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

# Migrating from Sonic 3 to Sonic 3.5

> What's new in Sonic 3.5 and what to know before you switch.

Sonic 3.5 is a drop-in replacement for Sonic 3 for most customers. Your existing voice IDs, request shape, and prompts work as-is.

## Switching the model ID

```python lines theme={null}
# Previous
model_id = "sonic-3"

# Current
model_id = "sonic-3.5"
```

## What's new in Sonic 3.5

Compared with `sonic-3`:

* **More natural speech, pacing, and emotional expression**, especially on expressive, conversational, and support-style transcripts.
* **Cleaner audio quality** across all languages and voices.
* **Dramatically better alphanumeric read-out** — confirmation codes, order numbers, phone numbers, IDs, and emails sound meaningfully more natural across all supported languages.
* **Step-change multilingual performance**, particularly Hebrew, Japanese, Spanish, Hindi, German, Korean, and French.
* **English heteronyms** like *read*, *bass*, and *bow* now have more accurate pronunciation in context.

## What to know before you switch

* **Spell tags work the same way.** If you already wrap alphanumerics in `<spell>...</spell>`, you don't need to change anything — you'll just get better-sounding output. If you use punctuation (commas, periods, spaces) instead of spell tags, the recommended format has changed; see [prompting tips](/build-with-cartesia/capability-guides/prompting-tips#controlling-pacing-and-spelling).
* **Timestamps.** End-of-word timestamps used for interruption handling should be unchanged. If you depend on beginning-of-word timestamps, test carefully and reach out if you see regressions.

## Tips for best results

* **Providing proper context to the model improves naturalness.** See our [buffering guide](/use-the-api/tts-websocket/buffering) for details.
* **Keep prompts in their natural written form.** Heavy pre-processing (stripping punctuation, forcing all caps) generally hurts output quality.