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[Bugfix][V1] Validate prompt_logprobs + kv_sharing_fast_prefill incompatibility at request admission - #49622
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…patibility at request admission Signed-off-by: liuzhenlong <dragonliu2018@gmail.com> Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Hi, @njhill Please review this PR when you have a chance. Thanks! |
test_kv_sharing_fast_prefill_rejects_prompt_logprobs_zero asserted pytest.raises(ValueError, ...), copied from the existing test pattern in this file (added by vllm-project#49622, whose code raises a plain ValueError). This PR's own LLMEngine.add_request guard raises VLLMValidationError instead, to match AsyncLLM's existing sibling guard -- so the copied assertion never matched what this code actually raises. Caught by Copilot's review on the PR. Co-authored-by: Claude <noreply@anthropic.com> Signed-off-by: Thippaluri Yaseen Basha <yasenroycey6@gmail.com>
Purpose
Fix a gap in the kv_sharing_fast_prefill + prompt_logprobs incompatibility check on the sync LLMEngine path.
--kv-sharing-fast-prefill produces incorrect logprobs for prompt tokens, so it must not be combined with prompt_logprobs. The async path (vllm/v1/engine/async_llm.py) already validates this at request-admission time, but the sync path (vllm/v1/engine/llm_engine.py, used by LLM.generate()) had no such guard. Users of LLM.generate() would only hit a cryptic AssertionError deep inside the GPU model runner after model execution had already started, instead of a clear upfront error.
The original fast-prefill feature (#22628) added the early ValueError only to async_llm.py and left a typo ("prompt tokens, tokens,") in the defensive assert inside gpu_model_runner.py. This PR:
Test Plan
# Regression test for the sync LLMEngine path pytest tests/v1/engine/test_llm_engine.py::test_kv_sharing_fast_prefill_rejects_prompt_logprobs -vThe test boots LLM(model=facebook/opt-125m, kv_sharing_fast_prefill=True, enforce_eager=True, gpu_memory_utilization=0.5) and asserts that llm.generate(..., SamplingParams(prompt_logprobs=5)) raises ValueError matching "kv-sharing-fast-prefill".
Test Result
tests/v1/engine/test_llm_engine.py::test_kv_sharing_fast_prefill_rejects_prompt_logprobs PASSED [100%] 1 passed, 18 warnings in 33.00sEssential Elements of an Effective PR Description Checklist
supported_models.mdandexamplesfor a new model.