load_balancing_loss(probs).item()
+    config = _make_tiny_moe_config()
+    model = AutoRegressiveLM(config).to(device)
+    wrapped = ForwardOnlyWrapper(model)
+    wrapped.train()
 
-    # Very skewed routing should give higher loss
-    skewed = torch.zeros(64, 8)
-    skewed[:, 0] = 1.0
-    skewed[:, 1] = 1.0
-    skewed = skewed / skewed.sum(dim=-1, keepdim=True)
-    skewed_loss = _load_balancing_loss(skewed).item()
+    strategy = SEQStrategy(wrapped, device, moe_aux_loss_coef=0.01)
+    output = strategy.compute_loss_output(
+        {
+            "input_ids": torch.randint(0, config.vocab_size, (2, 8)),
+            "target_ids": torch.randint(0, config.vocab_size, (2, 8)),
+        }
+    )
 
-    assert loss < skewed_loss
-
-
-# ── SEQStrategy integration tests ────────────────────────────────────
+    assert "moe_aux_loss" in output["metrics"]
+    assert "router_entropy" in strategy._moe_metrics
 
 
 class TestSEQStrategyMoE:
@@ -255,9 +257,6 @@ class TestSEQStrategyMoE:
         assert strategy._moe_metrics == {}
 
 
-# ── SFTStrategy integration tests ────────────────────────────────────
-
-
 class TestSFTStrategyMoE:
     """End‑to‑end tests for SFTStrategy with MoE aux loss."""