[Perf] Skip repeated sampling params validation for streaming input

[sherkevin]

Summary

• avoid re-running full SamplingParams/PoolingParams validation for streaming input chunks that reuse the request-level sampling params
• keep validation enabled for explicit per-chunk sampling_params, so changed chunk-level parameters are still checked before being sent to engine core
• add focused async streaming tests covering both reused request-level params and explicit per-chunk params

Why this is useful

Streaming input sessions can submit many small chunks while reusing the same request-level SamplingParams. The request-level params are already validated when _add_streaming_input_request() creates the final sentinel request. Re-validating those same params for every chunk repeats work in the frontend hot path, including logprob/logit-bias/logits-processor/structured-output checks inside SamplingParams.verify().

This change only skips validation for chunks that omit StreamingInput.sampling_params and therefore reuse the already-validated request-level params. Explicit per-chunk params continue to validate normally.

Duplicate-work check

I checked open PRs before opening this:

• Avoid re-validating reused sampling parameters: no open PRs
• streaming SamplingParams validation: no open PRs
• validate_params process_inputs: no open PRs
• streaming input sampling params: returned [EC Connector] Add EC Transfer Params #42433, fix(scheduler): update max_tokens from StreamingUpdate in session #37843, [Responses API] Structured output + reasoning via structural tag embedding #35904; these cover EC transfer params, streaming max_tokens session updates, and structured output/reasoning, not repeated params validation
• streaming input validation: returned related streaming/realtime PRs such as [Bugfix] Fix engine crash when realtime streaming input is empty (#34532) #34793, but those fix empty-stream/realtime error behavior rather than skipping repeated params validation

No linked issue; this addresses the existing TODO in vllm/v1/engine/async_llm.py.

Tests

• uvx ruff check vllm/v1/engine/async_llm.py vllm/v1/engine/input_processor.py tests/v1/streaming_input/test_async_llm_streaming.py
• uvx ruff format --check vllm/v1/engine/async_llm.py vllm/v1/engine/input_processor.py tests/v1/streaming_input/test_async_llm_streaming.py
• python -m pytest tests/v1/streaming_input/test_async_llm_streaming.py -q

AI assistance

Used OpenAI Codex to inspect the streaming input path, implement the small patch, run duplicate-work searches, and execute the targeted tests.

[claude]

Claude Code Review

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[github-actions]

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[gemini-code-assist]

Code Review

This pull request optimizes the V1 engine's input processing by avoiding redundant validation of sampling parameters when they are reused across streaming input chunks. It introduces a validate_params flag in the process_inputs method and updates the AsyncLLM to skip validation for reused parameters. Additionally, new unit tests were added to verify this behavior. I have no feedback to provide.

[Copilot]

Pull request overview

This PR improves the v1 async streaming-input hot path by avoiding repeated SamplingParams/PoolingParams validation for streaming chunks that reuse the already-validated request-level sampling params, while still validating any explicit per-chunk sampling_params. It also adds targeted async tests to ensure the new behavior is enforced.

Changes:

• Add a validate_params: bool = True flag to InputProcessor.process_inputs() to allow skipping parameter verification when safe.
• In AsyncLLM._add_streaming_input_request(), skip repeated params validation for chunks that omit StreamingInput.sampling_params (reuse request-level params), but keep validation for explicit per-chunk params.
• Add focused streaming-input tests asserting correct validate_params behavior for reused vs per-chunk sampling params.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
vllm/v1/engine/input_processor.py	Adds an opt-out flag to skip _validate_params() when callers know params were already validated.
vllm/v1/engine/async_llm.py	Uses the new flag to avoid redundant validation on streaming chunks that reuse request-level params, preserving validation for per-chunk overrides.
tests/v1/streaming_input/test_async_llm_streaming.py	Adds async tests that verify validation is skipped only for reused request-level params and remains enabled for explicit per-chunk params.

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[sherkevin]

Closing after re-evaluating the PR against the project contribution policy and the current checks. The remaining failures are gate-related, but the change itself is too small to justify keeping this as a contribution target without stronger evidence/benchmarking.