fix: correct weight loading prefix mapping for Qwen3-VL (#18024)

Co-authored-by: liuxiaoming <liuxiaoming@modelbest.cn>
Co-authored-by: Xinyuan Tong <115166877+JustinTong0323@users.noreply.github.com>

python/sglang/srt/models/qwen3_vl.py

@@ -959,7 +959,13 @@ class Qwen3VLForConditionalGeneration(nn.Module):
                 name = name.replace(r"model.language_model.", r"model.")
             layer_id = get_layer_id(name)
 
-            if self.pp_group.is_last_rank and "model.embed_tokens.weight" in name:
+            # Only copy embed_tokens to lm_head when tie_word_embeddings=True
+            # For models with tie_word_embeddings=False (e.g. 8B), lm_head has independent weights
+            if (
+                self.pp_group.is_last_rank
+                and "model.embed_tokens.weight" in name
+                and self.config.tie_word_embeddings
+            ):
                 if "lm_head.weight" in params_dict:
                     lm_head_param = params_dict["lm_head.weight"]
                     weight_loader = getattr(