diff --git a/verl/trainer/ppo/core_algos.py b/verl/trainer/ppo/core_algos.py
index 7a57b2c80cc..a95e5e71849 100644
--- a/verl/trainer/ppo/core_algos.py
+++ b/verl/trainer/ppo/core_algos.py
@@ -781,21 +781,15 @@ def agg_loss(
     """
     Aggregate the loss across global batch to ensure the loss is invariant to fsdp/megatron parallelism.
 
-    NOTE: ``dp_size``, ``batch_num_tokens``, and ``global_batch_size`` are only compatible with the new model engine
-        for now, while the legacy model engines conduct the aggregation outside ``agg_loss``.
-
     NOTE: The returned loss has different behaviors for different backend:
     - FSDP: the loss is directly used for backward.
     - Megatron: the loss should be scaled by `num_microbatches` and `cp_size` for pp schedule.
 
-    # TODO: Consider the numerical stability?
-
     Args:
         loss_mat: micro batch loss matrix, (bs, response_length)
         loss_mask: micro batch loss mask, (bs, response_length)
         loss_agg_mode: method to aggregate the loss matrix into a scalar
-        dp_size: data parallel size. When appling manual aggregation,
-            scaling up the ``loss`` by ``dp_size`` can cancel out FSDP averaging.
+        dp_size: data parallel size
         batch_num_tokens: number of valid tokens in global batch
         global_batch_size: global batch size
         loss_scale_factor: scale factor for "seq-mean-token-sum-norm" mode. If None, uses loss_mask.shape[-1].
@@ -805,39 +799,30 @@ def agg_loss(
         loss: `a scalar torch.Tensor`
             aggregated loss
     """
-    # NOTE: `masked_sum` is more robust than multiplying the `mask`.
     if loss_agg_mode == "token-mean":
         if batch_num_tokens is None:
             batch_num_tokens = loss_mask.sum()
         loss = verl_F.masked_sum(loss_mat, loss_mask) / batch_num_tokens * dp_size
-    elif loss_agg_mode.startswith("seq-mean"):
-        # TODO: Correct and unify the denominator logic.
-        if global_batch_size is not None:
-            seq_denominator = global_batch_size * dp_size
-        else:  # The default logic which is only correct when the batch sizes are even.
-            local_bsz = loss_mat.shape[0]
-            seq_denominator = local_bsz
-
-        if loss_agg_mode.startswith("seq-mean-token-sum"):
-            seq_losses = verl_F.masked_sum(loss_mat, loss_mask, axis=-1)  # token-sum per sequence
-
-            if loss_agg_mode == "seq-mean-token-sum":
-                pass  # TODO: Add assertation.
-            elif loss_agg_mode == "seq-mean-token-sum-norm":
-                if loss_scale_factor is None:
-                    loss_scale_factor = loss_mask.shape[-1]
-                seq_losses = seq_losses / loss_scale_factor
-            else:
-                raise ValueError(f"Invalid {loss_agg_mode=}")
-        elif loss_agg_mode == "seq-mean-token-mean":
-            token_counts = torch.sum(loss_mask, dim=-1)  # per-sequence token count
-            # token-mean per sequence
-            seq_losses = verl_F.masked_sum(loss_mat, loss_mask, axis=-1) / (token_counts + 1e-8)
-        else:
-            raise ValueError(f"Invalid {loss_agg_mode=}")
-        loss = torch.sum(seq_losses) / seq_denominator  # seq-mean
+    elif loss_agg_mode == "seq-mean-token-sum":
+        seq_losses = torch.sum(loss_mat * loss_mask, dim=-1)  # token-sum
+        seq_mask = (torch.sum(loss_mask, dim=-1) > 0).float()  # exclude fully masked sequences
+        if global_batch_size is None:
+            global_batch_size = seq_mask.sum()
+        loss = verl_F.masked_sum(seq_losses, seq_mask) / global_batch_size * dp_size  # seq-mean
+    elif loss_agg_mode == "seq-mean-token-mean":
+        seq_mask = torch.sum(loss_mask, dim=-1)  # per-sequence token count
+        seq_losses = torch.sum(loss_mat * loss_mask, dim=-1) / (seq_mask + 1e-8)  # token-mean
+        seq_mask = (seq_mask > 0).float()  # exclude fully masked sequences
+        if global_batch_size is None:
+            global_batch_size = seq_mask.sum()
+        loss = verl_F.masked_sum(seq_losses, seq_mask) / global_batch_size * dp_size  # seq-mean
+    elif loss_agg_mode == "seq-mean-token-sum-norm":
+        seq_losses = torch.sum(loss_mat * loss_mask, dim=-1)
+        if loss_scale_factor is None:
+            loss_scale_factor = loss_mask.shape[-1]
+        loss = torch.sum(seq_losses) / loss_scale_factor
     else:
-        raise ValueError(f"Invalid {loss_agg_mode=}")
+        raise ValueError(f"Invalid loss_agg_mode: {loss_agg_mode}")
 
     return loss
 
diff --git a/verl/workers/actor/megatron_actor.py b/verl/workers/actor/megatron_actor.py
index 6b279fc8903..5b64f1901f0 100644
--- a/verl/workers/actor/megatron_actor.py
+++ b/verl/workers/actor/megatron_actor.py
@@ -484,6 +484,7 @@ def loss_func(output, data, meta_info):
 
                 entropy_coeff = self.config.entropy_coeff
                 loss_agg_mode = self.config.loss_agg_mode
+
                 loss_mode = self.config.policy_loss.get("loss_mode", "vanilla")
 
                 policy_loss_fn = get_policy_loss_fn(loss_mode)