fix: guided decoding arg placement, None guard, test cleanup (#6617)

Signed-off-by: Yifan Jiang <19356972+yifjiang@users.noreply.github.com>

components/src/dynamo/trtllm/backend_args.py

@@ -198,6 +198,17 @@ class DynamoTrtllmArgGroup(ArgGroup):
             help="Maximum size of downloadable embedding files/Image URLs.",
         )
 
+        # --- Guided Decoding ---
+        add_argument(
+            g,
+            flag_name="--guided-decoding-backend",
+            env_var="DYN_TRTLLM_GUIDED_DECODING_BACKEND",
+            default=None,
+            choices=["xgrammar", "llguidance"],
+            help="Backend to use for guided decoding (structured output). "
+            "Options: xgrammar, llguidance.",
+        )
+
         diffusion_group = parser.add_argument_group(
             "Diffusion Options [Experimental]",
             "Options for video_diffusion modality",
@@ -409,17 +420,6 @@ class DynamoTrtllmArgGroup(ArgGroup):
             arg_type=int,
             help="FSDP size for DiT.",
         )
-        # --- Guided Decoding ---
-        add_argument(
-            g,
-            flag_name="--guided-decoding-backend",
-            env_var="DYN_TRTLLM_GUIDED_DECODING_BACKEND",
-            default=None,
-            choices=["xgrammar", "llguidance"],
-            help="Backend to use for guided decoding (structured output). "
-            "Options: xgrammar, llguidance.",
-        )
-
         add_negatable_bool_argument(
             diffusion_group,
             flag_name="--enable-async-cpu-offload",

components/src/dynamo/trtllm/request_handlers/handler_base.py

@@ -883,7 +883,9 @@ class HandlerBase(BaseGenerativeHandler):
             regex = guided_decoding.get("regex")
             choice = guided_decoding.get("choice")
             if choice and not regex:
-                regex = "(" + "|".join(re.escape(c) for c in choice) + ")"
+                valid_choices = [c for c in choice if c is not None]
+                if valid_choices:
+                    regex = "(" + "|".join(re.escape(c) for c in valid_choices) + ")"
 
             overrides["guided_decoding"] = GuidedDecodingParams(
                 json=guided_decoding.get("json"),

components/src/dynamo/trtllm/tests/test_trtllm_handler_base.py

@@ -2,6 +2,7 @@
 # SPDX-License-Identifier: Apache-2.0
 
 import asyncio
+import re as re_mod
 from dataclasses import dataclass
 from unittest import mock
 from unittest.mock import MagicMock
@@ -245,8 +246,6 @@ class TestGuidedDecodingFromToolChoice:
 
     def test_choice_with_special_chars_escaped(self):
         """Choice values with regex special characters must be escaped."""
-        import re as re_mod
-
         sampling_params = MockSamplingParams()
         request = {
             "sampling_options": {
@@ -297,6 +296,37 @@ class TestGuidedDecodingFromToolChoice:
 
         assert result.guided_decoding.regex is None
 
+    def test_choice_with_none_items_filtered(self):
+        """Choice list with None items should filter them out."""
+        sampling_params = MockSamplingParams()
+        request = {
+            "sampling_options": {
+                "guided_decoding": {
+                    "choice": [None, "yes", None, "no"],
+                },
+            }
+        }
+
+        result = HandlerBase._override_sampling_params(sampling_params, request)
+
+        assert not isinstance(result.guided_decoding, dict)
+        assert result.guided_decoding.regex == "(yes|no)"
+
+    def test_choice_all_none_items_no_regex(self):
+        """Choice list with all None items should not produce a regex."""
+        sampling_params = MockSamplingParams()
+        request = {
+            "sampling_options": {
+                "guided_decoding": {
+                    "choice": [None, None],
+                },
+            }
+        }
+
+        result = HandlerBase._override_sampling_params(sampling_params, request)
+
+        assert result.guided_decoding.regex is None
+
 
 class _ConcreteHandler(HandlerBase):
     """Concrete subclass of HandlerBase for testing (satisfies abstract method)."""