ElevenLabs Speech Engine

Speech Engine is the server-side product that lets you keep your existing agent (any LLM/workflow that produces text) and bolt a real-time voice interface in front of it. ElevenLabs runs everything between the microphone and the LLM call; you only handle "transcript in, reply text out."

[Browser / Mobile]            [ElevenLabs Cloud]               [Your Server]
       │                              │                              │
       │ 1. signed URL request ─────▶ │                              │
       │ ◀──── signed URL (15 min) ─  │                              │
       │ 2. WebRTC audio ──────────▶  │                              │
       │                              │ 3. STT + VAD + turn detect   │
       │                              │ 4. WS open ─────────────────▶│
       │                              │ 5. user_transcript event    │
       │                              │    + AbortSignal (TS) /      │
       │                              │    asyncio cancel (Py)       │
       │                              │                              │ 6. LLM call (stream)
       │                              │ 7. session.sendResponse() ◀──│
       │                              │ 8. TTS                       │
       │ 9. WebRTC audio ◀──────────  │                              │
       │                              │                              │
       │ — user interrupts — ───────▶ │                              │
       │                              │ 10. signal aborted /         │
       │                              │     in-flight LLM cancelled  │
       │                              │ 11. new user_transcript ────▶│

The SDK gives you the WebSocket server (speechEngine.attach(), SpeechEngine.Server, Python engine.serve() / engine.create_session()) plus client libraries (@elevenlabs/react, @elevenlabs/client, @elevenlabs/react-native) for the user-facing side.

Speech Engine vs ElevenAgents — pick the right product first

These look similar in marketing but are different runtime paths. Choosing the wrong one means rewriting both server and dashboard config.

Engine IDs are seng_*. Agent IDs are agent_*. If the user shows you an agent_* ID and a server URL setting, they may actually be using "Custom LLM" under ElevenAgents (an OpenAI-compatible endpoint contract) rather than Speech Engine (which uses a WebSocket protocol that the SDK implements for you). Confirm before writing code — the two are not interchangeable.

The four moving parts

1. Server (this is your code) — a WebSocket endpoint that receives transcripts and streams text back. Node or Python SDK does the protocol.
2. Dashboard config — create a Speech Engine in the ElevenLabs dashboard, note its seng_* ID, set the server URL it should connect to, pick the voice and TTS model.
3. Client (browser/mobile) — uses @elevenlabs/react, @elevenlabs/client, or @elevenlabs/react-native to open a WebRTC audio session against ElevenLabs (not against your server directly).
4. Auth bridge — your server generates short-lived signed URLs so the client can connect without ever seeing your ELEVENLABS_API_KEY.

Skip any reference below that doesn't match the current task — every file is loadable on its own.

Minimum viable conversation

Server (TypeScript, attach to Express):

import express from "express";
import { createServer } from "http";
import { ElevenLabsClient } from "@elevenlabs/elevenlabs-js";
import OpenAI from "openai";

const app = express();
const httpServer = createServer(app);

const elevenlabs = new ElevenLabsClient({ apiKey: process.env.ELEVENLABS_API_KEY });
const openai = new OpenAI({ apiKey: process.env.OPENAI_API_KEY });

// 1. Speech Engine WS endpoint — ElevenLabs connects in here per conversation
//    attach() is synchronous; it returns a handle, not a Promise.
elevenlabs.speechEngine.attach("seng_123", httpServer, "/api/speech-engine/ws", {
  async onTranscript(transcript, signal, session) {
    const stream = await openai.responses.create(
      {
        model: "gpt-4o",
        input: transcript.map(m => ({
          role: m.role === "agent" ? "assistant" : m.role,  // <-- role mapping
          content: m.content,
        })),
        stream: true,
      },
      { signal },                                            // <-- barge-in cancels LLM
    );
    session.sendResponse(stream);                            // SDK extracts text
  },
});

// 2. Signed-URL endpoint — client calls this to get a 15-min token
app.get("/api/voice/signed-url", async (_req, res) => {
  const url = await elevenlabs.conversationalAi.conversations.getSignedUrl({
    agentId: "agent_abc",
  });
  res.json({ signedUrl: url });
});

httpServer.listen(3000);

Client (React):

import { ConversationProvider, useConversationControls, useConversationStatus, useConversationMode } from "@elevenlabs/react";

function App() {
  return (
    <ConversationProvider>
      <VoiceUI />
    </ConversationProvider>
  );
}

function VoiceUI() {
  const { startSession, endSession } = useConversationControls();
  const { status } = useConversationStatus();
  const { isSpeaking } = useConversationMode();

  const start = async () => {
    const { signedUrl } = await fetch("/api/voice/signed-url").then(r => r.json());
    startSession({ signedUrl });
  };

  return (
    <>
      <p>Status: {status} {isSpeaking && "(agent speaking)"}</p>
      <button onClick={start}>Talk</button>
      <button onClick={() => endSession()}>Stop</button>
    </>
  );
}

That's a full-duplex voice loop in front of your existing agent. The signal is the only thing wired in for barge-in — the SDK does the rest.

Critical conventions and gotchas

Trip-wires for an agent who hasn't used Speech Engine before. Skim before writing code.

• transcript[i].role is 'user' | 'agent', not 'user' | 'assistant'. OpenAI/Anthropic/Gemini expect 'assistant'. Map agent → assistant when you forward the history, or the model will see a malformed conversation and answer worse.
• In TypeScript, you MUST pass { signal } to the LLM SDK call. Without it, barge-in only stops audio playback — the LLM keeps generating, you keep paying tokens, and the next turn fights stale context. OpenAI, Anthropic, and Vercel ai SDK all accept an AbortSignal option.
• In Python, barge-in raises asyncio.CancelledError inside your handler. Let it propagate. Don't wrap await session.send_response(...) in try/except Exception that swallows cancellation, or you'll leak in-flight LLM requests.
• Never put your ELEVENLABS_API_KEY in client code. The browser asks your server for a signedUrl (WebSocket) or conversationToken (WebRTC). Tokens are valid for 15 minutes — the session can run longer, but the initial connect must happen inside that window.
• session.sendResponse() auto-detects OpenAI / Anthropic / Gemini streams. You can pass the raw await openai.responses.create({ stream: true }) result directly — no manual extraction. Strings and arbitrary async iterables of strings also work.
• The server's WebSocket auth header (X-Elevenlabs-Speech-Engine-Authorization) is verified automatically by both attach() and SpeechEngine.Server, using apiKey from options or ELEVENLABS_API_KEY env. Don't add your own auth on the path — you'll fight the SDK.
• VAD thresholds, interrupt eagerness, silence detection are NOT per-conversation API knobs. They live in the dashboard ("Conversation flow" → turn eagerness Eager/Normal/Patient, turn timeout 1–30s). If a reviewer asks "tune the interrupt sensitivity," that's a dashboard change, not code.
• React Native requires Expo dev build, not Expo Go. WebRTC needs native modules (@livekit/react-native, @livekit/react-native-webrtc). Tell the user up front if they're on Expo Go — they need to switch.
• ConversationProvider is mandatory before any conversation hook. Calling useConversationControls() outside it throws at render. Wrap at the app root or the smallest subtree that contains the voice UI.
• Prefer the granular hooks for performance. useConversation() (the combined hook) re-renders on every state change in any sub-context. useConversationStatus, useConversationMode, useConversationInput, useConversationFeedback each subscribe to only their slice — use them in leaf components.
• Status has four values, not three: 'disconnected' | 'connecting' | 'connected' | 'disconnecting'. Don't forget 'disconnecting' when rendering connection state — typically render it like 'connecting' (spinner) but with a "hanging up" label.
• onAudio payload is base64-encoded text, not a Uint8Array. If you need bytes, decode it: Uint8Array.from(atob(base64Audio), c => c.charCodeAt(0)). Most apps don't need this — audio playback is handled by the SDK.
• The token method is getWebrtcToken (TS) / get_webrtc_token (Py), but the client-side option you pass into startSession is still spelled conversationToken. Old docs and tutorials may say getConversationToken — that name no longer exists.
• Local dev needs a tunnel. ElevenLabs connects INTO your server, so localhost:3000 is not reachable. ngrok http 3000 (or Cloudflare Tunnel, Tailscale Funnel, etc.); paste the public URL into the Speech Engine's server URL field in the dashboard. The tunnel must stay up for the duration of testing.
• Voice/model picks dominate latency. eleven_flash_v2_5 gives ~135 ms end-to-end first-byte audio; eleven_multilingual_v2 is higher quality but slower. Default to Flash for chat/assistant feel, Multilingual v2 only when audio quality is non-negotiable. See full-duplex.md.
• sendContextualUpdate(text) vs sendUserMessage(text) — contextual updates are background info the agent receives but does not reply to (e.g., "user just navigated to checkout"). User messages prompt a reply (e.g., a typed message in a chat fallback). Don't confuse them or the agent will start narrating page navigations.

Diagnostics first

When voice "doesn't work", check in this order:

1. Does your server's WS receive init then user_transcript? Add onInit and a log in onTranscript. No init = ElevenLabs can't reach your URL (tunnel down, wrong path, or auth header rejected). No user_transcript = client never connected (signed URL expired? wrong agent ID? mic permission denied?).
2. Does session.sendResponse(stream) see anything? Log for await (const chunk of stream) once before passing — confirms the LLM is actually streaming.
3. Browser DevTools → Console for onError / onDebug output. The client SDK surfaces auth, network, and audio errors there. Use debug: true in the React hook options. Inspect onDisconnect(details) — details.reason is 'error' | 'agent' | 'user' and tells you who closed (the agent server-side, the user clicking End, or a fatal error).
4. onVadScore callback — if it's never firing, the mic isn't capturing. Check getUserMedia permission and changeInputDevice() selection.
5. onModeChange — confirms the agent is transitioning listening → speaking → listening. If stuck in listening, your server isn't returning a response.

Picking the right next reference

• "Write the server" → server-typescript.md or server-python.md
• "Build the browser UI" → client-web.md
• "Build the mobile app" → client-mobile.md
• "Conversation feels laggy / agent talks over me / interrupts me too eagerly" → full-duplex.md
• "Need transcript bubbles, mic indicator, send a typed message, push background context" → ui-patterns.md
• "Where do I configure the engine / signed URLs / ngrok / production deploy" → dashboard-and-deployment.md