[Bugfix] Fix Qwen3-ASR transcription streaming postprocessing

[BWAAEEEK]

Fix Qwen3-ASR transcription streaming leaking raw ASR prefixes.

Related to #35767.

Why this is not a duplicate

I checked the related open PRs and issues before opening this:

• gh issue view 35767 --repo vllm-project/vllm --comments
  • Confirmed [Enhancement]: Qwen3-ASR realtime endpoint produces degraded output — stateless segments, no cross-segment context, raw format leaks #35767 discusses the broader Qwen3-ASR realtime/raw-format leak problem and links adjacent work.
• gh pr list --repo vllm-project/vllm --state open --search "35767 in:body"
  • Found [Model] Qwen3-ASR realtime: SDK-style streaming with cross-segment context #35894 and [Feature] Add response_prefix parameter to audio transcription/translation endpoints #36018.
  • [Model] Qwen3-ASR realtime: SDK-style streaming with cross-segment context #35894 targets the realtime Qwen3-ASR streaming path.
  • [Feature] Add response_prefix parameter to audio transcription/translation endpoints #36018 adds a response_prefix parameter for audio transcription/translation endpoints.
• gh pr list --repo vllm-project/vllm --state open --search "speech_to_text post_process_output streaming"
  • No open PRs found.
• gh pr list --repo vllm-project/vllm --state open --search "Qwen3-ASR audio transcriptions streaming asr_text prefix"
  • Found [Model] Qwen3-ASR realtime: SDK-style streaming with cross-segment context #35894 and [Feature] Add response_prefix parameter to audio transcription/translation endpoints #36018, both adjacent but not the same scope.

This PR is scoped to the REST /v1/audio/transcriptions streaming path in speech_to_text, where RequestOutputKind.DELTA chunks were sent without model-specific post-processing. It does not implement realtime SDK-style streaming or add new request parameters.

What changed

• Added a default SupportsTranscription.get_streaming_post_processor() hook.
  • Default behavior is a no-op, so existing transcription models keep their current streaming behavior.
• Added a Qwen3-ASR streaming post-processor.
  • Buffers language ...<asr_text> when the prefix/tag is split across streaming deltas, including leading whitespace before the prefix.
  • Emits only the cleaned transcription text after <asr_text>.
  • Passes through normal text immediately when the output does not look like the Qwen3-ASR structured prefix.
• Updated the speech-to-text streaming generator to apply model-specific streaming post-processing before emitting SSE deltas.
• Moved inter-chunk separators to apply to the first non-empty cleaned delta, rather than raw model text.
• Added regression coverage for split Qwen3-ASR prefix/tag streaming, leading-whitespace prefix buffering, and plain text passthrough.

Tests

.venv/bin/python -m pytest tests/entrypoints/speech_to_text/transcription/test_transcription_inter_chunk_spacing.py -q

Result:

15 passed, 16 warnings

.venv/bin/pre-commit run ruff-check --files \
  vllm/model_executor/models/interfaces.py \
  vllm/model_executor/models/qwen3_asr.py \
  vllm/entrypoints/speech_to_text/base/serving.py \
  tests/entrypoints/speech_to_text/transcription/test_transcription_inter_chunk_spacing.py

Result:

Passed

.venv/bin/pre-commit run ruff-format --files \
  vllm/model_executor/models/interfaces.py \
  vllm/model_executor/models/qwen3_asr.py \
  vllm/entrypoints/speech_to_text/base/serving.py \
  tests/entrypoints/speech_to_text/transcription/test_transcription_inter_chunk_spacing.py

Result:

Passed

git diff --check

Result: passed.

AI assistance

This PR was prepared with AI assistance. I reviewed the changed code and ran the tests listed above.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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[mergify]

Hi @BWAAEEEK, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

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[gemini-code-assist]

Code Review

This pull request introduces a stateful streaming post-processor for speech-to-text models to handle structured outputs, specifically addressing the language prefix generated by Qwen3-ASR. The SupportsTranscription interface now includes a get_streaming_post_processor method, and the base serving logic has been updated to utilize this processor to strip prefixes and buffer deltas appropriately during streaming. New tests verify the correct handling of these prefixes across multiple stream chunks. I have no feedback to provide as no review comments were submitted.

[gemini-code-assist]

Code Review

This pull request introduces a stateful streaming post-processor for speech-to-text models, specifically to handle prefix stripping for models like Qwen3-ASR. The changes include updating the base serving logic to apply these post-processors to output deltas and adding unit tests for verification. A critical logic issue was identified in the Qwen3-ASR post-processor where transcription text could be swallowed if the length of the processed text decreases after the ASR tag is detected; a fix was suggested to reset the emitted length counter in such cases.

[gemini-code-assist]

The current logic for calculating new_text using emitted_len can lead to swallowed transcription text if the model produces plain text before the <asr_text> tag. If processed_text suddenly becomes shorter than emitted_len (which happens when the tag appears and post_process_output switches from returning the full raw text to just the transcription), new_text will be empty until the transcription length exceeds the previously emitted raw text length.

While waiting_for_asr_tag mitigates this for the expected prefix, a more robust approach is to reset emitted_len to 0 whenever the processed text length decreases, ensuring the full transcription is emitted upon transition.

Suggested change

	processed_text = cls.post_process_output(raw_text)
	new_text = processed_text[emitted_len:]
	emitted_len = len(processed_text)
	processed_text = cls.post_process_output(raw_text)
	if len(processed_text) < emitted_len:
	emitted_len = 0
	new_text = processed_text[emitted_len:]
	emitted_len = len(processed_text)
	return new_text

[gemini-code-assist]

Code Review

This pull request introduces a stateful streaming post-processor for speech-to-text models, specifically designed to strip model-specific prefixes like Qwen3-ASR's 'language ... <asr_text>' during streaming. The changes update the base serving generator to apply these processors and include a concrete implementation for Qwen3-ASR along with supporting unit tests. Feedback indicates that the buffering logic for the Qwen3-ASR prefix is currently too aggressive and could lead to high latency if the expected tag is missing; a suggestion was provided to add a length limit and newline check to the buffering condition.

[gemini-code-assist]

The current buffering logic for the Qwen3-ASR prefix is potentially too aggressive. If the model outputs text that starts with language but never produces the <asr_text> tag (e.g., due to hallucination or if the user actually spoke the word "language" at the start of a chunk), the processor will buffer the entire output until the request is finished. This breaks the streaming experience by introducing infinite latency for that chunk.

Consider adding a length limit (e.g., 50 characters) or a newline check to the waiting_for_asr_tag condition. The structured prefix is expected to be short and on a single line.

Suggested change

	waiting_for_asr_tag = _ASR_TEXT_TAG not in raw_text and (
	_LANGUAGE_PREFIX.startswith(raw_text)
	or raw_text.startswith(_LANGUAGE_PREFIX)
	)
	waiting_for_asr_tag = _ASR_TEXT_TAG not in raw_text and (
	_LANGUAGE_PREFIX.startswith(raw_text)
	or (raw_text.startswith(_LANGUAGE_PREFIX) and len(raw_text) < 50 and "\n" not in raw_text)
	)

[DarkLight1337]

@NickLucche PTAL

[NickLucche]

Thanks for the PR @BWAAEEEK .
As I commented below, I think we should find a cleaner way to avoid "nonlocal method-bound vars" that look like a hack.
Some alternatives I provided

• keep the actual state this function needs in serving.py, in a simple dataclass that we pass to this function (so we expand it if needs be without editing every instance). State is managed by the S2T serving class.
• we define a stateful *StreamingPostProcessor class that every model can implement and return through the getter (called once).

[NickLucche]

We should get this once in init or similar

[NickLucche]

this pattern of using nonlocal variables here is quite weird and likely signifies something's off about our (stateful) interface here.

We can either keep the actual state this function needs in serving.py, in a simple dataclass that we pass to this function (so we expand it if needs be without editing every instance).
OR we define a stateful *StreamingPostProcessor class that every model can implement and return through the getter (called once).

[BWAAEEEK]

Thanks @NickLucche, I refactored this to use a stateful streaming post-processor class instead of a closure with nonlocal state.

The model hook now returns a StreamingTranscriptionPostProcessor class, and the S2T serving class caches that class during init. I instantiate a fresh processor per audio chunk so mutable parsing state does not leak across chunks or concurrent requests, while avoiding repeated getter lookup inside the streaming loop.

For Qwen3-ASR, Qwen3ASRStreamingPostProcessor now owns the streaming parser state explicitly, and the non-streaming / streaming paths share the same tag-stripping helper.

I also kept the earlier edge-case handling around incomplete language ... prefixes:

• incomplete structured prefixes are emitted on finish instead of being swallowed
• long or newline-containing language ... plain text stops buffering instead of waiting indefinitely for <asr_text>
• independent processor state and delayed second-chunk prefix handling are covered by tests

Validation:

.venv/bin/python -m pytest tests/entrypoints/speech_to_text/transcription/test_transcription_inter_chunk_spacing.py -q
# 19 passed

.venv/bin/pre-commit run --files \
  vllm/model_executor/models/interfaces.py \
  vllm/model_executor/models/qwen3_asr.py \
  vllm/entrypoints/speech_to_text/base/serving.py \
  tests/entrypoints/speech_to_text/transcription/test_transcription_inter_chunk_spacing.py
# Passed

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @BWAAEEEK.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork