[Bugfix] Remove the last EOS token unless explicitly specified (#5077)

tests/engine/output_processor/test_stop_checker.py

+from unittest.mock import MagicMock
+
+import pytest
+from transformers import PreTrainedTokenizer
+
+from vllm.engine.output_processor.stop_checker import StopChecker
+from vllm.sampling_params import SamplingParams
+from vllm.sequence import Logprob, Sequence, SequenceStatus
+
+
+def sequence_with_eos(text: str, eos_token: str,
+                      eos_token_id: int) -> Sequence:
+    """
+    Create a Sequence that ends with an EOS token.
+    """
+    seq = Sequence(
+        seq_id=0,
+        prompt="",
+        prompt_token_ids=[],
+        block_size=16,
+        eos_token_id=eos_token_id,
+    )
+    seq.output_text = text + eos_token
+
+    offset = eos_token_id + 1
+    for i in range(offset, len(text) + offset):
+        seq.append_token_id(token_id=i, logprobs={i: Logprob(0.0)})
+    seq.append_token_id(token_id=eos_token_id,
+                        logprobs={eos_token_id: Logprob(0.0)})
+
+    seq.status = SequenceStatus.RUNNING
+
+    return seq
+
+
+@pytest.mark.parametrize(["text_wo_eos", "eos_token", "eos_token_id"], [
+    ("This text ends with EOS token", "</s>", 2),
+])
+@pytest.mark.parametrize("ignore_eos", [True, False, None])
+@pytest.mark.parametrize("include_stop_str_in_output", [True, False, None])
+@pytest.mark.skip_global_cleanup
+def test_stop_on_eos_token(text_wo_eos: str, eos_token: str, eos_token_id: int,
+                           ignore_eos: bool, include_stop_str_in_output: bool):
+    """
+    Test the behavior of the StopChecker's maybe_stop_sequence method
+    when an EOS token is encountered.
+
+    This test covers:
+    - When the EOS token should stop the sequence and be removed from the output
+    - When the EOS token should stop the sequence and be included in the output
+    - When the EOS token should be ignored, and the sequence continues
+    """
+
+    tokenizer = MagicMock(spec=PreTrainedTokenizer)
+    get_tokenizer_for_seq = MagicMock(return_value=tokenizer)
+    stop_checker = StopChecker(max_model_len=1024,
+                               get_tokenizer_for_seq=get_tokenizer_for_seq)
+
+    seq = sequence_with_eos(
+        text=text_wo_eos,
+        eos_token=eos_token,
+        eos_token_id=eos_token_id,
+    )
+    new_char_count = len(eos_token)
+
+    # Note that `stop` and `stop_token_ids` are not specified
+    sampling_params = SamplingParams(
+        min_tokens=1,
+        ignore_eos=ignore_eos,
+        include_stop_str_in_output=include_stop_str_in_output)
+
+    stop_checker.maybe_stop_sequence(
+        seq=seq,
+        new_char_count=new_char_count,
+        sampling_params=sampling_params,
+    )
+
+    if ignore_eos:
+        assert seq.status == SequenceStatus.RUNNING
+        assert seq.output_text == text_wo_eos + eos_token
+    elif include_stop_str_in_output:
+        assert seq.status == SequenceStatus.FINISHED_STOPPED
+        assert seq.output_text == text_wo_eos + eos_token
+    else:
+        assert seq.status == SequenceStatus.FINISHED_STOPPED
+        assert seq.output_text == text_wo_eos

vllm/engine/output_processor/stop_checker.py

@@ -48,6 +48,11 @@ class StopChecker:
         # Check if the sequence has generated the EOS token.
         if ((not sampling_params.ignore_eos)
                 and seq.get_last_token_id() == seq.eos_token_id):
+            # Remove the last EOS token unless explicitly specified
+            # This prevents unintended exposure of the EOS token
+            if new_char_count and (
+                    not sampling_params.include_stop_str_in_output):
+                seq.output_text = seq.output_text[:-new_char_count]
             seq.status = SequenceStatus.FINISHED_STOPPED
             return