[Model] Add support for JetBrains' Mellum v2 code generation model (#27375)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: Jiminator <69131491+Jiminator@users.noreply.github.com>
Co-authored-by: Alex Nails <alex.nails@radixark.ai>

python/sglang/srt/configs/model_config.py

@@ -1901,6 +1901,7 @@ def is_hybrid_swa_model(
         "Gemma4ForConditionalGeneration",
         "Gemma4UnifiedForConditionalGeneration",
         "LagunaForCausalLM",
+        "MellumForCausalLM",
         "UnlimitedOCRForCausalLM",
     }
     if any(arch in hybrid_swa_archs for arch in model_architectures):
@@ -1972,6 +1973,8 @@ def get_hybrid_layer_ids(
         "Gemma4ForCausalLM" in model_architectures
         or "Gemma4ForConditionalGeneration" in model_architectures
         or "Gemma4UnifiedForConditionalGeneration" in model_architectures
+        or "LagunaForCausalLM" in model_architectures
+        or "MellumForCausalLM" in model_architectures
     ):
         layer_types = getattr(hf_text_config, "layer_types", [])
         swa_attention_layer_ids = [
@@ -1980,14 +1983,6 @@ def get_hybrid_layer_ids(
         full_attention_layer_ids = [
             i for i, x in enumerate(layer_types) if x == "full_attention"
         ]
-    elif "LagunaForCausalLM" in model_architectures:
-        layer_types = getattr(hf_text_config, "layer_types", [])
-        swa_attention_layer_ids = [
-            i for i, x in enumerate(layer_types) if x == "sliding_attention"
-        ]
-        full_attention_layer_ids = [
-            i for i, x in enumerate(layer_types) if x == "full_attention"
-        ]
     elif "UnlimitedOCRForCausalLM" in model_architectures:
         swa_attention_layer_ids = list(range(num_hidden_layers))
         full_attention_layer_ids = []

python/sglang/srt/models/mellum.py

@@ -0,0 +1,595 @@
+# Copyright 2023-2026 SGLang Team
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ==============================================================================
+
+"""
+Inference-only Mellum (JetBrains/Mellum2-12B-A2.5B family) Qwen3-MoE variant with
+interleaved sliding-window/full attention, per-layer-type RoPE and mixed dense/MoE
+MLP layers.
+"""
+
+import logging
+from typing import Any, Dict, Optional, Tuple, cast
+
+import torch
+from torch import nn
+from transformers import PretrainedConfig
+
+from sglang.srt.distributed import get_pp_group
+from sglang.srt.layers.communicator import LayerCommunicator, LayerScatterModes
+from sglang.srt.layers.layernorm import RMSNorm
+from sglang.srt.layers.linear import QKVParallelLinear, RowParallelLinear
+from sglang.srt.layers.quantization.base_config import QuantizationConfig
+from sglang.srt.layers.radix_attention import RadixAttention
+from sglang.srt.layers.rotary_embedding import MRotaryEmbedding, get_rope
+from sglang.srt.layers.rotary_embedding.yarn import (
+    yarn_find_correction_range,
+    yarn_get_mscale,
+)
+from sglang.srt.layers.vocab_parallel_embedding import ParallelLMHead
+from sglang.srt.model_executor.forward_batch_info import ForwardBatch, PPProxyTensors
+from sglang.srt.models.qwen3_moe import (
+    Qwen3MoeAttention,
+    Qwen3MoeDecoderLayer,
+    Qwen3MoeForCausalLM,
+    Qwen3MoeMLP,
+    Qwen3MoeModel,
+    Qwen3MoeSparseMoeBlock,
+)
+from sglang.srt.models.utils import (
+    apply_qk_norm,
+    create_fused_set_kv_buffer_arg,
+    enable_fused_set_kv_buffer,
+)
+from sglang.srt.runtime_context import get_parallel, get_server_args
+from sglang.srt.utils import add_prefix, is_cuda
+
+_is_cuda = is_cuda()
+
+if _is_cuda:
+    from sglang.jit_kernel.fused_qknorm_rope import (
+        can_use_fused_qk_norm_rope,
+        fused_qk_norm_rope,
+    )
+
+logger = logging.getLogger(__name__)
+
+
+def _get_rope_type(rope_params: Dict[str, Any]) -> str:
+    return rope_params.get("rope_type") or rope_params.get("type") or "default"
+
+
+def _compute_yarn_from_rope_params(
+    rope_params: Dict[str, Any],
+    head_dim: int,
+    max_position_embeddings: int,
+) -> Dict[str, float]:
+    _default = {"factor": 1.0, "low": 0, "high": 0, "attention_factor": 1.0}
+    if rope_params is None:
+        return _default
+
+    if _get_rope_type(rope_params) == "default":
+        return _default
+
+    base = rope_params.get("rope_theta", 10000)
+    dim = head_dim
+    factor = rope_params.get("factor", 1.0)
+    attention_factor = rope_params.get("attention_factor")
+    mscale = rope_params.get("mscale")
+    mscale_all_dim = rope_params.get("mscale_all_dim")
+
+    original_max_position_embeddings = rope_params.get(
+        "original_max_position_embeddings", max_position_embeddings
+    )
+    if "original_max_position_embeddings" in rope_params:
+        factor = max_position_embeddings / original_max_position_embeddings
+
+    if attention_factor is None:
+        if mscale and mscale_all_dim:
+            attention_factor = float(
+                yarn_get_mscale(factor, mscale)
+                / yarn_get_mscale(factor, mscale_all_dim)
+            )
+        else:
+            attention_factor = yarn_get_mscale(factor)
+
+    beta_fast = rope_params.get("beta_fast") or 32
+    beta_slow = rope_params.get("beta_slow") or 1
+
+    truncate = rope_params.get("truncate", True)
+    low, high = yarn_find_correction_range(
+        beta_fast, beta_slow, dim, base, original_max_position_embeddings, truncate
+    )
+    return {
+        "factor": factor,
+        "low": low,
+        "high": high,
+        "attention_factor": attention_factor,
+    }
+
+
+def get_attention_sliding_window_size(config: PretrainedConfig) -> Optional[int]:
+    sw = getattr(config, "sliding_window", None)
+    if sw is not None:
+        return sw - 1
+    return None
+
+
+class MellumMLP(Qwen3MoeMLP):
+    # Qwen3MoeDecoderLayer.forward calls self.mlp(x, forward_batch).
+    def forward(self, x, forward_batch=None):
+        return super().forward(x)
+
+
+class MellumAttention(Qwen3MoeAttention):
+    """
+    Qwen3MoeAttention with per-layer sliding window and per-layer-type RoPE.
+    """
+
+    def __init__(
+        self,
+        hidden_size: int,
+        num_heads: int,
+        num_kv_heads: int,
+        layer_id: int = 0,
+        start_layer: int = 0,
+        rope_params: Optional[Dict[str, Any]] = None,
+        max_position_embeddings: int = 8192,
+        head_dim: Optional[int] = None,
+        rms_norm_eps: float = 1e-06,
+        attention_bias: bool = False,
+        config: Optional[PretrainedConfig] = None,
+        quant_config: Optional[QuantizationConfig] = None,
+        prefix: str = "",
+        sliding_window_size: int = -1,
+        alt_stream: Optional[torch.cuda.Stream] = None,
+    ) -> None:
+        # Skip Qwen3MoeAttention.__init__ so that we can pass the correct
+        # correct per-layer rope_theta / rope_scaling and sliding window.
+        nn.Module.__init__(self)
+
+        self.hidden_size = hidden_size
+        self.start_layer = start_layer
+
+        attn_tp_rank = get_parallel().attn_tp_rank
+        attn_tp_size = get_parallel().attn_tp_size
+
+        self.config = config
+        self.total_num_heads = num_heads
+        assert self.total_num_heads % attn_tp_size == 0
+        self.num_heads = self.total_num_heads // attn_tp_size
+        self.total_num_kv_heads = num_kv_heads
+        if self.total_num_kv_heads >= attn_tp_size:
+            assert self.total_num_kv_heads % attn_tp_size == 0
+        else:
+            assert attn_tp_size % self.total_num_kv_heads == 0
+        self.num_kv_heads = max(1, self.total_num_kv_heads // attn_tp_size)
+        self.head_dim = head_dim or hidden_size // self.total_num_heads
+        self.q_size = self.num_heads * self.head_dim
+        self.kv_size = self.num_kv_heads * self.head_dim
+        self.scaling = self.head_dim**-0.5
+        self.max_position_embeddings = max_position_embeddings
+
+        rope_params = rope_params or {}
+        self.rope_theta = rope_params.get("rope_theta", 10000.0)
+        rope_scaling = rope_params if _get_rope_type(rope_params) != "default" else None
+
+        self.tp_rank = get_parallel().tp_rank
+
+        self.qkv_proj = QKVParallelLinear(
+            hidden_size,
+            self.head_dim,
+            self.total_num_heads,
+            self.total_num_kv_heads,
+            bias=attention_bias,
+            quant_config=quant_config,
+            tp_rank=attn_tp_rank,
+            tp_size=attn_tp_size,
+            prefix=add_prefix("qkv_proj", prefix),
+        )
+
+        self.o_proj = RowParallelLinear(
+            self.total_num_heads * self.head_dim,
+            hidden_size,
+            bias=attention_bias,
+            quant_config=quant_config,
+            tp_rank=attn_tp_rank,
+            tp_size=attn_tp_size,
+            reduce_results=False,
+            prefix=add_prefix("o_proj", prefix),
+        )
+
+        self.rotary_emb = get_rope(
+            self.head_dim,
+            rotary_dim=self.head_dim,
+            max_position=max_position_embeddings,
+            base=self.rope_theta,
+            rope_scaling=rope_scaling,
+        )
+        self.compatible_with_fused_kv_buffer = not isinstance(
+            self.rotary_emb, MRotaryEmbedding
+        )
+        self.compatible_with_fused_qk_norm_rope = not isinstance(
+            self.rotary_emb, MRotaryEmbedding
+        ) and self.head_dim in (64, 128, 256)
+
+        # TODO: Precompute YaRN parameters in Qwen3MoeAttention for all
+        # inheriting models instead of recomputing them in every forward pass.
+        self._yarn_params = _compute_yarn_from_rope_params(
+            rope_params, self.head_dim, max_position_embeddings
+        )
+        _yarn_factor = self._yarn_params["factor"]
+
+        self.use_fused_qk_norm_rope = (
+            get_server_args().enable_fused_qk_norm_rope
+            and self.compatible_with_fused_qk_norm_rope
+            and _is_cuda
+            and can_use_fused_qk_norm_rope(
+                self.head_dim,
+                self.rotary_emb.is_neox_style,
+                torch.bfloat16,
+                _yarn_factor != 1.0,
+            )
+        )
+        self._used_fused_qk_norm_rope_last_call = False
+
+        self.attn = RadixAttention(
+            self.num_heads,
+            self.head_dim,
+            self.scaling,
+            num_kv_heads=self.num_kv_heads,
+            layer_id=layer_id,
+            sliding_window_size=sliding_window_size,
+            prefix=add_prefix("attn", prefix),
+        )
+
+        self.q_norm = RMSNorm(self.head_dim, eps=rms_norm_eps)
+        self.k_norm = RMSNorm(self.head_dim, eps=rms_norm_eps)
+        self.alt_stream = alt_stream
+
+    def forward_prepare_npu(self, positions, hidden_states, forward_batch):
+        raise NotImplementedError(
+            "Mellum per-layer RoPE is incompatible with the shared rotary "
+            "cos/sin priming in Qwen3MoeAttention.forward_prepare_npu"
+        )
+
+    def apply_qk_norm_rope(self, qkv, positions, forward_batch):
+        # Overridden to use pre-computed per-layer YaRN params.
+        use_fused = self.use_fused_qk_norm_rope and qkv.dtype == torch.bfloat16
+        if use_fused:
+            theta = self.rope_theta
+            fused_qk_norm_rope(
+                qkv,
+                self.num_heads,
+                self.num_kv_heads,
+                self.num_kv_heads,
+                self.head_dim,
+                self.q_norm.variance_epsilon,
+                self.q_norm.weight,
+                self.k_norm.weight,
+                theta,
+                self.rotary_emb.is_neox_style,
+                positions,
+                self._yarn_params["factor"],
+                self._yarn_params["low"],
+                self._yarn_params["high"],
+                self._yarn_params["attention_factor"],
+            )
+            q, k, v = qkv.split([self.q_size, self.kv_size, self.kv_size], dim=-1)
+            self._used_fused_qk_norm_rope_last_call = True
+        else:
+            q, k, v = qkv.split([self.q_size, self.kv_size, self.kv_size], dim=-1)
+            q, k = apply_qk_norm(
+                q=q,
+                k=k,
+                q_norm=self.q_norm,
+                k_norm=self.k_norm,
+                head_dim=self.head_dim,
+                alt_stream=self.alt_stream,
+            )
+            q, k = self.rotary_emb(
+                positions,
+                q,
+                k,
+                fused_set_kv_buffer_arg=(
+                    create_fused_set_kv_buffer_arg(
+                        value=v,
+                        layer=self.attn,
+                        forward_batch=forward_batch,
+                    )
+                    if enable_fused_set_kv_buffer(forward_batch)
+                    and self.compatible_with_fused_kv_buffer
+                    else None
+                ),
+            )
+            self._used_fused_qk_norm_rope_last_call = False
+        return q, k, v
+
+
+class MellumDecoderLayer(Qwen3MoeDecoderLayer):
+    """
+    Qwen3MoeDecoderLayer with per-layer attention type, RoPE and dense/sparse MLP.
+    """
+
+    def __init__(
+        self,
+        config: PretrainedConfig,
+        layer_id: int,
+        start_layer: int = 0,
+        quant_config: Optional[QuantizationConfig] = None,
+        prefix: str = "",
+        alt_stream: Optional[torch.cuda.Stream] = None,
+    ) -> None:
+        # As with MellumAttention, skip parent __init__ to wire up
+        # per-layer attention params, RoPE params, sliding window
+        # and mixed dense/MoE MLP.
+        nn.Module.__init__(self)
+
+        self.config = config
+        cfg = cast(Any, config)
+        self.hidden_size = cfg.hidden_size
+        self.layer_id = layer_id
+
+        layer_types = cfg.layer_types
+        if len(layer_types) != cfg.num_hidden_layers:
+            raise ValueError(
+                "Expected len(layer_types) == num_hidden_layers, got "
+                f"{len(layer_types)} and {cfg.num_hidden_layers}"
+            )
+        layer_type = layer_types[layer_id]
+        if layer_type not in ("sliding_attention", "full_attention"):
+            raise ValueError(
+                f"Unsupported layer_types[{layer_id}]={layer_type}; "
+                "expected 'sliding_attention' or 'full_attention'"
+            )
+
+        rope_parameters = cfg.rope_parameters
+        rope_params = rope_parameters.get(layer_type)
+        if rope_params is None:
+            raise ValueError(
+                f"Missing rope_parameters[{layer_type}] for Mellum layer {layer_id}"
+            )
+
+        # Mellum routes SWA per-layer via layer_types. Preserve the configured
+        # window regardless of legacy use_sliding_window post-init side effects.
+        if layer_type == "sliding_attention":
+            sliding_window_size = get_attention_sliding_window_size(config)
+            if sliding_window_size is None:
+                raise ValueError(
+                    "Missing config.sliding_window for Mellum "
+                    f"sliding_attention layer {layer_id}"
+                )
+        else:
+            sliding_window_size = -1
+
+        max_position_embeddings = cfg.max_position_embeddings
+        head_dim = cfg.head_dim
+        rms_norm_eps = cfg.rms_norm_eps
+        attention_bias = cfg.attention_bias
+
+        self.self_attn = MellumAttention(
+            hidden_size=self.hidden_size,
+            num_heads=cfg.num_attention_heads,
+            num_kv_heads=cfg.num_key_value_heads,
+            layer_id=layer_id,
+            start_layer=start_layer,
+            rope_params=rope_params,
+            max_position_embeddings=max_position_embeddings,
+            head_dim=head_dim,
+            rms_norm_eps=rms_norm_eps,
+            attention_bias=attention_bias,
+            config=config,
+            quant_config=quant_config,
+            prefix=add_prefix("self_attn", prefix),
+            sliding_window_size=sliding_window_size,
+            alt_stream=alt_stream,
+        )
+
+        self.attn_tp_size = get_parallel().attn_tp_size
+        self.attn_tp_rank = get_parallel().attn_tp_rank
+
+        mlp_layer_types = cfg.mlp_layer_types
+        num_experts = cfg.num_experts
+
+        if len(mlp_layer_types) != cfg.num_hidden_layers:
+            raise ValueError(
+                "Expected len(mlp_layer_types) == num_hidden_layers, got "
+                f"{len(mlp_layer_types)} and {cfg.num_hidden_layers}"
+            )
+
+        def _is_sparse(lid: int) -> bool:
+            if lid < 0 or lid >= cfg.num_hidden_layers:
+                return False
+            mlp_type = mlp_layer_types[lid]
+            if mlp_type not in ("sparse", "dense"):
+                raise ValueError(
+                    f"Unsupported mlp_layer_types[{lid}]={mlp_type}; "
+                    "expected 'sparse' or 'dense'"
+                )
+            return mlp_type == "sparse"
+
+        self.is_layer_sparse = _is_sparse(layer_id)
+
+        if self.is_layer_sparse:
+            if num_experts <= 0:
+                raise ValueError(
+                    "Sparse MLP requested but num_experts <= 0 in Mellum config"
+                )
+            self.mlp = Qwen3MoeSparseMoeBlock(
+                layer_id=layer_id,
+                config=cfg,
+                quant_config=quant_config,
+                prefix=add_prefix("mlp", prefix),
+            )
+        else:
+            self.mlp = MellumMLP(
+                hidden_size=cfg.hidden_size,
+                intermediate_size=cfg.intermediate_size,
+                hidden_act=cfg.hidden_act,
+                quant_config=quant_config,
+                prefix=add_prefix("mlp", prefix),
+            )
+
+        is_previous_layer_sparse = _is_sparse(layer_id - 1)
+        is_next_layer_sparse = _is_sparse(layer_id + 1)
+
+        self.layer_scatter_modes = LayerScatterModes.init_new(
+            layer_id=layer_id,
+            num_layers=cfg.num_hidden_layers,
+            is_layer_sparse=self.is_layer_sparse,
+            is_previous_layer_sparse=is_previous_layer_sparse,
+            is_next_layer_sparse=is_next_layer_sparse,
+        )
+
+        self.input_layernorm = RMSNorm(cfg.hidden_size, eps=rms_norm_eps)
+        self.post_attention_layernorm = RMSNorm(cfg.hidden_size, eps=rms_norm_eps)
+
+        self.layer_communicator = LayerCommunicator(
+            layer_scatter_modes=self.layer_scatter_modes,
+            input_layernorm=self.input_layernorm,
+            post_attention_layernorm=self.post_attention_layernorm,
+            allow_reduce_scatter=True,
+            is_last_layer=(layer_id == cfg.num_hidden_layers - 1),
+        )
+
+
+class MellumModel(Qwen3MoeModel):
+    def __init__(
+        self,
+        config: PretrainedConfig,
+        quant_config: Optional[QuantizationConfig] = None,
+        prefix: str = "",
+    ) -> None:
+        super().__init__(
+            config=config,
+            quant_config=quant_config,
+            prefix=prefix,
+            decoder_layer_type=MellumDecoderLayer,
+        )
+
+
+class MellumForCausalLM(Qwen3MoeForCausalLM):
+    packed_modules_mapping = {
+        "qkv_proj": ["q_proj", "k_proj", "v_proj"],
+        "gate_up_proj": ["gate_proj", "up_proj"],
+    }
+
+    def __init__(
+        self,
+        config: PretrainedConfig,
+        quant_config: Optional[QuantizationConfig] = None,
+        prefix: str = "",
+    ) -> None:
+        nn.Module.__init__(self)
+
+        from sglang.srt.layers.logits_processor import LogitsProcessor
+
+        self.pp_group = get_pp_group()
+        cfg = cast(Any, config)
+        self.config = cfg
+        self.quant_config = quant_config
+
+        if hasattr(cfg, "max_window_layers"):
+            logger.warning(
+                "Mellum ignores config.max_window_layers and relies on "
+                "config.layer_types for per-layer SWA routing."
+            )
+
+        self.model = MellumModel(cfg, quant_config, prefix=add_prefix("model", prefix))
+        # Over-approximation is safe: the non-fused path accepts int32 positions.
+        self.use_fused_qk_norm_rope = any(
+            getattr(
+                getattr(layer, "self_attn", None),
+                "use_fused_qk_norm_rope",
+                False,
+            )
+            for layer in self.model.layers
+        )
+        self.lm_head = ParallelLMHead(
+            cfg.vocab_size,
+            cfg.hidden_size,
+            quant_config=quant_config,
+            prefix=add_prefix("lm_head", prefix),
+            use_attn_tp_group=get_server_args().enable_dp_lm_head,
+        )
+        self.logits_processor = LogitsProcessor(cfg)
+        self.capture_aux_hidden_states = False
+
+        self.attn_cp_size = get_parallel().attn_cp_size
+        self.attn_cp_rank = get_parallel().attn_cp_rank
+        self.moe_dp_size = get_parallel().moe_dp_size
+
+        assert self.attn_cp_size % self.moe_dp_size == 0, (
+            f"attn_cp_size ({self.attn_cp_size}) must be divisible by "
+            f"moe_dp_size ({self.moe_dp_size})"
+        )
+
+    def _prepare_positions(
+        self,
+        positions: torch.Tensor,
+        device: torch.device,
+    ) -> torch.Tensor:
+        if self.use_fused_qk_norm_rope:
+            return positions.view(-1).to(dtype=torch.int32, device=device).contiguous()
+        return positions
+
+    @torch.no_grad()
+    def forward(
+        self,
+        input_ids: torch.Tensor,
+        positions: torch.Tensor,
+        forward_batch: ForwardBatch,
+        input_embeds: Optional[torch.Tensor] = None,
+        pp_proxy_tensors: Optional[PPProxyTensors] = None,
+    ) -> torch.Tensor:
+        if pp_proxy_tensors is not None:
+            positions_device = pp_proxy_tensors["hidden_states"].device
+        elif input_embeds is not None:
+            positions_device = input_embeds.device
+        else:
+            positions_device = input_ids.device
+        positions = self._prepare_positions(positions, positions_device)
+        return super().forward(
+            input_ids,
+            positions,
+            forward_batch,
+            input_embeds,
+            pp_proxy_tensors=pp_proxy_tensors,
+        )
+
+    @torch.no_grad()
+    def forward_split_prefill(
+        self,
+        input_ids: torch.Tensor,
+        positions: torch.Tensor,
+        forward_batch: ForwardBatch,
+        split_interval: Tuple[int, int],
+        input_embeds: Optional[torch.Tensor] = None,
+    ):
+        positions_device = (
+            input_embeds.device if input_embeds is not None else input_ids.device
+        )
+        positions = self._prepare_positions(positions, positions_device)
+        return super().forward_split_prefill(
+            input_ids,
+            positions,
+            forward_batch,
+            split_interval,
+            input_embeds,
+        )
+
+    def get_attention_sliding_window_size(self) -> Optional[int]:
+        return get_attention_sliding_window_size(self.config)
+
+
+EntryClass = MellumForCausalLM

python/sglang/srt/utils/hf_transformers/common.py

@@ -142,6 +142,36 @@ try:
 except ImportError:
     pass
 
+# Newer transformers versions (>=5.10.2) expose MellumConfig directly,
+# but fallback to Qwen3MoeConfig for older versions.
+try:
+    import transformers as _hf_transformers
+
+    _HFMellumConfig = getattr(_hf_transformers, "MellumConfig", None)
+
+    if _HFMellumConfig is not None:
+        _CONFIG_REGISTRY["mellum"] = _HFMellumConfig
+    else:
+        from transformers import Qwen3MoeConfig as _HFQwen3MoeConfig
+
+        class _MellumConfigAlias(_HFQwen3MoeConfig):
+            model_type = "mellum"
+
+            def __post_init__(self, **kwargs):
+                # Qwen3MoeConfig.__post_init__ wipes sliding_window unless
+                # use_sliding_window=True. Mellum gates sliding attention
+                # per-layer via layer_types, so preserve sliding_window
+                # regardless of the legacy use_sliding_window flag.
+                sliding_window = getattr(self, "sliding_window", None)
+                super().__post_init__(**kwargs)
+                self.sliding_window = sliding_window
+
+        _CONFIG_REGISTRY["mellum"] = _MellumConfigAlias
+
+except ImportError:
+    pass
+
+
 try:
     from transformers import Gemma4Config as _HFGemma4Config