diff --git a/python/sglang/multimodal_gen/runtime/models/dits/cosmos3video.py b/python/sglang/multimodal_gen/runtime/models/dits/cosmos3video.py
index 6ff98af59..9f76363ea 100644
--- a/python/sglang/multimodal_gen/runtime/models/dits/cosmos3video.py
+++ b/python/sglang/multimodal_gen/runtime/models/dits/cosmos3video.py
@@ -231,6 +231,8 @@ def _apply_qwen3_qk_norm_rope(
     head_dim: int,
     cos_sin_cache: torch.Tensor,
     rope_cache_positions: torch.Tensor,
+    *,
+    round_norm_before_rope: bool = False,
 ) -> tuple[torch.Tensor, torch.Tensor]:
     return apply_qk_norm_rope(
         q=q,
@@ -242,6 +244,7 @@ def _apply_qwen3_qk_norm_rope(
         is_neox=True,
         positions=rope_cache_positions,
         allow_strided_qk=True,
+        round_norm_before_rope=round_norm_before_rope,
     )
 
 
@@ -256,6 +259,8 @@ def _apply_qwen3_qk_norm_rope_pack_kv(
     head_dim: int,
     cos_sin_cache: torch.Tensor,
     rope_cache_positions: torch.Tensor,
+    *,
+    round_norm_before_rope: bool = False,
 ) -> tuple[torch.Tensor, torch.Tensor, torch.Tensor]:
     batch_size, suffix_tokens, _, _ = q.shape
     prefix_tokens = k_prefix.shape[1]
@@ -283,6 +288,7 @@ def _apply_qwen3_qk_norm_rope_pack_kv(
         eps=q_norm.variance_epsilon,
         head_dim=head_dim,
         rope_dim=cos_sin_cache.shape[-1],
+        round_norm_before_rope=round_norm_before_rope,
     )
     return q, packed_kv[0], packed_kv[1]
 
@@ -746,6 +752,7 @@ class Cosmos3CrossAttention(nn.Module):
         cos_sin_cache: torch.Tensor,
         rope_cache_positions: torch.Tensor,
         use_fused_qk_norm_rope: bool,
+        round_norm_before_rope: bool = False,
     ) -> torch.Tensor:
         """Cross-attention from GEN to cached UND K/V.
 
@@ -795,6 +802,7 @@ class Cosmos3CrossAttention(nn.Module):
                 True,
                 q.dtype,
                 cos_sin_cache.dtype,
+                round_norm_before_rope=round_norm_before_rope,
                 pack_kv=True,
             )
         )
@@ -810,6 +818,7 @@ class Cosmos3CrossAttention(nn.Module):
                 self.head_dim,
                 cos_sin_cache,
                 rope_cache_positions,
+                round_norm_before_rope=round_norm_before_rope,
             )
             out = self.attn.forward(q, packed_k, packed_v)
         elif use_fused_qk_norm_rope:
@@ -821,6 +830,7 @@ class Cosmos3CrossAttention(nn.Module):
                 self.head_dim,
                 cos_sin_cache,
                 rope_cache_positions,
+                round_norm_before_rope=round_norm_before_rope,
             )
         else:
             q, k = _apply_qwen3_qk_norm_rope_split(
@@ -960,6 +970,7 @@ class Cosmos3GenDecoderLayer(nn.Module):
         cos_sin_cache: torch.Tensor,
         rope_cache_positions: torch.Tensor,
         use_fused_qk_norm_rope: bool,
+        round_norm_before_rope: bool = False,
         residual: torch.Tensor | None = None,
     ) -> tuple[torch.Tensor, torch.Tensor]:
         # Fused add+rmsnorm: each `(hidden_states, residual) = norm(...)`
@@ -979,6 +990,7 @@ class Cosmos3GenDecoderLayer(nn.Module):
             cos_sin_cache,
             rope_cache_positions,
             use_fused_qk_norm_rope,
+            round_norm_before_rope,
         )
 
         hidden_states, residual = self.post_attention_layernorm(hidden_states, residual)
@@ -1126,6 +1138,7 @@ class Cosmos3OmniTransformer(CachableDiT, LayerwiseOffloadableModuleMixin):
         self.rms_norm_eps = arch.rms_norm_eps
         self.hidden_act = arch.hidden_act
         self.rope_theta = arch.rope_theta
+        self._gen_layers_torch_compiled = False
 
         # The checkpoint may override the activation (and thus the MLP weight
         # layout), so bind the arch-derived mappings on the instance.
@@ -1618,11 +1631,21 @@ class Cosmos3OmniTransformer(CachableDiT, LayerwiseOffloadableModuleMixin):
                 vis_pos_ids = vis_pos_ids.view(
                     3, batch_size, self.sp_size, local_seq_len
                 )[:, :, self.sp_rank, :]
-            self.cached_gen_rope_inputs[cache_key] = (
+            cos_sin_gen, gen_rope_cache_positions = (
                 self.language_model.rotary_emb.build_rope_cache_inputs(
                     vis_pos_ids, cache_dtype=hidden_gen.dtype
                 )
             )
+            if T == 1 and not self._gen_layers_torch_compiled:
+                # build_rope_cache_inputs already rounds through cache_dtype
+                # before returning FP32 storage. Keep that rounded cache in the
+                # activation dtype so the exact fused QKNorm+RoPE kernel can
+                # consume it without repeating the cast in every GEN layer.
+                cos_sin_gen = cos_sin_gen.to(hidden_gen.dtype)
+            self.cached_gen_rope_inputs[cache_key] = (
+                cos_sin_gen,
+                gen_rope_cache_positions,
+            )
 
         cos_sin_gen, gen_rope_cache_positions = self.cached_gen_rope_inputs[cache_key]
 
@@ -1631,7 +1654,22 @@ class Cosmos3OmniTransformer(CachableDiT, LayerwiseOffloadableModuleMixin):
         # fused add+rmsnorm path instead of separate add + norm kernels.
         cached_kv_for_key = self.cached_kv[cache_key]
         residual: torch.Tensor | None = None
-        use_fused_qk_norm_rope = T > 1
+        round_norm_before_rope = T == 1
+        use_fused_qk_norm_rope = T > 1 or (
+            hidden_gen.device.type == "cuda"
+            and not torch.compiler.is_compiling()
+            and not self._gen_layers_torch_compiled
+            and get_sp_world_size() == 1
+            and can_use_fused_inplace_qknorm_rope(
+                self.head_dim,
+                cos_sin_gen.shape[-1],
+                True,
+                hidden_gen.dtype,
+                cos_sin_gen.dtype,
+                round_norm_before_rope=True,
+                pack_kv=True,
+            )
+        )
         for i, layer in enumerate(self.gen_layers):
             k_und, v_und = cached_kv_for_key[i]
             hidden_gen, residual = layer(
@@ -1641,6 +1679,7 @@ class Cosmos3OmniTransformer(CachableDiT, LayerwiseOffloadableModuleMixin):
                 cos_sin_gen,
                 gen_rope_cache_positions,
                 use_fused_qk_norm_rope,
+                round_norm_before_rope,
                 residual=residual,
             )
 
diff --git a/python/sglang/multimodal_gen/runtime/pipelines_core/stages/model_specific_stages/cosmos3.py b/python/sglang/multimodal_gen/runtime/pipelines_core/stages/model_specific_stages/cosmos3.py
index 6d8e0cb59..697ce3bbf 100644
--- a/python/sglang/multimodal_gen/runtime/pipelines_core/stages/model_specific_stages/cosmos3.py
+++ b/python/sglang/multimodal_gen/runtime/pipelines_core/stages/model_specific_stages/cosmos3.py
@@ -870,6 +870,7 @@ class Cosmos3DenoisingStage(PipelineStage):
                 len(gen_layers),
                 compile_kwargs,
             )
+            transformer._gen_layers_torch_compiled = True
             for i, layer in enumerate(gen_layers):
                 gen_layers[i] = torch.compile(layer, **compile_kwargs)
         else:
diff --git a/test/registered/kernels/ops/diffusion/test_qknorm_rope.py b/test/registered/kernels/ops/diffusion/test_qknorm_rope.py
index be3b67d6e..867d23c78 100644
--- a/test/registered/kernels/ops/diffusion/test_qknorm_rope.py
+++ b/test/registered/kernels/ops/diffusion/test_qknorm_rope.py
@@ -413,6 +413,99 @@ def test_qknorm_rope_pack_kv_matches_separate_ops() -> None:
     assert torch.equal(packed_v_ref, packed_kv[1])
 
 
+def test_qknorm_rope_pack_kv_preserves_split_bf16_rounding() -> None:
+    from sgl_kernel import rotary_embedding
+
+    from sglang.kernels.ops.diffusion.qknorm_rope import (
+        fused_qknorm_rope_pack_kv,
+    )
+    from sglang.kernels.ops.layernorm.norm import fused_inplace_qknorm
+
+    batch_size = 1
+    prefix_tokens, suffix_tokens = 17, 1024
+    num_q_heads, num_kv_heads, head_dim = 32, 8, 64
+    num_heads = num_q_heads + 2 * num_kv_heads
+    qkv = torch.randn(
+        batch_size,
+        suffix_tokens,
+        num_heads,
+        head_dim,
+        device=DEVICE,
+        dtype=DTYPE,
+    )
+    prefix_qkv = torch.randn(
+        batch_size,
+        prefix_tokens,
+        num_heads,
+        head_dim,
+        device=DEVICE,
+        dtype=DTYPE,
+    )
+    k_prefix = prefix_qkv[:, :, num_q_heads : num_q_heads + num_kv_heads]
+    v_prefix = prefix_qkv[:, :, num_q_heads + num_kv_heads :]
+    q_weight = torch.randn(head_dim, device=DEVICE, dtype=DTYPE)
+    k_weight = torch.randn(head_dim, device=DEVICE, dtype=DTYPE)
+    positions = torch.arange(
+        batch_size * suffix_tokens, device=DEVICE, dtype=torch.int64
+    )
+    cos_sin_cache = create_cos_sin_cache(head_dim, batch_size * suffix_tokens).to(DTYPE)
+
+    qkv_ref = qkv.clone()
+    q_ref = qkv_ref[:, :, :num_q_heads]
+    k_ref = qkv_ref[:, :, num_q_heads : num_q_heads + num_kv_heads]
+    v_ref = qkv_ref[:, :, num_q_heads + num_kv_heads :]
+    fused_inplace_qknorm(
+        q_ref.view(-1, num_q_heads, head_dim),
+        k_ref.view(-1, num_kv_heads, head_dim),
+        q_weight,
+        k_weight,
+        eps=1e-6,
+    )
+    rotary_embedding(
+        positions,
+        q_ref.view(-1, num_q_heads * head_dim),
+        k_ref.view(-1, num_kv_heads * head_dim),
+        head_dim,
+        cos_sin_cache,
+        True,
+    )
+    packed_k_ref = torch.cat([k_prefix, k_ref], dim=1)
+    packed_v_ref = torch.cat([v_prefix, v_ref], dim=1)
+
+    qkv_fused = qkv.clone()
+    q_fused = qkv_fused[:, :, :num_q_heads]
+    k_fused = qkv_fused[:, :, num_q_heads : num_q_heads + num_kv_heads]
+    v_fused = qkv_fused[:, :, num_q_heads + num_kv_heads :]
+    packed_kv = torch.empty(
+        2,
+        batch_size,
+        prefix_tokens + suffix_tokens,
+        num_kv_heads,
+        head_dim,
+        device=DEVICE,
+        dtype=DTYPE,
+    )
+    fused_qknorm_rope_pack_kv(
+        q_fused,
+        k_fused,
+        v_fused,
+        k_prefix,
+        v_prefix,
+        packed_kv,
+        q_weight,
+        k_weight,
+        cos_sin_cache,
+        positions,
+        is_neox=True,
+        rope_dim=head_dim,
+        round_norm_before_rope=True,
+    )
+
+    assert torch.equal(q_ref, q_fused)
+    assert torch.equal(packed_k_ref, packed_kv[0])
+    assert torch.equal(packed_v_ref, packed_kv[1])
+
+
 def test_qknorm_rope_accepts_empty_token_dimension() -> None:
     from sglang.kernels.ops.diffusion.qknorm_rope import fused_inplace_qknorm_rope