> ## Documentation Index
> Fetch the complete documentation index at: https://docs.zeroruntime.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Ultravox

> Use the Ultravox speech-to-speech model in a Zero Runtime pipeline. Setup and usage in Python.

Ultravox is a **speech-to-speech** model. It handles transcription, reasoning, and voice
synthesis end-to-end in a single model, so it goes in the pipeline's `llm` slot with no
separate STT or TTS.

## Setup

Set your Ultravox API key in the worker environment. Generate a key from the [Ultravox dashboard](https://app.ultravox.ai/):

```bash theme={null}
export ULTRAVOX_API_KEY=<key>
```

## Usage

Pass the realtime model to the pipeline's `llm` slot, no `stt` or `tts` needed.

```python theme={null}
from zrt import Pipeline
from zrt.plugins import UltravoxRealtime, UltravoxLiveConfig

model = UltravoxRealtime(
    config=UltravoxLiveConfig(
        model="fixie-ai/ultravox",
        voice="54ebeae1-88df-4d66-af13-6c41283b4332",
        language_hint="en",
    ),
)

pipeline = Pipeline(llm=model)
```

## Parameters

*Constructor parameters for the Python SDK (`UltravoxRealtime`). Model behavior is configured
through the `config` object (`UltravoxLiveConfig`).*

### Constructor

| Parameter | Type                 | Default | Description                                                                                     |
| --------- | -------------------- | ------- | ----------------------------------------------------------------------------------------------- |
| `api_key` | `str`                | `None`  | Ultravox API key. Falls back to the `ULTRAVOX_API_KEY` environment variable when unset.         |
| `config`  | `UltravoxLiveConfig` | `None`  | Model behavior configuration (see below). When omitted, a default `UltravoxLiveConfig` is used. |

### `UltravoxLiveConfig`

| Field                                  | Type    | Default                | Description                                                                 |
| -------------------------------------- | ------- | ---------------------- | --------------------------------------------------------------------------- |
| `model`                                | `str`   | `"fixie-ai/ultravox"`  | Ultravox model to use.                                                      |
| `voice`                                | `str`   | `None`                 | Voice ID for the synthesized speech.                                        |
| `language_hint`                        | `str`   | `"en"`                 | Hint for the conversation's language.                                       |
| `temperature`                          | `float` | `None`                 | Controls the randomness of responses (0.0 to 1.0).                          |
| `max_duration`                         | `str`   | `None`                 | Maximum duration of the call (e.g. `"600s"`).                               |
| `time_exceeded_message`                | `str`   | `None`                 | Message spoken when the maximum duration is exceeded.                       |
| `input_sample_rate`                    | `int`   | `48000`                | Sample rate (Hz) of the input audio.                                        |
| `output_sample_rate`                   | `int`   | `24000`                | Sample rate (Hz) of the synthesized output audio.                           |
| `client_buffer_size_ms`                | `int`   | `30000`                | Client-side audio buffer size in milliseconds.                              |
| `vad_turn_endpoint_delay_ms`           | `int`   | `800`                  | Milliseconds of silence before voice activity detection ends a turn.        |
| `vad_minimum_turn_duration_ms`         | `int`   | `600`                  | Minimum duration in milliseconds for a valid speech turn.                   |
| `vad_minimum_interruption_duration_ms` | `int`   | `None`                 | Minimum duration in milliseconds of speech required to interrupt the agent. |
| `vad_frame_activation_threshold`       | `float` | `0.4`                  | Frame activation threshold for voice activity detection.                    |
| `first_speaker`                        | `str`   | `"FIRST_SPEAKER_USER"` | Determines who speaks first.                                                |
| `enable_greeting_prompt`               | `bool`  | `False`                | Whether to enable an initial greeting prompt.                               |
| `base_url`                             | `str`   | `None`                 | Override the Ultravox streaming endpoint.                                   |

To pair the realtime model with an external STT or TTS, see [Hybrid mode](/build/configure-a-pipeline/modes#hybrid).