[https://nvbugs/5625743][fix] Fix output corruption when enable n>1 and streaming

tensorrt_llm/executor/result.py

@@ -624,6 +624,9 @@ def __init__(self,
         self._streaming = streaming
 
     def _handle_response(self, response: "GenerationExecutor.Response"):
+        # Save token lengths before processing to detect which outputs received new tokens
+        prev_token_lens = {id(o): len(o.token_ids) for o in self._outputs}
+
         GenerationResultBase._handle_response(self, response)
 
         # The postprocess has been performed, return directly
@@ -638,7 +641,15 @@ def _handle_response(self, response: "GenerationExecutor.Response"):
         }
         if self.sampling_params.detokenize and self.tokenizer is not None:
             for beam_output in self.outputs:
+                # Always update _last_text_len to prevent stale text_diff
                 beam_output._last_text_len = len(beam_output.text)
+                # For n > 1 streaming: only detokenize outputs that received new tokens
+                # to prevent re-decoding the same tokens multiple times
+                output_received_new_tokens = len(
+                    beam_output.token_ids) != prev_token_lens.get(
+                        id(beam_output), 0)
+                if not output_received_new_tokens:
+                    continue
                 if hasattr(
                         self.tokenizer, 'decode_incrementally'
                 ) and self._streaming and not self.sampling_params.use_beam_search:

tests/unittest/llmapi/apps/_test_openai_completions.py

@@ -5,6 +5,7 @@
 
 import openai
 import pytest
+from utils.util import similar
 
 from ..test_llm import get_model_path
 from .openai_server import RemoteOpenAIServer
@@ -204,6 +205,39 @@ async def test_batch_completions_streaming(async_client: openai.AsyncOpenAI,
     assert texts[0] == texts[1]
 
 
+@pytest.mark.asyncio(loop_scope="module")
+@pytest.mark.parametrize("prompts", [["Hello, my name is"] * 2])
+async def test_batch_completions_with_option_n_streaming(
+        async_client: openai.AsyncOpenAI, model_name, prompts):
+    # Use non-stream single generation as reference
+    completion_ref = await async_client.completions.create(
+        model=model_name,
+        prompt=prompts[0],
+        max_tokens=5,
+        temperature=0.0001,
+    )
+    text_ref = completion_ref.choices[0].text
+
+    # test n>1 with streaming
+    batch = await async_client.completions.create(
+        model=model_name,
+        prompt=prompts,
+        n=3,  # number of completions to generate for each prompt.
+        max_tokens=5,
+        temperature=0.0001,
+        stream=True,
+    )
+    texts = [""] * 6  # 2 prompts × 3 generations per prompt = 6 choices
+    async for chunk in batch:
+        assert len(chunk.choices) == 1
+        choice = chunk.choices[0]
+        texts[choice.index] += choice.text
+
+    # Check all generations are consistent with the reference
+    for text in texts:
+        assert similar(text, text_ref, threshold=0.8)
+
+
 @pytest.mark.asyncio(loop_scope="module")
 async def test_completion_stream_options(async_client: openai.AsyncOpenAI,
                                          model_name: str):