# Gemini TTS API: voices, tone control, and audio formats for speech synthesis

Source: https://ofox.ai/docs/develop/models/gemini-tts

Google's Gemini speech synthesis models, called through the OpenAI-compatible `POST /v1/audio/speech`. You can describe tone and style in natural language.

**Common model IDs (copy them exactly)**

- Gemini 3.8 Flash TTS: `google/gemini-3.8-flash-tts`
- Gemini 3.8 Flash-Lite TTS: `google/gemini-3.8-flash-lite-tts` — Lower rate

For all TTS models, see the [models page](https://ofox.ai/models?provider=google). For the common endpoint parameters, see [Audio Speech API](https://ofox.ai/docs/api/openai/audio-speech).

## Examples

**Python**

```python filename="gemini_tts.py"
import os
from openai import OpenAI

client = OpenAI(base_url="https://api.ofox.ai/v1", api_key=os.environ["OFOX_API_KEY"])

with client.audio.speech.with_streaming_response.create(
    model="google/gemini-3.8-flash-tts",
    voice="Kore",
    input="The meeting today has moved to 3 p.m. Please be on time.",
    instructions="Read in a calm, clear tone, like a news broadcast.",
    response_format="wav",
) as resp:
    resp.stream_to_file("speech.wav")
```

**cURL**

```bash filename="Terminal"
curl https://api.ofox.ai/v1/audio/speech \
  -H "Authorization: Bearer $OFOX_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "google/gemini-3.8-flash-tts",
    "voice": "Kore",
    "input": "The meeting today has moved to 3 p.m. Please be on time.",
    "instructions": "Read in a calm, clear tone, like a news broadcast.",
    "response_format": "wav"
  }' \
  --output speech.wav
```

## Parameter values

**Key point**:voice is required and takes a Google voice name; only wav and pcm output formats are supported.

| Parameter | Description |
|---|---|
| `voice` | **Required.** A Google voice name, such as `Kore`, `Puck`, or `Zephyr`. For the full list, see [Gemini official docs · Voices](https://ai.google.dev/gemini-api/docs/speech-generation#voices). OpenAI voice names (such as `alloy`) cannot be used |
| `response_format` | `wav` (default) or `pcm`, both 24 kHz, 16-bit, mono. `mp3`, `opus`, `aac`, and `flac` are not supported |
| `instructions` | Describes tone, pace, and style in natural language, such as "in a cheerful tone" or "slow down" |
| `language_code` | Language code, such as `cmn-CN` or `en-US`; detected automatically from the text if omitted |
| `stream_format` | Set to `sse` to receive audio chunks as a stream, suited to playing while generating |

If you need mp3, fetch wav first and convert it with a tool such as ffmpeg: `ffmpeg -i speech.wav speech.mp3`.

## Latency and long text
In our tests, a Chinese sentence of about 20 characters returned in about 3 seconds. About 6,000 characters of English produced about 9 minutes of audio in about 105 seconds, with a wav file of about 26 MB. For long text, set the client timeout to 300 seconds or more, or use `stream_format: "sse"` to fetch it as a stream.

## Common errors
| HTTP | Error message | Cause | Fix |
|---|---|---|---|
| 400 | `voice is required for speech completion` | `voice` was not sent | Pass a voice name, such as `Kore` |
| 400 | `Request contains an invalid argument.` | The voice name does not exist, e.g. `alloy` was passed | Use a Google voice name |
| 400 | `unsupported Gemini response_format: mp3` | Unsupported format | Use `wav` or `pcm` |

For more errors, see [Audio Speech API](https://ofox.ai/docs/api/openai/audio-speech#errors).

## Official docs
- [Gemini API · Speech generation](https://ai.google.dev/gemini-api/docs/speech-generation)

## FAQ

**What do I put in voice for Gemini TTS?**

A Google voice name, such as Kore, Puck, or Zephyr; the full list is in the official Gemini docs. voice is required, and OpenAI voices such as alloy cannot be used.

**Can Gemini TTS output mp3?**

No. Only wav and pcm (24 kHz, 16-bit, mono) are supported. If you need mp3, convert it yourself, e.g. ffmpeg -i speech.wav speech.mp3.

**How do I control tone with Gemini TTS?**

Describe it in natural language with the instructions parameter, for example "read in a cheerful tone" or "calm and steady, like a news broadcast".

**Gemini TTS returns voice is required?**

Gemini TTS requires voice, for example "voice": "Kore".