[Quantization] Support FP8 MoE bias for models like GPT-OSS (#34906)

Signed-off-by: jasperjiaguo <jasperg662@gmail.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vllm/model_executor/layers/quantization/fp8.py

@@ -758,6 +758,25 @@ class Fp8MoEMethod(FusedMoEMethodBase):
         layer.register_parameter("w2_weight", w2_weight)
         set_weight_attrs(w2_weight, extra_weight_attrs)
 
+        # BIASES (for models like GPT-OSS that have biased MoE)
+        if self.moe.has_bias:
+            w13_bias = torch.nn.Parameter(
+                torch.zeros(
+                    num_experts,
+                    2 * intermediate_size_per_partition,
+                    dtype=layer.orig_dtype,
+                ),
+                requires_grad=False,
+            )
+            layer.register_parameter("w13_bias", w13_bias)
+            set_weight_attrs(w13_bias, extra_weight_attrs)
+            w2_bias = torch.nn.Parameter(
+                torch.zeros(num_experts, hidden_size, dtype=layer.orig_dtype),
+                requires_grad=False,
+            )
+            layer.register_parameter("w2_bias", w2_bias)
+            set_weight_attrs(w2_bias, extra_weight_attrs)
+
         # WEIGHT_SCALES
         if not self.block_quant:
             # For per-tensor quant, the scales are per expert and weight.
@@ -939,7 +958,7 @@ class Fp8MoEMethod(FusedMoEMethodBase):
         a1_scale = layer.w13_input_scale
         a2_scale = layer.w2_input_scale
 
-        return make_fp8_moe_quant_config(
+        quant_config = make_fp8_moe_quant_config(
             fp8_backend=self.fp8_backend,
             w1_scale=w1_scale,
             w2_scale=w2_scale,
@@ -948,6 +967,18 @@ class Fp8MoEMethod(FusedMoEMethodBase):
             block_shape=self.weight_block_size,
         )
 
+        # Inject biases into the quant config if the model has them
+        # (e.g. GPT-OSS biased MoE)
+        if quant_config is not None and self.moe.has_bias:
+            w13_bias = getattr(layer, "w13_bias", None)
+            w2_bias = getattr(layer, "w2_bias", None)
+            if w13_bias is not None:
+                quant_config._w1.bias = w13_bias
+            if w2_bias is not None:
+                quant_config._w2.bias = w2_bias
+
+        return quant_config
+
     @property
     def supports_eplb(self) -> bool:
         return True
@@ -1168,6 +1199,28 @@ class Fp8OnlineMoEMethod(Fp8MoEMethod):
         # stash the correct device for `patched_weight_loader`
         layer._load_device = torch.get_default_device()
 
+        # BIASES (for models like GPT-OSS that have biased MoE)
+        if self.moe.has_bias:
+            # Use the original weight_loader (not patched) for biases
+            orig_extra_weight_attrs = dict(extra_weight_attrs)
+            orig_extra_weight_attrs["weight_loader"] = weight_loader
+            w13_bias = torch.nn.Parameter(
+                torch.zeros(
+                    num_experts,
+                    2 * intermediate_size_per_partition,
+                    dtype=layer.orig_dtype,
+                ),
+                requires_grad=False,
+            )
+            layer.register_parameter("w13_bias", w13_bias)
+            set_weight_attrs(w13_bias, orig_extra_weight_attrs)
+            w2_bias = torch.nn.Parameter(
+                torch.zeros(num_experts, hidden_size, dtype=layer.orig_dtype),
+                requires_grad=False,
+            )
+            layer.register_parameter("w2_bias", w2_bias)
+            set_weight_attrs(w2_bias, orig_extra_weight_attrs)
+
         # WEIGHT_SCALES
         # Allocate 2 scales for w1 and w3 respectively.
         # They will be combined to a single scale after weight loading.