> ## 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 and JavaScript.

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.

<CodeGroup>
  ```python Python theme={null}
  from zeroruntime import Pipeline
  from zeroruntime.plugins import UltravoxRealtime

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

  pipeline = Pipeline(llm=model)
  ```

  ```typescript Node JS theme={null}
  import { Pipeline } from '@zeroruntime/js-sdk';
  import { UltravoxRealtime } from '@zeroruntime/js-sdk/plugins';

  const model = UltravoxRealtime({
    model: 'fixie-ai/ultravox',
    config: {
      voice: '54ebeae1-88df-4d66-af13-6c41283b4332',
      language_hint: 'en',
    },
  });

  const pipeline = Pipeline({ llm: model });
  ```
</CodeGroup>

## Parameters

*Constructor parameters for `UltravoxRealtime`. The Python and Node JS SDKs share these field names. Model behavior is configured
through the `config` mapping.*

### Constructor

| Parameter | Type   | Default               | Description                                                                             |
| --------- | ------ | --------------------- | --------------------------------------------------------------------------------------- |
| `model`   | `str`  | `"fixie-ai/ultravox"` | Ultravox model to use.                                                                  |
| `api_key` | `str`  | `None`                | Ultravox API key. Falls back to the `ULTRAVOX_API_KEY` environment variable when unset. |
| `config`  | `dict` | `None`                | Model behavior configuration (see below). When omitted, provider defaults are used.     |

### `config` keys

| Field                                  | Type    | Default                | Description                                                                 |
| -------------------------------------- | ------- | ---------------------- | --------------------------------------------------------------------------- |
| `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).