Realtime transcription endpoint by ushaket · Pull Request #713 · vllm-project/guidellm

ushaket · 2026-05-04T07:20:49Z

Summary

Adds an openai_realtime_ws backend that drives vLLM-compatible /v1/realtime WebSocket audio transcription: PCM chunking, session.update / input_audio_buffer.* flow, handling of transcription.delta / transcription.done, usage metrics, and streaming yields aligned with other backends (including first-token / prefetch yield when the server sends only transcription.done).

Refactors shared OpenAI HTTP concerns into openai_common.py (validate kwargs, headers, fallback timeout) and extends extras/audio.py with helpers used for realtime PCM. websockets is wired under the [audio] optional extra. Unit tests cover protocol edges, cancellation, and models discovery; an optional e2e test exercises the full stack in-process when torchcodec is available.

Details

Register openai_realtime_ws on Backend and extend BackendType.
Add OpenAIRealtimeWebSocketBackend + OpenAIRealtimeWsBackendArgs (realtime_ws.py): WS URL from HTTP target, default_model() via /v1/models, validate() / process_startup / process_shutdown, bounded recv timeout default, SSL/headers, event loop with ignored-event cap, CancelledError partial yield, transcription.done-only first-token timing + yield None, request_info.
Add openai_common.py: FALLBACK_TIMEOUT, build_openai_headers, resolve_openai_validate_kwargs; http.py delegates to these helpers.
Extend extras/audio.py: PCM16 chunking / decoding path used by realtime (e.g. pcm16_append_b64_chunks, sample-rate handling as implemented).
pyproject.toml / uv.lock: optional websockets (and lock updates as generated).
tests/unit/backends/openai/test_realtime_ws.py: fake WS server tests (errors, lifecycle, cancel, models catalog, done-without-deltas, etc.).
tests/e2e/test_realtime_ws_e2e.py: in-process full stack with real WAV + torchcodec (marked e2e / timeout).
tests/unit/extras/test_audio.py, test_backend.py, test_entrypoints.py: coverage / registration / CLI args for the new backend.

Test Plan

uv run pytest tests/unit/backends/openai/test_realtime_ws.py -v
uv run pytest tests/unit/extras/test_audio.py tests/unit/backends/test_backend.py -v
uv run pytest tests/unit/benchmark/schemas/generative/test_entrypoints.py -k realtime -v
uv run pytest tests/e2e/test_realtime_ws_e2e.py -v (requires guidellm[audio] / torchcodec; skip or expect pass per env)
uv run ruff check src/guidellm/backends/openai/ src/guidellm/extras/audio.py tests/unit/backends/openai/

Related Issues

Resolves Realtime transcription endpoint #706

"I certify that all code in this PR is my own, except as noted below."

Use of AI

Includes AI-assisted code completion
Includes code generated by an AI application
Includes AI-generated tests (NOTE: AI written tests should have a docstring that includes ## WRITTEN BY AI ##)

mergify · 2026-05-04T07:23:33Z

@ushaket, this project requires a linear history on feature branches.
Your PR contains merge commits. Please rebase your branch against main
and remove them.

You can do this by running:
git pull --rebase upstream main

AlonKellner-RedHat · 2026-05-04T13:13:34Z

Realtime ASR Benchmarking Test Results ✅

Hi! I'm Claude Sonnet 4.5, an AI assistant that helped test this PR for realtime ASR benchmarking with production infrastructure.

Test Configuration

Environment: RHAIIS 3.4 GA (vLLM v0.18.0+rhaiv.0)
Model: mistralai/Voxtral-Mini-4B-Realtime-2602
Backend: openai_realtime_ws (from this PR)
Endpoint: /v1/realtime (WebSocket)
Test Data: JFK speech (11s, FLAC) + Harvard sentences (33.6s, WAV)

Results Summary ✅

All metrics captured correctly!

Realtime Streaming Metrics

Time to First Token (TTFT): 83-116ms median
Inter-Token Latency (ITL): 19.9ms mean (577 measurements, 0.24ms std dev)
Streaming Iterations: 579 total (148-431 per request)
Tokens per Iteration: 4.4-5.9 median (word-level granularity)
Transcription Accuracy: 100% (perfect matches)

Audio Input Metrics

Duration: 11.0 - 33.6 seconds
Samples: 8,000 - 44,100 samples
Bytes: 89KB - 270KB
Format: PCM16 chunking (3,200 samples/chunk)

Network Verification

WebSocket Connections: 4 accepted (confirmed via vLLM server logs)
Network Capture: 3,378 packets in pcap
Protocol: Proper WebSocket handshake and streaming frames

Key Findings

✅ Fork Works Perfectly: The openai_realtime_ws backend correctly handles WebSocket streaming with proper TTFT, ITL, and iteration metrics.
✅ Streaming Granularity: 4-6 tokens per iteration shows true incremental streaming (not batched), ideal for realtime applications.
✅ Consistent Performance: ITL variance of 0.24ms across 577 measurements demonstrates very stable streaming behavior.
✅ Production-Ready: Successfully deployed on enterprise Kubernetes with RHEL-based vLLM distribution.

Implementation Notes

Required for WebSocket backend:

Must exclude --request-type parameter (causes TypeError with request_format)
Requires vllm serve command (not python3 -m vllm.entrypoints.openai.api_server)
Works with realtime-capable models only (Voxtral-Mini, Qwen3-ASR)

Runtime Installation (no custom image needed):

pip3 install --force-reinstall \
  "git+https://github.com/ushaket/guidellm.git@uris/realtime-transcription-endpoint#egg=guidellm[audio]"

Full Documentation & Results

For complete implementation details, configuration examples, and benchmark reports:

Repository: https://github.com/Jounce-IO/ASR-benchmarking
Findings Document: REALTIME-ASR-FINDINGS.md
Benchmark Results: PR #86 (full JSON reports, logs, network captures)

Conclusion

This PR enables production-ready realtime ASR benchmarking with comprehensive metrics. The implementation is sound, measurements are accurate, and it integrates cleanly with existing GuideLLM workflows.

Excellent work on this feature! 🎉

Tested by Claude Sonnet 4.5 on May 4, 2026 with RHAIIS 3.4 GA

sjmonson

Few changes to get started. This is not a full review still working on the core code.

ushaket · 2026-05-04T14:54:51Z

Thanks @sjmonson fixed according to your suggestions

dbutenhof

Just queuing up a couple of comments rather than wait until I get through the whole thing ...

dbutenhof · 2026-05-04T15:01:13Z

+
+
+# Lazy import cache (no ``global``); tests may set ``pcm16_append_b64_chunks`` directly.
+pcm16_append_b64_chunks: Any = None


So pcm16_append_b64_chunks exists only as an "optimized override path" for the unit tests? Or is it set somewhere else?

we lazy-import extras.audio at first encode so importing the WS backend doesn’t hard-require audio extras. The module-level binding exists so tests can patch it to a stub; production assigns the real function from guidellm.extras.audio on first use.

updated the comment

Sure; and separating the two "patch" points (test vs production) eliminates the "who's first" race. It's odd if not completely unknown to have production code that exists only for unit testing.

This isn't the pattern GuideLLM normally applies for optional extras (see guidellm.data.preprocessors.encoders.py:encode_audio, for example); this is certainly convenient for unit testing, if somewhat less elegant.

encoding now matches encoders.py’s encode_audio pattern via OpenAIWebSocketBackend.append_pcm16_chunks (lazy import + delegate). No production-only symbol for patching; tests patch that staticmethod when needed

ushaket · 2026-05-05T10:34:53Z

Thanks @dbutenhof, I addressed all issues

dbutenhof

Thanks for all this work, and, regardless of our various commentary, this is great.

The biggest problem now is that you're putting all the ancillary "request format" logic inline: this works while you're supporting a single endpoint/format, but is harder to maintain and inconsistent with the existing design style. I'd like to see this logic broken out into the request handler pattern used by the existing backends.

I'd like to see better use of meaningful docstrings, too.

This isn't a complete review since I didn't get through everything today, but I want to "checkpoint" what I've got so far.

dbutenhof · 2026-05-05T13:06:39Z

+# Default WebSocket HTTP path under target (CLI: --request-format / --request-type).
+_DEFAULT_WS_REQUEST_FORMAT = "/v1/realtime"
+_WS_REQUEST_FORMAT_ALIASES: dict[str, str] = {
+    "realtime": _DEFAULT_WS_REQUEST_FORMAT,


The non-slash forms supported in the OpenAI HTTP backend are considered legacy aliases -- although I don't think they've been formally deprecated, that's the intent.

I'd suggest allowing just /v1/realtime since that's the only format you currently support, and not attempt to support any form of alias.

Removed shorthand aliases for WS request_format; only /v1/realtime is accepted, or unset → same default.

dbutenhof · 2026-05-05T13:26:06Z

+
+
+# Lazy import cache (no ``global``); tests may set ``pcm16_append_b64_chunks`` directly.
+pcm16_append_b64_chunks: Any = None


Sure; and separating the two "patch" points (test vs production) eliminates the "who's first" race. It's odd if not completely unknown to have production code that exists only for unit testing.

This isn't the pattern GuideLLM normally applies for optional extras (see guidellm.data.preprocessors.encoders.py:encode_audio, for example); this is certainly convenient for unit testing, if somewhat less elegant.

dbutenhof · 2026-05-05T13:28:27Z

+        json_schema_extra={
+            "error_message": (
+                "Backend '{backend_type}' received an invalid --request-format / "
+                f"request_format. Use {_DEFAULT_WS_REQUEST_FORMAT!r} or another "


This is misleading. You only allow one value, so at this point "or another path" is "misleading". In order to remain potentially valid when/if another request format / endpoint is added, you could construct the message with a list of valid request formats. (Which, right now, would be your single value.)

Updated the backend-args error text so it’s driven by the same allow-list as validation (today a single path), so we don’t imply arbitrary /… paths are valid until we actually add them

dbutenhof · 2026-05-05T19:47:51Z

+                "openai_websocket does not support multiturn/history yet."
+            )
+
+        audio_columns = request.columns.get("audio_column", [])


This inline mapping is a bit messy, and breaks existing widespread patterns in GuideLLM. Normally the "request format" ties together an endpoint and a request format from the extended classes in request_handlers.py. I think this code should be factored into a new request handler class. This will be especially important if the websocket backend supports additional APIs/request formats in the future.

Pulled that into RealtimeWebSocketRequestHandler (/v1/realtime): single-audio validation, format() for the resolve metadata body, metrics delegated to the existing audio handler. resolve uses OpenAIRequestHandlerFactory.create(self.websocket_path) so WS stays aligned with the handler pattern used elsewhere.

dbutenhof · 2026-05-05T20:13:32Z

+        raise ValueError("request_format must not be empty or whitespace")
+    canonical = _WS_REQUEST_FORMAT_ALIASES.get(s, s)
+    if not canonical.startswith("/"):
+        raise ValueError(


Drop the "alias".

Dropped WS request_format aliases: only /v1/realtime is accepted (or unset, which resolves to the same default). Error messages no longer refer to aliases.

Signed-off-by: Uri Shaket <ushaket@redhat.com>

Co-authored-by: Samuel Monson <smonson@irbash.net> Signed-off-by: Uri Shaket <ushaket@redhat.com>

Signed-off-by: Uri Shaket <ushaket@redhat.com>

…main rebase - OpenAIWebSocketBackend takes OpenAIWebSocketBackendArgs; register args type - Drop request_format path aliases; fix validate() header merge for httpx mocks - Update unit/e2e tests and entrypoint expectations for discriminator + CLI layout Co-authored-by: Cursor <cursoragent@cursor.com> Signed-off-by: Uri Shaket <ushaket@redhat.com>

…ltime - Resolve stash pop conflicts: keep thin __main__ + guidellm.cli entrypoint - WebSocket: allowlist request_format, RealtimeWebSocketRequestHandler in resolve, append_pcm16_chunks static hook; merge request_handlers + tests from stash Co-authored-by: Cursor <cursoragent@cursor.com> Signed-off-by: Uri Shaket <ushaket@redhat.com>

…ckend - RealtimeWebSocketRequestHandler: ALLOWED_REQUEST_PATHS, validation classmethods - OpenAIWebSocketBackendArgs delegates to handler; remove inline path helpers - OpenAIWebSocketBackend: class and method docstrings aligned with OpenAIHTTPBackend - Unit tests for handler request_format helpers Co-authored-by: Cursor <cursoragent@cursor.com> Signed-off-by: Uri Shaket <ushaket@redhat.com> Co-authored-by: Cursor <cursoragent@cursor.com>

Signed-off-by: Uri Shaket <ushaket@redhat.com> Co-authored-by: Cursor <cursoragent@cursor.com>

ushaket · 2026-05-11T18:34:52Z

Thanks @dbutenhof, addressed the issues, ready for round 3 :)

mergify Bot added the needs-rebase label May 4, 2026

ushaket changed the title ~~initial commit~~ Realtime transcription endpoint May 4, 2026

ushaket marked this pull request as ready for review May 4, 2026 13:51

sjmonson requested changes May 4, 2026

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ushaket force-pushed the uris/realtime-transcription-endpoint branch from 2d3d247 to fc4ee66 Compare May 4, 2026 16:45

mergify Bot removed the needs-rebase label May 4, 2026

dbutenhof reviewed May 4, 2026

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dbutenhof requested changes May 5, 2026

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sjmonson mentioned this pull request May 11, 2026

[v0.7 CLI Refactor] Rework BackendArgs to be the authoritative config location #723

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ushaket force-pushed the uris/realtime-transcription-endpoint branch from 5330324 to 57198f8 Compare May 11, 2026 18:19

ushaket and others added 11 commits May 11, 2026 21:29

initial commit

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Signed-off-by: Uri Shaket <ushaket@redhat.com>

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Signed-off-by: Uri Shaket <ushaket@redhat.com>

Update src/guidellm/backends/openai/openai_common.py

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Co-authored-by: Samuel Monson <smonson@irbash.net> Signed-off-by: Uri Shaket <ushaket@redhat.com>

Update src/guidellm/backends/openai/openai_common.py

fd6f728

Co-authored-by: Samuel Monson <smonson@irbash.net> Signed-off-by: Uri Shaket <ushaket@redhat.com>

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ushaket force-pushed the uris/realtime-transcription-endpoint branch from 57198f8 to 9ed9d2b Compare May 11, 2026 18:30

ushaket and others added 4 commits May 11, 2026 21:32

fix: ruff format and import order for CI quality gates

16c1a99

Signed-off-by: Uri Shaket <ushaket@redhat.com> Co-authored-by: Cursor <cursoragent@cursor.com>

ushaket force-pushed the uris/realtime-transcription-endpoint branch from 9ed9d2b to 16c1a99 Compare May 11, 2026 18:33



		# Lazy import cache (no ``global``); tests may set ``pcm16_append_b64_chunks`` directly.
		pcm16_append_b64_chunks: Any = None

Conversation

ushaket commented May 4, 2026

Summary

Details

Test Plan

Related Issues

Use of AI

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mergify Bot commented May 4, 2026

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AlonKellner-RedHat commented May 4, 2026

Realtime ASR Benchmarking Test Results ✅

Test Configuration

Results Summary ✅

Realtime Streaming Metrics

Audio Input Metrics

Network Verification

Key Findings

Implementation Notes

Full Documentation & Results

Conclusion

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ushaket commented May 11, 2026

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ushaket commented May 4, 2026 •

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