[Bugfix][PD] set max_completion_tokens=1 if req has this value

[Abirdcfly]

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Fix #21712

vllm/vllm/entrypoints/utils.py

Lines 289 to 294 in 04e3850

	def get_max_tokens(max_model_len: int, request: Union[ChatCompletionRequest,
	CompletionRequest],
	input_length: int, default_sampling_params: dict) -> int:
	max_tokens = getattr(request, "max_completion_tokens",
	None) or request.max_tokens

func get_max_tokens used in chat or completions, It first looks for the value of max_completion_tokens, followed by the value of max_tokens, which means that if max_completion_tokens has a value, no matter how much max_tokens is set, it will not take effect.

when use python3 benchmark_serving.py --backend openai-chat --endpoint /v1/chat/completions the req has max_completion_tokens. Therefore, the proxy.py setting max_tokens=1 becomes useless, and only the value of max_completion_tokens is used in vllm serve. :

vllm/benchmarks/backend_request_func.py

Line 376 in ad341c5

"max_completion_tokens": request_func_input.output_len,

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[gemini-code-assist]

Code Review

This pull request addresses a bug in the disaggregated serving examples where prefill requests were not correctly limited to generating a single token if the max_completion_tokens parameter was present. The fix correctly sets max_completion_tokens to 1 in addition to max_tokens.

The change is correctly applied for chat completion endpoints. However, in disagg_proxy_demo.py, the fix is missing for the standard completion endpoint (create_completion method), which could lead to the same bug persisting for those requests. I've added a comment to highlight this for a more complete fix.

[chaunceyjiang]

Good catch!!

curl http://localhost:10001/v1/chat/completions \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -d '{
    "messages": [
      {"role": "system", "content": "You are a helpful assistant."},
      {"role": "user", "content": "Hello! What can you do?"}
    ],
    "temperature": 0.7,
    "max_completion_tokens": 20
  }'

Indeed, using "max_completion_tokens": 20" can reliably reproduce the issue.

Does tests/v1/kv_connector/nixl_integration/toy_proxy_server.py have the same issue?

[Abirdcfly]

Does tests/v1/kv_connector/nixl_integration/toy_proxy_server.py have the same issue?

I just checked these tests, and it seems that there are no tests added for /v1/chat/completions at all. I will add this change in another PR.

[chaunceyjiang]

LGTM.

Thanks~