Hydra

Smallest AI is a voice-AI platform offering text-to-speech, speech-to-text, speech-to-speech, and orchestrated voice-agent models for real-time deployments.

AssemblyAI is a voice AI infrastructure platform that provides APIs and models for transcribing, understanding, and interacting with speech. Its products include pre-recorded, realtime, and synchronous speech-to-text APIs; speech understanding for capabilities such as speaker identification, sentiment, chapters, and summaries; inline audio and transcript guardrails; an LLM Gateway with fallback; and a Voice Agent API with turn detection and interruption handling. The platform can combine speech-to-text, an LLM, and text-to-speech into a real-time voice-agent stack, and supports voice applications such as notetakers, call analytics, medical transcription, dictation, agent assist, and AI scribes. AssemblyAI states that its speech-to-text APIs support 99 languages and that its infrastructure processes about 2 million hours of audio per day.

Speech To Speech is an open-source, modular voice-agent pipeline from Hugging Face. It chains voice activity detection, speech-to-text, a language model, and text-to-speech as separate stages connected by queues; each stage has interchangeable backends. The language-model stage supports OpenAI-compatible protocols and can use hosted providers, Hugging Face Inference Providers, vLLM, or llama.cpp, while the system streams transcripts, generated text, tool calls, and synthesized audio. The project exposes the core OpenAI Realtime event set through WebSocket and WebRTC and includes commands for running a realtime server, a microphone-and-speaker client, or a local setup. It is distributed as a Python package and requires Python 3.10 or newer. The repository says the pipeline is used as the conversation backend for Reachy Mini robots.

Speechify is an AI voice and text-to-speech platform that reads written material aloud and also provides speech-to-text, conversational, and business-focused AI tools. Its SpeechifyAI service offers text-to-speech and realtime voice-agent APIs through a single interface, with streaming synthesis, zero-shot voice cloning from a consented reference clip, SSML-based emotion control, and multilingual output for English, German, Mexican Spanish, French, Italian, and Brazilian Portuguese. The service includes the Simba model family, which the site describes as streaming-native speech models designed to model voice identity, expression, and language for realtime conversation. Speechify was co-founded by Cliff Weitzman, who initially built it to consume written material through audio.

AssemblyAI's Voice Agent API is an API for building real-time voice agents. It connects speech-to-text, an LLM, and text-to-speech through a single WebSocket, with built-in turn detection and interruption handling.