AssemblyAI: Universal-3.5 Pro

AssemblyAI TTS 1 is a text-to-speech model from AssemblyAI. It is suited for voice agents, narration, and multilingual applications, and draws on a public library of several thousand voices across languages. Voices are selected by slug or ID, and the spoken language follows the chosen voice or can be set explicitly through the language provider option.

Universal-3.6 Pro is AssemblyAI's speech-to-text model and the successor to Universal-3.5 Pro, served here through AssemblyAI's Sync API. Compared with Universal-3.5 Pro, AssemblyAI reports 30% fewer errors on short conversational utterances, 12% fewer errors on average across supported languages, 14 newly supported languages, and better recognition and formatting of entities such as phone numbers, addresses, emails, and codes. It returns a complete transcript with word-level timestamps in a single synchronous response for audio up to 20 minutes, accepts WAV and compressed input (MP3, M4A, AAC, Ogg/Opus, FLAC, and WebM), and supports free-text prompting, keyterms, and conversation context to steer transcription toward domain vocabulary.