[#11625][fix] handle list-typed eos_token_id in SamplingParams._setup

tensorrt_llm/sampling_params.py

@@ -396,7 +396,21 @@ def _setup(
         self, tokenizer, hf_model_config, generation_config, add_special_tokens: bool = False
     ) -> "SamplingParams":
         if self.end_id is None:
-            self.end_id = tokenizer.eos_token_id
+            eos_token_id = tokenizer.eos_token_id
+            if isinstance(eos_token_id, list):
+                # HuggingFace tokenizers may return a list of EOS token ids
+                # (e.g. Llama 3.1 has eos_token_id=[128001, 128009]).
+                # Use the first token as end_id and register the rest as stop tokens.
+                self.end_id = eos_token_id[0] if eos_token_id else None
+                extra_eos = eos_token_id[1:]
+                if extra_eos:
+                    if not self.stop_token_ids:
+                        self.stop_token_ids = []
+                    for tok in extra_eos:
+                        if tok not in self.stop_token_ids:
+                            self.stop_token_ids.append(tok)
+            else:
+                self.end_id = eos_token_id
             self.pad_id = tokenizer.pad_token_id
 
             if self.pad_id is None:

tests/unittest/llmapi/test_sampling_params.py

@@ -0,0 +1,102 @@
+# Copyright 2026 NVIDIA CORPORATION & AFFILIATES
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# SPDX-License-Identifier: Apache-2.0
+"""Unit tests for SamplingParams that do not require GPU or model weights."""
+
+from tensorrt_llm.sampling_params import SamplingParams
+
+
+class _MockTokenizer:
+    """Minimal tokenizer stub for _setup() without real models."""
+
+    def __init__(self, eos_token_id, pad_token_id=None):
+        self.eos_token_id = eos_token_id
+        self.pad_token_id = pad_token_id
+
+
+def test_setup_scalar_eos_token_id():
+    """_setup with a scalar eos_token_id sets end_id correctly."""
+    tokenizer = _MockTokenizer(eos_token_id=2, pad_token_id=0)
+    params = SamplingParams()
+    params._setup(tokenizer, hf_model_config=None, generation_config=None)
+
+    assert params.end_id == 2
+    assert params.pad_id == 0
+    assert not params.stop_token_ids
+
+
+def test_setup_list_eos_token_id_single_element():
+    """_setup with a single-element list behaves like a scalar."""
+    tokenizer = _MockTokenizer(eos_token_id=[128001], pad_token_id=0)
+    params = SamplingParams()
+    params._setup(tokenizer, hf_model_config=None, generation_config=None)
+
+    assert params.end_id == 128001
+    assert params.pad_id == 0
+    assert not params.stop_token_ids
+
+
+def test_setup_list_eos_token_id_multiple_elements():
+    """Regression for GitHub issue #11625.
+
+    _setup with a multi-element eos_token_id list (e.g. Llama 3.1) must set
+    end_id to a scalar int — not a list — to avoid std::bad_cast in the C++
+    binding.  Extra EOS tokens go into stop_token_ids so generation stops on
+    all of them.
+    """
+    tokenizer = _MockTokenizer(eos_token_id=[128001, 128009], pad_token_id=0)
+    params = SamplingParams()
+    params._setup(tokenizer, hf_model_config=None, generation_config=None)
+
+    assert isinstance(params.end_id, int), (
+        "end_id must be int, not list, to avoid std::bad_cast in C++ binding"
+    )
+    assert params.end_id == 128001
+    assert params.stop_token_ids is not None
+    assert 128009 in params.stop_token_ids
+    assert 128001 not in params.stop_token_ids
+
+
+def test_setup_list_eos_token_id_no_duplicate_stop_tokens():
+    """Extra EOS tokens are not added to stop_token_ids twice.
+
+    When the user already specified them explicitly, no duplicate should appear.
+    """
+    tokenizer = _MockTokenizer(eos_token_id=[128001, 128009], pad_token_id=0)
+    params = SamplingParams(stop_token_ids=[128009])
+    params._setup(tokenizer, hf_model_config=None, generation_config=None)
+
+    assert params.end_id == 128001
+    assert params.stop_token_ids.count(128009) == 1
+
+
+def test_setup_end_id_already_set_ignores_tokenizer_eos():
+    """When end_id is explicitly provided, _setup must not overwrite it."""
+    tokenizer = _MockTokenizer(eos_token_id=[128001, 128009], pad_token_id=0)
+    params = SamplingParams(end_id=99)
+    params._setup(tokenizer, hf_model_config=None, generation_config=None)
+
+    assert params.end_id == 99
+    assert not params.stop_token_ids
+
+
+def test_setup_list_eos_token_id_pad_fallback():
+    """When pad_token_id is None, pad_id falls back to end_id (an int)."""
+    tokenizer = _MockTokenizer(eos_token_id=[128001, 128009], pad_token_id=None)
+    params = SamplingParams()
+    params._setup(tokenizer, hf_model_config=None, generation_config=None)
+
+    assert params.end_id == 128001
+    assert params.pad_id == 128001