[Bugfix] Fix prefix creation for Qwen3.5 (#34723)

Signed-off-by: mgoin <mgoin64@gmail.com>

vllm/model_executor/models/qwen3_5.py

@@ -676,9 +676,10 @@ class Qwen3_5ForCausalLMBase(
         super().__init__()
         self.config = config
         self.scheduler_config = scheduler_config
-        self.model = Qwen3_5Model(
-            vllm_config=vllm_config, prefix=maybe_prefix(prefix, "model")
-        )
+        # Deal with the case where the prefix is already "language_model" since
+        # Qwen/Qwen3.5-397B-A17B has naming like: model.language_model.layers.0
+        model_prefix = prefix if "model" in prefix else "model"
+        self.model = Qwen3_5Model(vllm_config=vllm_config, prefix=model_prefix)
 
         if get_pp_group().is_last_rank:
             if config.tie_word_embeddings:
@@ -754,7 +755,7 @@ class Qwen3_5MoeForCausalLM(Qwen3_5ForCausalLMBase, QwenNextMixtureOfExperts):
     dummy_inputs=Qwen3VLDummyInputsBuilder,
 )
 class Qwen3_5ForConditionalGeneration(Qwen3VLForConditionalGeneration, IsHybrid):
-    def __init__(self, *, vllm_config: VllmConfig, prefix: str = ""):
+    def __init__(self, *, vllm_config: VllmConfig, prefix: str = "model"):
         # protocols have not __init__ method, so we need to use nn.Module.__init__
         nn.Module.__init__(self)
         config: Qwen3_5Config = vllm_config.model_config.hf_config
@@ -962,7 +963,7 @@ class Qwen3_5_MoeMixtureOfExperts(MixtureOfExperts):
 class Qwen3_5MoeForConditionalGeneration(
     Qwen3_5ForConditionalGeneration, Qwen3_5_MoeMixtureOfExperts
 ):
-    def __init__(self, *, vllm_config: VllmConfig, prefix: str = ""):
+    def __init__(self, *, vllm_config: VllmConfig, prefix: str = "model"):
         # protocols have not __init__ method, so we need to use nn.Module.__init__
         nn.Module.__init__(self)
         config: Qwen3_5MoeConfig = vllm_config.model_config.hf_config