Fish Audio open-sources S2: expressive 80+ language TTS with inline emotion tags
Fish Audio released S2 (S2 Pro) on 2026-03-09 with weights, fine-tuning code and an SGLang-based production inference stack: a Dual-AR TTS on a Qwen3-4B backbone trained on 10M+ hours in ~80 languages, with free-form [bracket] emotion and paralinguistic cues and multi-speaker dialogue. It led open-weights TTS on Artificial Analysis until Breeze TTS 2 (Aug 2026). The closed follow-up S2.1 Pro (June 2026) was offered as a free API.
Key facts
- Dual-AR: 4B time-axis + 400M depth-axis; RTF 0.195, ~100 ms TTFA
- Seed-TTS Eval WER 0.54% (zh) / 0.99% (en); EmergentTTS-Eval win rate 81.88%
- API id s2-pro, $15 per 1M UTF-8 bytes; weights under Fish Audio Research License (non-commercial)
- S2.1 Pro (2026-06-23): free API tier `s2.1-pro-free` through 2026-11-30, ~90 ms TTFA, 83 languages; weights not released
What happened
Fish Audio shipped S2 as a complete system: weights, fine-tuning code and a serving stack compatible with LLM-inference optimizations (SGLang). Emotion is controlled inline with natural-language tags.
Why it matters
It made open TTS with fine-grained, LLM-style prompt control and production streaming available to the public, and set the open-weights bar for most of 2026. Fish Audio's later move to a free closed API (S2.1 Pro) shows price pressure in hosted TTS.
Changelog
- 2026-09-29: created
Models
- Fish Audio S2 Pro / S2.1 Pro Fish Audio · current
Related events
- BreezeBlue releases Breeze TTS 2, the new top open-weights text-to-speech model ★★★
- Mistral releases Voxtral TTS, an open-weight 4B text-to-speech model with 3-second voice cloning ★★★
- Alibaba open-sources Qwen3-TTS (voice design, 3-second cloning, 97 ms streaming) and, a week later, Qwen3-ASR ★★★
Sources (4)
- officialFish Audio: open-sourcing S2
- paperFish Audio S2 Technical Report (arXiv 2603.08823)
- codeHugging Face: fishaudio/s2-pro
- officialFish Audio: S2.1 Pro free API
id: 2026-03-09-fish-audio-s2-open-source · updated 2026-09-29 · open in the interactive timeline