Streaming Video Input API¶

vLLM-Omni provides a WebSocket API for streaming video frames and optional audio chunks into Qwen3-Omni, then asking questions over the buffered session context.

Each server instance runs a single model specified at startup with vllm serve <model> --omni.

Quick Start¶

Start the Server¶

vllm serve Qwen/Qwen3-Omni-30B-A3B-Instruct \
    --deploy-config vllm_omni/deploy/qwen3_omni_moe.yaml \
    --omni \
    --port 8000 \
    --trust-remote-code

Run the Example Client¶

python examples/online_serving/qwen3_omni/streaming_video_client.py \
    --url ws://localhost:8000/v1/video/chat/stream \
    --video /path/to/video.mp4 \
    --query "Describe what is happening in the video."

API Reference¶

Endpoint¶

WebSocket /v1/video/chat/stream

Protocol¶

Direction	Type	Required fields	Description
Client -> Server	`session.config`	none	First message. Configures output modalities, frame sampling, EVS, and prompts.
Client -> Server	`video.frame`	`data`	Base64 JPEG/PNG frame.
Client -> Server	`audio.chunk`	`data`	Base64 PCM16 16 kHz mono audio bytes.
Client -> Server	`video.query`	`text`	Ask a question over the buffered frames and audio.
Client -> Server	`video.done`	none	End the WebSocket session.
Server -> Client	`response.start`	none	Query generation started.
Server -> Client	`response.text.delta`	`delta`	Incremental text output.
Server -> Client	`response.text.done`	`text`	Final text output for the query.
Server -> Client	`response.output_audio.delta`	`data`, `format`	Incremental generated audio, base64 WAV.
Server -> Client	`response.output_audio.done`	none	Audio output finished.
Server -> Client	`session.done`	none	Session closed.
Server -> Client	`error`	`message`	Recoverable protocol or generation error.

Frame Consumption Reporting¶

When buffered frames include client-supplied frame_id values, the server emits video.frames.consumed after the first engine output. Its frame_ids, frames, and latest_pts_ms describe the images included in that query's prompt.

Queries stride-sample the buffered frames, keeping the last frame, then exclude frames already known to have failed decoding. Excluded frames are not replaced with other buffered frames. Frames whose background decoding has not finished remain eligible through the image URL path.

Bugfix compatibility note: video.frames.consumed now excludes known decode failures that were already excluded from the prompt. Older versions could report those frames and their timestamps as consumed. The event name and fields are unchanged; an empty selection reports empty lists and latest_pts_ms: null.

`session.config` Fields¶

Field	Type	Default	Description
`model`	string or null	null	Optional model name. Usually omitted because the server hosts one model.
`modalities`	list[string]	`["text", "audio"]`	Output modalities. Use `["text"]`, `["audio"]`, or both.
`num_frames`	integer, 1-128	`4`	Number of buffered frames sampled for each query.
`max_frames`	integer, 1-256	`50`	Maximum retained frame buffer size. Oldest frames are evicted first.
`system_prompt`	string or null	null	Optional custom system prompt.
`use_audio_in_video`	bool	`true`	Include streamed audio chunks in multimodal video understanding when audio is present.
`sampling_params_list`	list or null	null	Optional per-stage parameter dictionaries. Each provided entry replaces that stage's deployment sampling settings.
`enable_frame_filter`	bool	`true`	Enable EVS near-duplicate frame filtering.
`frame_filter_threshold`	float, 0.0-1.0	`0.95`	EVS similarity threshold. Higher keeps more frames; lower drops more near-duplicates.

Sampling parameter bugfix: sampling_params_list is now forwarded to the engine; earlier versions accepted this field but silently used deployment defaults instead. Each provided entry constructs a fresh SamplingParams for that stage in pipeline order; it is not merged with that stage's deployment defaults. Fields absent from a provided entry use SamplingParams constructor defaults. Only omitted trailing stages keep their deployment defaults. An omitted, null, or empty list keeps the engine defaults. Invalid sampling parameters return an error when the query is submitted.

For example, [{"temperature": 0.2, "max_tokens": 64}] configures the thinker while the talker and code2wav retain their deployment defaults. With only [{"temperature": 0.2}], the thinker's max_tokens and top_p use constructor defaults, not its YAML values.

Legacy Aliases¶

The server accepts these legacy field names and rewrites them before validation. New clients should send the canonical names above.

Legacy field	Canonical field
`num_sample_frames`	`num_frames`
`evs_enabled`	`enable_frame_filter`
`evs_threshold`	`frame_filter_threshold`

Environment Variables¶

Variable	Values	Default	Description
`VLLM_VIDEO_ASYNC_CHUNK`	`on`, `off`	`on`	Wire-level streaming switch. `off` buffers server-side deltas and emits coalesced outputs at the end of a query.
`VLLM_VIDEO_AUDIO_DELTA_MODE`	`fast`, `slow`	`fast`	Audio delta extraction strategy. `fast` emits only newly produced chunks; `slow` recomputes from accumulated audio and exists for A/B verification.

EVS Semantics¶

EVS compares downsampled frames and drops near-duplicate frames before they enter the session frame buffer. frame_filter_threshold controls retention: higher values are more permissive and keep more frames; lower values are more aggressive and drop more similar frames.

Known Limitations¶

Session KV reuse and incremental prefill are not implemented in this PR. Each video.query rebuilds the model prompt from the retained frame and audio buffers.
Back-to-back short replies can still expose an engine-layer scheduler race. The PR notes an observed workaround of at least 200 ms idle between turns when clients repeatedly see idle timeouts.
If the audio buffer exceeds the server limit, the server emits Audio buffer overflow and clears the currently buffered audio for the session.
The API is intended for Qwen3-Omni streaming video understanding; other models may not support the same multimodal processor arguments.