[Test] Add tests for n parameter in chat completions API

tests/entrypoints/openai/test_chat.py

@@ -3,6 +3,7 @@
 
 # imports for structured outputs tests
 import json
+from collections import defaultdict
 
 import jsonschema
 import openai  # use the official client for correctness check
@@ -13,6 +14,11 @@
 import torch
 from openai import BadRequestError
 
+from vllm.entrypoints.openai.chat_completion.protocol import (
+    ChatCompletionRequest,
+)
+from vllm.sampling_params import SamplingParams
+
 from ...utils import RemoteOpenAIServer
 
 # any model with a chat template should work here
@@ -815,3 +821,203 @@ async def test_invocations(server: RemoteOpenAIServer, client: openai.AsyncOpenA
 
     assert chat_output.keys() == invocation_output.keys()
     assert chat_output["choices"] == invocation_output["choices"]
+
+
+# Test n parameter for chat completions
+# Tests that the n parameter works correctly for regular sampling
+# (non-beam search) in chat completions, addressing issue #34305.
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "model_name",
+    [MODEL_NAME],
+)
+async def test_chat_completion_n_parameter_non_streaming(
+    client: openai.AsyncOpenAI, model_name: str
+):
+    """Test that n parameter returns multiple choices for non-streaming requests."""
+    messages = [
+        {"role": "system", "content": "You are a helpful assistant."},
+        {"role": "user", "content": "What is the opposite of big?"},
+    ]
+
+    # Test with n=3
+    chat_completion = await client.chat.completions.create(
+        model=model_name,
+        messages=messages,
+        max_completion_tokens=20,
+        temperature=0.7,
+        n=3,
+        stream=False,
+    )
+
+    assert len(chat_completion.choices) == 3
+
+    # Verify each choice has content and correct index
+    for i, choice in enumerate(chat_completion.choices):
+        assert choice.index == i
+        assert choice.message.content is not None
+        assert len(choice.message.content) > 0
+
+    # Verify all responses are different (highly likely with temperature > 0)
+    contents = [choice.message.content for choice in chat_completion.choices]
+    assert len(set(contents)) > 1, "Expected different responses with n=3"
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "model_name",
+    [MODEL_NAME],
+)
+async def test_chat_completion_n_parameter_streaming(
+    client: openai.AsyncOpenAI, model_name: str
+):
+    """Test that n parameter returns multiple choices for streaming requests."""
+    messages = [
+        {"role": "system", "content": "You are a helpful assistant."},
+        {"role": "user", "content": "What is the capital of France?"},
+    ]
+
+    stream = await client.chat.completions.create(
+        model=model_name,
+        messages=messages,
+        max_completion_tokens=15,
+        temperature=0.7,
+        n=2,
+        stream=True,
+    )
+
+    # Collect all chunks using defaultdict for dynamic handling
+    chunks_by_index = defaultdict(list)
+    async for chunk in stream:
+        for choice in chunk.choices:
+            if choice.delta.content:
+                chunks_by_index[choice.index].append(choice.delta.content)
+
+    # Verify both choices received content
+    assert len(chunks_by_index[0]) > 0, "Choice 0 received no content chunks"
+    assert len(chunks_by_index[1]) > 0, "Choice 1 received no content chunks"
+
+    # Reconstruct full responses
+    response_0 = "".join(chunks_by_index[0])
+    response_1 = "".join(chunks_by_index[1])
+
+    assert len(response_0) > 0, "Choice 0 has empty response"
+    assert len(response_1) > 0, "Choice 1 has empty response"
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "model_name",
+    [MODEL_NAME],
+)
+async def test_chat_completion_n_with_seed(client: openai.AsyncOpenAI, model_name: str):
+    """Test that n parameter works correctly with seed parameter."""
+    messages = [
+        {"role": "user", "content": "Say hello."},
+    ]
+
+    # Test that seed parameter is accepted and works with n > 1
+    chat_completion = await client.chat.completions.create(
+        model=model_name,
+        messages=messages,
+        max_completion_tokens=10,
+        temperature=0.8,
+        n=2,
+        seed=42,
+        stream=False,
+    )
+
+    # Verify we get n=2 choices
+    assert len(chat_completion.choices) == 2
+
+    # Verify both choices have valid content
+    for i, choice in enumerate(chat_completion.choices):
+        assert choice.index == i
+        assert choice.message.content is not None
+        assert len(choice.message.content) > 0
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "model_name",
+    [MODEL_NAME],
+)
+async def test_chat_completion_n_equals_1(client: openai.AsyncOpenAI, model_name: str):
+    """Test that n=1 (default) still works correctly."""
+    messages = [
+        {"role": "user", "content": "Hello!"},
+    ]
+
+    chat_completion = await client.chat.completions.create(
+        model=model_name,
+        messages=messages,
+        max_completion_tokens=10,
+        temperature=0.7,
+        n=1,
+        stream=False,
+    )
+
+    assert len(chat_completion.choices) == 1
+    assert chat_completion.choices[0].index == 0
+    assert chat_completion.choices[0].message.content is not None
+
+
+# Unit tests for n parameter in ChatCompletionRequest.to_sampling_params()
+def test_chat_completion_request_n_parameter_to_sampling_params():
+    """Test that n parameter is correctly passed to SamplingParams."""
+    # Test with n=3
+    request = ChatCompletionRequest(
+        model="test-model",
+        messages=[{"role": "user", "content": "Hello"}],
+        n=3,
+        max_tokens=10,
+    )
+
+    sampling_params = request.to_sampling_params(
+        max_tokens=10,
+        default_sampling_params={},
+    )
+
+    assert isinstance(sampling_params, SamplingParams)
+    assert sampling_params.n == 3, f"Expected n=3, got n={sampling_params.n}"
+
+
+def test_chat_completion_request_n_parameter_default():
+    """Test that n parameter defaults to 1."""
+    request = ChatCompletionRequest(
+        model="test-model",
+        messages=[{"role": "user", "content": "Hello"}],
+        # n not specified, should default to 1
+        max_tokens=10,
+    )
+
+    assert request.n == 1, "n should default to 1"
+    sampling_params = request.to_sampling_params(
+        max_tokens=10,
+        default_sampling_params={},
+    )
+
+    # SamplingParams.from_optional converts None to 1
+    assert sampling_params.n == 1, f"Expected n=1 (default), got n={sampling_params.n}"
+
+
+def test_chat_completion_request_n_parameter_various_values():
+    """Test n parameter with various values."""
+    for n_value in [1, 2, 5, 10]:
+        request = ChatCompletionRequest(
+            model="test-model",
+            messages=[{"role": "user", "content": "Test"}],
+            n=n_value,
+            max_tokens=10,
+        )
+
+        sampling_params = request.to_sampling_params(
+            max_tokens=10,
+            default_sampling_params={},
+        )
+
+        assert sampling_params.n == n_value, (
+            f"Expected n={n_value}, got n={sampling_params.n}"
+        )