xAI's Grok text to speech API (POST https://api.x.ai/v1/tts) has 28 built-in voices, 20 languages and a set of speech tags you put straight into the text. The docs list the tags; they don't say how much each one changes. I measured a couple.
const res = await fetch('https://api.x.ai/v1/tts', {
method: 'POST',
headers: {
Authorization: `Bearer ${process.env.XAI_API_KEY}`,
'Content-Type': 'application/json',
},
body: JSON.stringify({
text: 'Stand back. <loud>This is not a drill.</loud>',
voice_id: 'eve',
language: 'en',
output_format: { codec: 'mp3', sample_rate: 24000, bit_rate: 128000 },
}),
});
const mp3 = Buffer.from(await res.arrayBuffer());
language also takes auto. Price is $15 per million characters (xAI pricing page, checked 2026-10-30).
[], [long-], [hum-tune], [laugh], [chuckle], [giggle], [cry], [tsk], [tongue-click], [lip-smack], [breath], [inhale], [exhale], [sigh].<soft>, <whisper>, <loud>, <build-intensity>, <decrease-intensity>, <higher-pitch>, <lower-pitch>, <slow>, <fast>, <sing-song>, <singing>, <emphasis>.
Same sentence, voice Eve, English, one clip with and one without the tag. One clip each, so this is a quick check rather than a benchmark.
| Tag | Length | Loudness (RMS) | Median pitch |
|---|---|---|---|
none → <loud> |
5.5 s → 5.5 s | -20.7 → -20.9 dBFS | 195 → 253 Hz |
none → <singing> |
6.5 s → 8.4 s | — | 189 → 250 Hz |
So <loud> is closer to "raised, urgent delivery" than a volume control: if you need a louder file, normalise it afterwards. <singing> stretches the words into a melody, which also makes the clip longer (and, on a per-character price, not more expensive).
If you want to hear the tags on all 28 voices before wiring up the API, I built a free front-end: Grok Text to Speech. The speech tag reference has a before/after sample for each group, and there are preset pages for whisper, shouting and singing. It's an independent project, not affiliated with xAI.