fix: sampling params parsing in vllm EPD flow (#5813)

Signed-off-by: Indrajit Bhosale <iamindrajitb@gmail.com>

components/src/dynamo/vllm/multimodal_handlers/worker_handler.py

@@ -274,7 +274,8 @@ class MultimodalPDWorkerHandler(BaseWorkerHandler):
                 )
 
         # Remove the image features from the request as they are not required
-        request.multimodal_inputs = None
+        # Use empty list instead of None to satisfy Pydantic validation on decode worker after vllm upgrade
+        request.multimodal_inputs = []
 
         logger.info(f"Prepared multimodal data size: {len(multi_modal_data['image'])}")
         logger.info(f"{multi_modal_data}")

components/src/dynamo/vllm/multimodal_utils/protocol.py

@@ -86,13 +86,32 @@ class vLLMGenerateRequest(BaseModel):
         if isinstance(v, str):
             v = json.loads(v)
         if isinstance(v, dict):
+            # Workaround for vLLM SamplingParams serialization/deserialization issue.
+            #
+            # Problem: When SamplingParams is serialized via msgspec.json.encode(),
+            # Python sets are converted to JSON arrays (lists). The serialized dict
+            # includes private fields like _all_stop_token_ids. Upon deserialization,
+            # passing this dict to SamplingParams(**dict) causes __post_init__ to fail
+            # because it expects _all_stop_token_ids to be a set (to call .update()),
+            # but it's now a list.
+            #
+            # Solution: Filter out private fields (starting with '_') which are
+            # internal state that should be computed by __post_init__, not passed
+            # from serialized data. Public fields like stop_token_ids are preserved.
+            v = {k: val for k, val in v.items() if not k.startswith("_")}
             return SamplingParams(**v)
         return v
 
     @field_serializer("sampling_params")
     def serialize_sampling_params(self, value: SamplingParams) -> dict[str, Any]:
-        """Serialize SamplingParams using msgspec and return as dict."""
-        return json.loads(msgspec.json.encode(value))
+        """Serialize SamplingParams, filtering out private fields.
+
+        This is the primary fix for the set→list serialization issue.
+        Private fields like _all_stop_token_ids are filtered out here
+        so they never get sent over the wire.
+        """
+        serialized = json.loads(msgspec.json.encode(value))
+        return {k: v for k, v in serialized.items() if not k.startswith("_")}
 
     model_config = ConfigDict(
         arbitrary_types_allowed=True,