From 739e5945dc4b3ba30d84bdf6e637657abd4136b8 Mon Sep 17 00:00:00 2001
From: Yongye Zhu <zyy1102000@gmail.com>
Date: Mon, 13 Apr 2026 12:53:58 -0400
Subject: [PATCH] [Quantization] [Refactor] Create special
 "GptOssMxfp4MoeMethod" (#39604)

Signed-off-by: Yongye Zhu <zyy1102000@gmail.com>
---
 docs/design/moe_kernel_features.md            |  2 +-
 vllm/config/model.py                          |  3 +-
 vllm/model_executor/layers/fused_moe/layer.py |  2 +-
 .../layers/fused_moe/oracle/mxfp4.py          |  9 ++-
 .../layers/quantization/__init__.py           |  4 +-
 .../layers/quantization/awq_marlin.py         |  2 +-
 .../layers/quantization/base_config.py        | 13 ++++-
 .../layers/quantization/cpu_wna16.py          |  2 +-
 .../layers/quantization/gguf.py               |  2 +-
 .../layers/quantization/gptq_marlin.py        |  2 +-
 .../model_executor/layers/quantization/inc.py |  2 +-
 .../layers/quantization/modelopt.py           |  8 +--
 .../layers/quantization/moe_wna16.py          |  2 +-
 .../layers/quantization/mxfp4.py              | 55 ++++++++++++++++---
 .../layers/quantization/quark/quark_moe.py    |  8 +--
 vllm/model_executor/models/config.py          | 17 ++++++
 vllm/model_executor/models/gpt_oss.py         | 28 +++++++++-
 .../models/transformers/base.py               |  2 +-
 18 files changed, 129 insertions(+), 34 deletions(-)

diff --git a/docs/design/moe_kernel_features.md b/docs/design/moe_kernel_features.md
index c9dc1292d0..e2fd5645f6 100644
--- a/docs/design/moe_kernel_features.md
+++ b/docs/design/moe_kernel_features.md
@@ -59,7 +59,7 @@ Modular kernels are supported by the following `FusedMoEMethodBase` classes.
 - [`Fp8MoEMethod`][vllm.model_executor.layers.quantization.fp8.Fp8MoEMethod]
 - [`CompressedTensorsW4A4Nvfp4MoEMethod`][vllm.model_executor.layers.quantization.compressed_tensors.compressed_tensors_moe.compressed_tensors_moe_w4a4_nvfp4.CompressedTensorsW4A4Nvfp4MoEMethod]
 - [`CompressedTensorsW8A8Fp8MoEMethod`][vllm.model_executor.layers.quantization.compressed_tensors.compressed_tensors_moe.compressed_tensors_moe_w8a8_fp8.CompressedTensorsW8A8Fp8MoEMethod]
-- [`Mxfp4MoEMethod`][vllm.model_executor.layers.quantization.mxfp4.Mxfp4MoEMethod]
+- [`GptOssMxfp4MoEMethod`][vllm.model_executor.layers.quantization.mxfp4.GptOssMxfp4MoEMethod]
 - [`UnquantizedFusedMoEMethod`][vllm.model_executor.layers.fused_moe.layer.UnquantizedFusedMoEMethod]
 
 ## Fused Experts Kernels
diff --git a/vllm/config/model.py b/vllm/config/model.py
index 1cce7f9d94..2b767b21a7 100644
--- a/vllm/config/model.py
+++ b/vllm/config/model.py
@@ -951,6 +951,7 @@ class ModelConfig:
                 # Ensure heavy backends are probed last to avoid unnecessary
                 # imports during override detection (e.g., MXFP4 imports Triton)
                 "mxfp4",
+                "gpt_oss_mxfp4",
                 "cpu_awq",
                 "gguf",
             ]
@@ -966,7 +967,7 @@ class ModelConfig:
             for name in quantization_methods:
                 method = me_quant.get_quantization_config(name)
                 quantization_override = method.override_quantization_method(
-                    quant_cfg, self.quantization
+                    quant_cfg, self.quantization, hf_config=self.hf_config
                 )
                 if quantization_override is not None:
                     # Raise error if the override is not custom (custom would
diff --git a/vllm/model_executor/layers/fused_moe/layer.py b/vllm/model_executor/layers/fused_moe/layer.py
index c4fc1fd255..422f9e4276 100644
--- a/vllm/model_executor/layers/fused_moe/layer.py
+++ b/vllm/model_executor/layers/fused_moe/layer.py
@@ -1063,7 +1063,7 @@ class FusedMoE(CustomOp):
         expert_id: int,
         return_success: bool = False,
     ) -> bool | None:
-        if self.quant_config and self.quant_config.get_name() == "mxfp4":
+        if self.quant_config and self.quant_config.get_name() == "gpt_oss_mxfp4":
             # (FIXME) for gpt-oss all experts are combined
             if "bias" in weight_name:
                 dim1 = loaded_weight.shape[1]
diff --git a/vllm/model_executor/layers/fused_moe/oracle/mxfp4.py b/vllm/model_executor/layers/fused_moe/oracle/mxfp4.py
index d4a0817e0b..e7cfb881c1 100644
--- a/vllm/model_executor/layers/fused_moe/oracle/mxfp4.py
+++ b/vllm/model_executor/layers/fused_moe/oracle/mxfp4.py
@@ -194,7 +194,7 @@ def _backend_activation_key(backend: Mxfp4MoeBackend) -> QuantKey | None:
     return None
 
 
-def select_mxfp4_moe_backend(
+def select_gpt_oss_mxfp4_moe_backend(
     config: FusedMoEConfig,
 ) -> tuple[Mxfp4MoeBackend, type[mk.FusedMoEExperts] | None]:
     """
@@ -400,7 +400,7 @@ def mxfp4_round_up_hidden_size_and_intermediate_size(
     return hidden_size, intermediate_size
 
 
-def convert_to_mxfp4_moe_kernel_format(
+def convert_gpt_oss_weight_to_mxfp4_moe_kernel_format(
     mxfp4_backend: Mxfp4MoeBackend,
     layer: torch.nn.Module,
     w13_weight: torch.Tensor,
@@ -426,7 +426,10 @@ def convert_to_mxfp4_moe_kernel_format(
 
     sf_block_size = 32  # mxfp4 block size
 
-    if mxfp4_backend in (Mxfp4MoeBackend.MARLIN, Mxfp4MoeBackend.BATCHED_MARLIN):
+    if mxfp4_backend in (
+        Mxfp4MoeBackend.MARLIN,
+        Mxfp4MoeBackend.BATCHED_MARLIN,
+    ):
         from vllm.model_executor.layers.quantization.utils.marlin_utils_fp4 import (
             prepare_moe_mxfp4_layer_for_marlin,
         )
diff --git a/vllm/model_executor/layers/quantization/__init__.py b/vllm/model_executor/layers/quantization/__init__.py
index 1ac0f9ee9c..6313db78a8 100644
--- a/vllm/model_executor/layers/quantization/__init__.py
+++ b/vllm/model_executor/layers/quantization/__init__.py
@@ -30,6 +30,7 @@ QuantizationMethods = Literal[
     "torchao",
     "inc",
     "mxfp4",
+    "gpt_oss_mxfp4",
     "mxfp8",
     "cpu_awq",
     "online",
@@ -133,7 +134,7 @@ def get_quantization_config(quantization: str) -> type[QuantizationConfig]:
         ModelOptNvFp4Config,
     )
     from .moe_wna16 import MoeWNA16Config
-    from .mxfp4 import Mxfp4Config
+    from .mxfp4 import GptOssMxfp4Config, Mxfp4Config
     from .mxfp8 import Mxfp8Config
     from .online.base import OnlineQuantizationConfig
     from .torchao import TorchAOConfig
@@ -160,6 +161,7 @@ def get_quantization_config(quantization: str) -> type[QuantizationConfig]:
         "auto-round": INCConfig,
         "inc": INCConfig,
         "mxfp4": Mxfp4Config,
+        "gpt_oss_mxfp4": GptOssMxfp4Config,
         "mxfp8": Mxfp8Config,
         "cpu_awq": CPUAWQConfig,
         "online": OnlineQuantizationConfig,
diff --git a/vllm/model_executor/layers/quantization/awq_marlin.py b/vllm/model_executor/layers/quantization/awq_marlin.py
index be3001a7fa..cfad1f86fa 100644
--- a/vllm/model_executor/layers/quantization/awq_marlin.py
+++ b/vllm/model_executor/layers/quantization/awq_marlin.py
@@ -232,7 +232,7 @@ class AWQMarlinConfig(QuantizationConfig):
 
     @classmethod
     def override_quantization_method(
-        cls, hf_quant_cfg, user_quant
+        cls, hf_quant_cfg, user_quant, hf_config=None
     ) -> "QuantizationMethods | None":
         # Skip override to marlin kernels, as they are not
         # batch invariant
diff --git a/vllm/model_executor/layers/quantization/base_config.py b/vllm/model_executor/layers/quantization/base_config.py
index eedc62f7d4..344ddd8abd 100644
--- a/vllm/model_executor/layers/quantization/base_config.py
+++ b/vllm/model_executor/layers/quantization/base_config.py
@@ -110,13 +110,22 @@ class QuantizationConfig(ABC):
 
     @classmethod
     def override_quantization_method(
-        cls, hf_quant_cfg, user_quant
+        cls,
+        hf_quant_cfg: dict[str, Any],
+        user_quant: str | None,
+        hf_config: Any = None,
     ) -> QuantizationMethods | None:
         """
         Detects if this quantization method can support a given checkpoint
         format by overriding the user specified quantization method --
         this method should only be overwritten by subclasses in exceptional
-        circumstances
+        circumstances.
+
+        Args:
+            hf_quant_cfg: The checkpoint's quantization config dict.
+            user_quant: The user-specified quantization method string.
+            hf_config: The HuggingFace model config object (e.g. for
+                model_type checks). May be None if not available.
         """
         return None
 
diff --git a/vllm/model_executor/layers/quantization/cpu_wna16.py b/vllm/model_executor/layers/quantization/cpu_wna16.py
index 8ec569042d..aea1067ff2 100644
--- a/vllm/model_executor/layers/quantization/cpu_wna16.py
+++ b/vllm/model_executor/layers/quantization/cpu_wna16.py
@@ -104,7 +104,7 @@ class CPUAWQConfig(QuantizationConfig):
 
     @classmethod
     def override_quantization_method(
-        cls, hf_quant_cfg, user_quant
+        cls, hf_quant_cfg, user_quant, hf_config=None
     ) -> "QuantizationMethods | None":
         quant_method = hf_quant_cfg.get("quant_method", "").lower()
         if current_platform.is_cpu() and (quant_method == "awq"):
diff --git a/vllm/model_executor/layers/quantization/gguf.py b/vllm/model_executor/layers/quantization/gguf.py
index 2a72da26cc..61eb6c912a 100644
--- a/vllm/model_executor/layers/quantization/gguf.py
+++ b/vllm/model_executor/layers/quantization/gguf.py
@@ -84,7 +84,7 @@ class GGUFConfig(QuantizationConfig):
 
     @classmethod
     def override_quantization_method(
-        cls, hf_quant_cfg: dict[str, Any], user_quant: str | None
+        cls, hf_quant_cfg: dict[str, Any], user_quant: str | None, hf_config=None
     ) -> "QuantizationMethods | None":
         # When user explicitly specifies --quantization gguf, override
         # whatever quantization method is in the HF model config (e.g. fp8).
diff --git a/vllm/model_executor/layers/quantization/gptq_marlin.py b/vllm/model_executor/layers/quantization/gptq_marlin.py
index ce0dc0f4e0..1ca551d635 100644
--- a/vllm/model_executor/layers/quantization/gptq_marlin.py
+++ b/vllm/model_executor/layers/quantization/gptq_marlin.py
@@ -214,7 +214,7 @@ class GPTQMarlinConfig(QuantizationConfig):
 
     @classmethod
     def override_quantization_method(
-        cls, hf_quant_cfg, user_quant
+        cls, hf_quant_cfg, user_quant, hf_config=None
     ) -> QuantizationMethods | None:
         can_convert = cls.is_gptq_marlin_compatible(hf_quant_cfg)
 
diff --git a/vllm/model_executor/layers/quantization/inc.py b/vllm/model_executor/layers/quantization/inc.py
index 93be5b7613..29d7392847 100644
--- a/vllm/model_executor/layers/quantization/inc.py
+++ b/vllm/model_executor/layers/quantization/inc.py
@@ -453,7 +453,7 @@ class INCConfig(QuantizationConfig):
 
     @classmethod
     def override_quantization_method(
-        cls, hf_quant_cfg, user_quant
+        cls, hf_quant_cfg, user_quant, hf_config=None
     ) -> "QuantizationMethods | None":
         """Override the `auto-round` method to `inc`."""
         is_auto_round_format = hf_quant_cfg.get("quant_method", None) == "auto-round"
diff --git a/vllm/model_executor/layers/quantization/modelopt.py b/vllm/model_executor/layers/quantization/modelopt.py
index 20965c77f0..0b8ad0cbc1 100644
--- a/vllm/model_executor/layers/quantization/modelopt.py
+++ b/vllm/model_executor/layers/quantization/modelopt.py
@@ -406,7 +406,7 @@ class ModelOptFp8Config(ModelOptQuantConfigBase):
 
     @classmethod
     def override_quantization_method(
-        cls, hf_quant_cfg, user_quant
+        cls, hf_quant_cfg, user_quant, hf_config=None
     ) -> QuantizationMethods | None:
         algo = cls._extract_modelopt_quant_algo(hf_quant_cfg)
         if algo is not None and algo == "FP8":
@@ -1028,7 +1028,7 @@ class ModelOptNvFp4Config(ModelOptQuantConfigBase):
 
     @classmethod
     def override_quantization_method(
-        cls, hf_quant_cfg, user_quant
+        cls, hf_quant_cfg, user_quant, hf_config=None
     ) -> QuantizationMethods | None:
         algo = cls._extract_modelopt_quant_algo(hf_quant_cfg)
         if algo is not None and ("NVFP4" in algo or "FP4" in algo):
@@ -1525,7 +1525,7 @@ class ModelOptMxFp8Config(ModelOptQuantConfigBase):
 
     @classmethod
     def override_quantization_method(
-        cls, hf_quant_cfg, user_quant
+        cls, hf_quant_cfg, user_quant, hf_config=None
     ) -> QuantizationMethods | None:
         algo = cls._extract_modelopt_quant_algo(hf_quant_cfg)
         if algo is not None and "MXFP8" in algo:
@@ -2052,7 +2052,7 @@ class ModelOptMixedPrecisionConfig(ModelOptQuantConfigBase):
 
     @classmethod
     def override_quantization_method(
-        cls, hf_quant_cfg, user_quant
+        cls, hf_quant_cfg, user_quant, hf_config=None
     ) -> QuantizationMethods | None:
         algo = cls._extract_modelopt_quant_algo(hf_quant_cfg)
         if algo is not None and algo == "MIXED_PRECISION":
diff --git a/vllm/model_executor/layers/quantization/moe_wna16.py b/vllm/model_executor/layers/quantization/moe_wna16.py
index a327ac17bb..e5ef3f4c31 100644
--- a/vllm/model_executor/layers/quantization/moe_wna16.py
+++ b/vllm/model_executor/layers/quantization/moe_wna16.py
@@ -130,7 +130,7 @@ class MoeWNA16Config(QuantizationConfig):
 
     @classmethod
     def override_quantization_method(
-        cls, hf_quant_cfg, user_quant
+        cls, hf_quant_cfg, user_quant, hf_config=None
     ) -> QuantizationMethods | None:
         can_convert = cls.is_moe_wna16_compatible(hf_quant_cfg)
         if can_convert and user_quant == "moe_wna16":
diff --git a/vllm/model_executor/layers/quantization/mxfp4.py b/vllm/model_executor/layers/quantization/mxfp4.py
index adb191b0a0..019bb45d65 100644
--- a/vllm/model_executor/layers/quantization/mxfp4.py
+++ b/vllm/model_executor/layers/quantization/mxfp4.py
@@ -19,11 +19,11 @@ from vllm.model_executor.layers.fused_moe.config import (
 from vllm.model_executor.layers.fused_moe.oracle.mxfp4 import (
     TRITON_BACKENDS,
     Mxfp4MoeBackend,
-    convert_to_mxfp4_moe_kernel_format,
+    convert_gpt_oss_weight_to_mxfp4_moe_kernel_format,
     make_mxfp4_moe_kernel,
     make_mxfp4_moe_quant_config,
     mxfp4_round_up_hidden_size_and_intermediate_size,
-    select_mxfp4_moe_backend,
+    select_gpt_oss_mxfp4_moe_backend,
 )
 from vllm.model_executor.layers.linear import LinearBase, UnquantizedLinearMethod
 from vllm.model_executor.layers.quantization import QuantizationMethods
@@ -38,6 +38,12 @@ logger = init_logger(__name__)
 
 
 class Mxfp4Config(QuantizationConfig):
+    """Canonical base config for MXFP4 quantization.
+
+    Subclasses override get_name() and override_quantization_method() to
+    register themselves as the handler for a specific checkpoint format.
+    """
+
     def __init__(self, ignored_layers: list[str] | None = None):
         super().__init__()
         self.ignored_layers = ignored_layers
@@ -62,6 +68,8 @@ class Mxfp4Config(QuantizationConfig):
     def get_config_filenames(cls) -> list[str]:
         return []
 
+    # TODO (zyongye) This is only temporaty fallback.
+    # We should have `Mxfp4MoEMethod` after this migration is complete.
     def get_quant_method(
         self, layer: torch.nn.Module, prefix: str
     ) -> "QuantizeMethodBase | None":
@@ -79,7 +87,7 @@ class Mxfp4Config(QuantizationConfig):
             )
             return UnquantizedLinearMethod()
         elif isinstance(layer, FusedMoE):
-            return Mxfp4MoEMethod(layer.moe_config)
+            return GptOssMxfp4MoEMethod(layer.moe_config)
         elif isinstance(layer, Attention):
             logger.debug_once(
                 "MXFP4 attention layer is not implemented. "
@@ -93,13 +101,46 @@ class Mxfp4Config(QuantizationConfig):
         return True
 
 
-class Mxfp4MoEMethod(FusedMoEMethodBase):
+class GptOssMxfp4Config(Mxfp4Config):
+    """MXFP4 config for GPT-OSS checkpoints.
+
+    Checkpoints carry ``"quant_method": "mxfp4"`` in their JSON config.
+    override_quantization_method() maps that to the canonical internal name
+    so that the rest of the loading path uses "gpt_oss_mxfp4" consistently.
+    """
+
+    @classmethod
+    def get_name(cls) -> QuantizationMethods:
+        return "gpt_oss_mxfp4"
+
+    @classmethod
+    def override_quantization_method(
+        cls, hf_quant_cfg, user_quant, hf_config=None
+    ) -> QuantizationMethods | None:
+        # Match both "mxfp4" (original checkpoint value) and "gpt_oss_mxfp4"
+        # (already normalized by verify_and_update_model_config) so that
+        # explicit --quantization mxfp4 from the user doesn't cause a mismatch.
+        if not (
+            isinstance(hf_quant_cfg, dict)
+            and hf_quant_cfg.get("quant_method") in ("mxfp4", "gpt_oss_mxfp4")
+        ):
+            return None
+        # Require explicit confirmation that this is a GPT-OSS model.
+        # Do NOT fall back to returning the override when hf_config is None,
+        # as that would silently claim all mxfp4 checkpoints.
+        model_type = getattr(hf_config, "model_type", None)
+        if model_type != "gpt_oss":
+            return None
+        return "gpt_oss_mxfp4"
+
+
+class GptOssMxfp4MoEMethod(FusedMoEMethodBase):
     """MXFP4 MoE quantization method."""
 
     def __init__(self, moe: FusedMoEConfig):
         super().__init__(moe)
-        self.weight_dtype = "mxfp4"
-        self.mxfp4_backend, self.experts_cls = select_mxfp4_moe_backend(moe)
+        self.weight_dtype = "gpt_oss_mxfp4"
+        self.mxfp4_backend, self.experts_cls = select_gpt_oss_mxfp4_moe_backend(moe)
 
         self.max_capture_size = (
             get_current_vllm_config().compilation_config.max_cudagraph_capture_size
@@ -281,7 +322,7 @@ class Mxfp4MoEMethod(FusedMoEMethodBase):
 
         # Convert weights to kernel format
         w13, w2, w13_scale, w2_scale, w13_bias, w2_bias = (
-            convert_to_mxfp4_moe_kernel_format(
+            convert_gpt_oss_weight_to_mxfp4_moe_kernel_format(
                 mxfp4_backend=self.mxfp4_backend,
                 layer=layer,
                 w13_weight=w13,
diff --git a/vllm/model_executor/layers/quantization/quark/quark_moe.py b/vllm/model_executor/layers/quantization/quark/quark_moe.py
index 58ed8940b3..d4db929eae 100644
--- a/vllm/model_executor/layers/quantization/quark/quark_moe.py
+++ b/vllm/model_executor/layers/quantization/quark/quark_moe.py
@@ -30,11 +30,11 @@ from vllm.model_executor.layers.fused_moe.fused_marlin_moe import fused_marlin_m
 from vllm.model_executor.layers.fused_moe.oracle.mxfp4 import (
     TRITON_BACKENDS,
     Mxfp4MoeBackend,
-    convert_to_mxfp4_moe_kernel_format,
+    convert_gpt_oss_weight_to_mxfp4_moe_kernel_format,
     make_mxfp4_moe_kernel,
     make_mxfp4_moe_quant_config,
     mxfp4_round_up_hidden_size_and_intermediate_size,
-    select_mxfp4_moe_backend,
+    select_gpt_oss_mxfp4_moe_backend,
 )
 from vllm.model_executor.layers.quantization.utils.marlin_utils_fp8 import (
     prepare_fp8_moe_layer_for_marlin,
@@ -995,7 +995,7 @@ class QuarkOCP_MX_MoEMethod(QuarkMoEMethod):
         self.w2_precision_config = None
 
         if self.ocp_mx_scheme == "w_mxfp4":
-            self.mxfp4_backend, self.experts_cls = select_mxfp4_moe_backend(moe)
+            self.mxfp4_backend, self.experts_cls = select_gpt_oss_mxfp4_moe_backend(moe)
         elif self.ocp_mx_scheme.startswith("w_mxfp4"):
             # TODO(bowenbao): refactor and introduce backends for other OCP MX schemes.
             self.mxfp4_backend = Mxfp4MoeBackend.NONE
@@ -1300,7 +1300,7 @@ class QuarkOCP_MX_MoEMethod(QuarkMoEMethod):
 
         # Convert weights to kernel format
         w13, w2, w13_scale, w2_scale, w13_bias, w2_bias = (
-            convert_to_mxfp4_moe_kernel_format(
+            convert_gpt_oss_weight_to_mxfp4_moe_kernel_format(
                 mxfp4_backend=self.mxfp4_backend,
                 layer=layer,
                 w13_weight=w13,
diff --git a/vllm/model_executor/models/config.py b/vllm/model_executor/models/config.py
index 9e919d3473..521184e4c6 100644
--- a/vllm/model_executor/models/config.py
+++ b/vllm/model_executor/models/config.py
@@ -108,6 +108,23 @@ class Gemma4Config(VerifyAndUpdateConfig):
 
 
 class GptOssForCausalLMConfig(VerifyAndUpdateConfig):
+    @staticmethod
+    def verify_and_update_model_config(model_config: "ModelConfig") -> None:
+        quant_config = getattr(model_config.hf_config, "quantization_config", None)
+        if quant_config is not None and quant_config.get("quant_method") == "mxfp4":
+            model_config.hf_config.quantization_config["quant_method"] = "gpt_oss_mxfp4"
+
+        hf_text_quant_config = getattr(
+            model_config.hf_text_config, "quantization_config", None
+        )
+        if (
+            hf_text_quant_config is not None
+            and hf_text_quant_config.get("quant_method") == "mxfp4"
+        ):
+            model_config.hf_text_config.quantization_config["quant_method"] = (
+                "gpt_oss_mxfp4"
+            )
+
     @staticmethod
     def verify_and_update_config(vllm_config: "VllmConfig") -> None:
         structured_outputs_config = vllm_config.structured_outputs_config
diff --git a/vllm/model_executor/models/gpt_oss.py b/vllm/model_executor/models/gpt_oss.py
index a9ec829742..4e4eb58184 100644
--- a/vllm/model_executor/models/gpt_oss.py
+++ b/vllm/model_executor/models/gpt_oss.py
@@ -560,6 +560,14 @@ class GptOssModel(nn.Module, EagleModelMixin):
                 pcp_rank=get_pcp_group().rank_in_group,
             )
 
+        def _is_mxfp4(weight_dtype: str | None) -> bool:
+            """Return True for any MXFP4 weight-dtype variant.
+
+            Covers "gpt_oss_mxfp4" (GptOssMxfp4MoEMethod) and "mxfp4"
+            (QuarkMoEMethod with fp4 weights) and any future variants.
+            """
+            return weight_dtype is not None and "mxfp4" in weight_dtype
+
         def _get_moe_weight_dtype(layer_id: int = 0) -> str | None:
             """Helper function to get MoE quantization weight dtype.
 
@@ -578,7 +586,7 @@ class GptOssModel(nn.Module, EagleModelMixin):
 
         moe_weight_dtype = _get_moe_weight_dtype(layer_id=0)
 
-        if moe_weight_dtype == "mxfp4":
+        if _is_mxfp4(moe_weight_dtype):
             # MXFP4 requires OCP_MX_BLOCK_SIZE alignment
             intermediate_size_block = intermediate_size // OCP_MX_BLOCK_SIZE
             per_rank_intermediate_size_block = cdiv(intermediate_size_block, tp_size)
@@ -682,7 +690,7 @@ class GptOssModel(nn.Module, EagleModelMixin):
                 continue
 
             # Unified handler for mxfp4 weights and scales
-            elif moe_quant_method == "mxfp4" and any(
+            elif _is_mxfp4(moe_quant_method) and any(
                 name.endswith(suffix)
                 for suffix in [
                     ".w13_weight_scale",
@@ -1116,8 +1124,22 @@ class GptOssModel(nn.Module, EagleModelMixin):
             if hasattr(self.config, "quantization_config")
             else None
         )
-
+        # Normalize the checkpoint's quant_method to the internal name.
+        # Note: there are three places where "mxfp4" -> "gpt_oss_mxfp4"
+        # normalization occurs, each serving a different data path:
+        #   1. GptOssMxfp4Config.override_quantization_method() — sets
+        #      ModelConfig.quantization (used to select the QuantizationConfig
+        #      class at model init time), reading from model_arch_config which
+        #      is a snapshot taken before verify_and_update_model_config runs.
+        #   2. GptOssForCausalLMConfig.verify_and_update_model_config() —
+        #      patches hf_config.quantization_config in-place (a separate copy
+        #      of the dict from model_arch_config) for later hf_config lookups.
+        #   3. Here — reads directly from self.config (the raw HF config) which
+        #      may still carry the original "mxfp4" string from the checkpoint.
         if quant_method == "mxfp4":
+            quant_method = "gpt_oss_mxfp4"
+
+        if quant_method == "gpt_oss_mxfp4":
             return self._load_weights_mxfp4(
                 ep_rank_end,
                 ep_rank_start,
diff --git a/vllm/model_executor/models/transformers/base.py b/vllm/model_executor/models/transformers/base.py
index 8b3ef56c80..b7967985f2 100644
--- a/vllm/model_executor/models/transformers/base.py
+++ b/vllm/model_executor/models/transformers/base.py
@@ -150,7 +150,7 @@ class Base(
         if self.quant_config:
             quant_method_name = self.quant_config.get_name()
             # Check for unsupported quantization methods.
-            if quant_method_name == "mxfp4":
+            if quant_method_name in ("mxfp4", "gpt_oss_mxfp4"):
                 raise NotImplementedError(
                     "Transformers modeling backend does "
                     "not support MXFP4 quantization yet."