A modular voice pipeline for streaming audio input, speech transcription, agent responses, and synthesized speech output. Built with FastAPI and asynchronous Python, with explicit session lifecycle, turn ownership, interruption, and provider cleanup.

V1 scope: NVIDIA STT and VoxCPM TTS are real providers; the Agent is a mock implementation. Production readiness and container deployment have not yet been validated.

Architecture
Quickstart
Configuration
WebSocket protocol
Lifecycle and interruption
Testing
Project structure
Known limitations

Architecture

flowchart TB
    C[Audio Client] <-->|WebSocket /ws/audio · JSON + binary PCM| G[Audio Gateway · FastAPI]
    G --> P[VoicePipeline]
    P --- S[SessionState · session and audio sequence]
    P --- T[TurnManager · semantic turn state]
    G --> STT[NVIDIA ASR · STT per turn]
    STT -->|Transcript events| P
    P --> A[Mock Agent · per session]
    A -->|Response text| P
    P --> V[VoxCPM TTS · per session]
    V -->|PCM chunks| G
    G -->|Streamed audio| C

The Audio Gateway handles transport and protocol messages. VoicePipeline coordinates processing and task ownership. SessionState tracks session and audio sequence; TurnManager enforces turn transitions. A new STT adapter is created for each turn, while Agent and TTS adapters are created per session.

Quickstart

Requirements

Python 3.12 or newer and uv
Accessible NVIDIA ASR and VoxCPM services
Run commands from the repository root (config/ is resolved relative to the working directory)

Install

uv sync

Start the gateway

With the configured provider services running:

APP_ENV=development uv run uvicorn \
  voice_assistant.audio_gateway.app.main:app \
  --host 127.0.0.1 --port 8001

Check availability:

curl http://127.0.0.1:8001/ready
# {"status":"ready"}

/ready reports Gateway availability; it does not verify NVIDIA ASR or VoxCPM health.

Configuration

APP_ENV selects config/<environment>.yaml; the default is development. config/providers.yaml maps provider names to Python implementations and capability metadata. YAML data is validated using Pydantic models.

Component	Development provider	Configuration
Speech-to-text	NVIDIA ASR	`127.0.0.1:9000`, `pt-BR`, punctuation enabled
Agent	Mock Agent	In-process; simulated text response
Text-to-speech	VoxCPM	`http://127.0.0.1:8000`, `openbmb/VoxCPM2`, `default` voice

To select another environment:

APP_ENV=staging uv run uvicorn \
  voice_assistant.audio_gateway.app.main:app \
  --host 127.0.0.1 --port 8001

The staging and production files are present, but deployment readiness for those environments has not been established. When services run in different containers or machines, replace loopback addresses with reachable service endpoints.

WebSocket protocol

Property	Value
Endpoint	`ws://127.0.0.1:8001/ws/audio`
Protocol version	`1.0`
Input audio	PCM signed 16-bit little-endian, 16 kHz, mono
Transport:	JSON control messages and binary PCM frames.

Typical exchange

sequenceDiagram
    participant C as Client
    participant G as Audio Gateway
    participant P as VoicePipeline / Providers
    C->>G: session.start
    G-->>C: session.ready
    C->>G: audio.start
    G-->>C: turn.started
    loop Audio input
        C->>G: audio.chunk (turn_id, sequence_number)
        C->>G: Binary PCM frame
        P-->>G: STT partial events
        G-->>C: stt.partial
    end
    C->>G: audio.stop (turn_id)
    P-->>G: STT final transcript
    G-->>C: stt.final
    P-->>G: Agent response
    G-->>C: agent.response
    P-->>G: TTS PCM chunks
    G-->>C: Binary audio frames
    G-->>C: turn.ended

STT events may arrive while input audio is still streaming. Binary audio input is preceded by an audio.chunk control message containing turn_id and sequence_number. The Gateway limits each binary chunk to 64 KiB.

Interruption and session closure

interrupt targets a turn_id. For a matching active turn, the Gateway cancels and awaits execution, aborts STT, releases turn state, and then acknowledges with interrupted.
session.end requests cleanup and session closure; successful completion is acknowledged with session.ended.
Errors are returned as structured error events (for example, STT_START_FAILED or TURN_CANCEL_FAILED).

Lifecycle and interruption

The system separates semantic state from asynchronous execution. TurnManager controls listening, processing, speaking, and completion; VoicePipeline owns the registered turn task. The Gateway coordinates provider shutdown and WebSocket delivery.

Cancellation is cooperative: task.cancel() requests termination, and the Gateway awaits task completion before confirming a successful interruption. Startup rollback attempts to cancel any created task and abort a partially initialized STT provider. Failed cleanup is reported rather than represented as a successful retry.

Testing

Run the automated suite:

uv run pytest -q

Run Gateway tests or targeted static checks:

uv run pytest -q tests/test_gateway.py
uv run ruff check src tests --select F401,F811,F821

End-to-end smoke test

With the Gateway, NVIDIA ASR, and VoxCPM services available:

uv run python tools/voice_pipeline_smoke_test.py

The script reads audios/chafik_normalize.wav (16 kHz, mono, 16-bit PCM), sends audio over WebSocket, and checks for a final transcript, an Agent response, synthesized audio, and turn.ended. It writes raw output to /tmp/voice_pipeline_response.pcm.

Last verified checkpoint: 142 automated tests passing; the full NVIDIA STT → Mock Agent → VoxCPM TTS smoke test completed successfully. These results describe the development environment, not a production SLA.

Project structure

config/                         Environment YAML and provider catalog
src/voice_assistant/
  audio_gateway/app/            WebSocket endpoint, protocol, session state
  audio/                        Canonical audio models
  stt/                          STT contract, mock and NVIDIA adapter
  agent/                        Agent contract and mock adapter
  tts/                          TTS contract, mock and VoxCPM adapter
  config/                       Typed configuration and loading
  voice_pipeline.py             Turn execution and orchestration
  turn_manager.py               Semantic turn state machine
  providers.py                  Provider factories
tests/                          Automated tests
tools/                          Integration and smoke scripts

Known limitations

Mock Agent: responses are simulated; the V1 does not yet include a production conversational agent.
Cooperative cancellation: a coroutine that ignores cancellation may remain active after a timeout. Logical session closure does not guarantee that all external resources have terminated.
Provider availability: readiness does not constitute an end-to-end health check.
Development-oriented deployment: loopback endpoints and a fixed smoke-test WAV are configured for local execution.
Containers: the repository's Dockerfile has not been implemented or validated for deployment.

Status: V1 development baseline · Python 3.12+ · WebSocket protocol 1.0

README.md

Voice Assistant

Contents