Fix Gemma4 NVFP4 MoE default attention backend (#26791)

python/sglang/srt/server_args.py

@@ -1809,7 +1809,8 @@ class ServerArgs:
         if parse_connector_type(self.model_path) == ConnectorType.INSTANCE:
             return
 
-        hf_config = self.get_model_config().hf_config
+        model_config = self.get_model_config()
+        hf_config = model_config.hf_config
         model_arch = hf_config.architectures[0]
 
         _hybrid_spec = get_linear_attn_spec_by_arch(model_arch)
@@ -2366,8 +2367,17 @@ class ServerArgs:
             "Gemma4ForCausalLM",
             "Gemma4UnifiedForConditionalGeneration",
         ):
+            is_gemma4_modelopt_fp4 = model_config.quantization == "modelopt_fp4"
+            is_gemma4_moe = getattr(
+                model_config.hf_text_config, "enable_moe_block", False
+            )
+            is_gemma4_modelopt_fp4_moe = is_gemma4_modelopt_fp4 and is_gemma4_moe
+            # TODO: switch Gemma4 modelopt_fp4 MoE back to trtllm_mha by default
+            # after the SM10X trtllm_mha accuracy issue is fixed.
             default_attention_backend = (
-                "trtllm_mha" if is_sm100_supported() else "triton"
+                "trtllm_mha"
+                if is_sm100_supported() and not is_gemma4_modelopt_fp4_moe
+                else "triton"
             )
             if self.is_attention_backend_not_set():
                 self.attention_backend = default_attention_backend
@@ -2392,7 +2402,7 @@ class ServerArgs:
             )
 
             if is_sm100_supported() and self.moe_runner_backend == "auto":
-                if self.get_model_config().quantization == "modelopt_fp4":
+                if is_gemma4_modelopt_fp4:
                     self.quantization = "modelopt_fp4"
                     self.moe_runner_backend = "flashinfer_trtllm"
                     logger.info(