Speech to Speech (Hydra)

Hydra - Speech to Speech

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Hydra is a realtime speech-to-speech model. The client streams microphone audio over a WebSocket, the model returns synthesised speech in the same socket, and turn-taking is handled server-side. There is no transcript on the wire - audio bytes are the payload.

If you’ve used the OpenAI Realtime API, Hydra fills the same role on the Smallest AI stack.

Use ?model=hydra-v1.1 on all new integrations. It’s the current release and the default (10 voices: zoe, maya, elena, ivy, grace, alex, aria, leo, sam, kai). ?model=hydra (the original release, 6 voices: wren, sloane, marlowe, reed, knox, tate) is deprecated: sessions still open, but the server emits a warning frame with code: "model_deprecated" on connect. Migrate to ?model=hydra-v1.1. Full per-version reference: Hydra model card. See also: Deprecation Notices.

Common use cases

Phone agents

Sub-second latency from end-of-user-speech to first audio chunk. Drop-in for outbound and inbound voice flows.

In-product voice copilots

Hands-free assistants embedded in web and mobile apps - barge-in handled by the model.

Kiosks & in-car

One WebSocket, predictable failure modes, no STT/LLM/TTS glue to maintain.

Voice-first consumer apps

Companions, audio diaries, language tutors - natural turn-taking out of the box.

What’s on the wire

Two things to know up front:

  • Stream audio continuously - no manual commit or end-of-turn. Hydra detects turn boundaries on its own.
  • Full-duplex - the user can speak over the model. The in-flight response cancels automatically with status: "cancelled", reason: "interrupted".

Next

Quickstart

Clone the reference client, paste your API key, and talk to Hydra in your browser.

WebSocket connection

Connect URL, auth, idle timeout, close codes. Python + Node + Browser snippets.

Managing sessions

Session lifecycle, persona, voice, mid-session updates, conversation items.

Turn detection & barge-in

How the model detects speech, how to handle barge-in cleanly on the client.

Tool calling

Declare tools, stream arguments, post results back, narrate the answer.

Prompting voice agents

System prompts, voice identity, length discipline, tool-call prompting.