Fix weight loading bug for Deepseek v3+nextn (#5684)

python/sglang/srt/models/deepseek_nextn.py

@@ -242,6 +242,12 @@ class DeepseekV3ForCausalLMNextN(DeepseekV3ForCausalLM):
             num_experts=self.config.n_routed_experts + self.n_share_experts_fusion,
         )
 
+        # Fuse q_a_proj and kv_a_proj_with_mqa along output dimension when q_lora_rank is not None
+        fuse_qkv_a_proj = hasattr(self.config, "q_lora_rank") and (
+            self.config.q_lora_rank is not None
+        )
+        cached_a_proj = {} if fuse_qkv_a_proj else None
+
         nextn_layer_prefix = "model.layers.0"
         nextn_spec_weight_names = [
             "shared_head.norm",
@@ -313,6 +319,46 @@ class DeepseekV3ForCausalLMNextN(DeepseekV3ForCausalLM):
                     if name.endswith(".bias") and name not in params_dict:
                         continue
 
+                    # Handle fused_qkv_a_proj
+                    if fuse_qkv_a_proj and (
+                        "q_a_proj" in name or "kv_a_proj_with_mqa" in name
+                    ):
+                        cached_a_proj[name] = loaded_weight
+                        q_a_proj_name = (
+                            name
+                            if "q_a_proj" in name
+                            else name.replace("kv_a_proj_with_mqa", "q_a_proj")
+                        )
+                        kv_a_proj_name = (
+                            name
+                            if "kv_a_proj_with_mqa" in name
+                            else name.replace("q_a_proj", "kv_a_proj_with_mqa")
+                        )
+
+                        # When both q_a_proj and kv_a_proj_with_mqa has been cached, load the fused weight to parameter
+                        if (
+                            q_a_proj_name in cached_a_proj
+                            and kv_a_proj_name in cached_a_proj
+                        ):
+
+                            q_a_proj_weight = cached_a_proj[q_a_proj_name]
+                            kv_a_proj_weight = cached_a_proj[kv_a_proj_name]
+                            fused_weight = torch.cat(
+                                [q_a_proj_weight, kv_a_proj_weight], dim=0
+                            )
+
+                            param_name = name.replace(
+                                "q_a_proj", "fused_qkv_a_proj_with_mqa"
+                            )
+                            param = params_dict[param_name]
+
+                            weight_loader = getattr(
+                                param, "weight_loader", default_weight_loader
+                            )
+                            weight_loader(param, fused_weight)
+                            cached_a_proj.pop(q_a_proj_name)
+                            cached_a_proj.pop(kv_a_proj_name)
+                    else:
                         param = params_dict[name]
                         weight_loader = getattr(
                             param, "weight_loader", default_weight_loader