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The realtime demo’s natural cadence comes from three things:
  1. inworld-tts-2 — the model that renders bracketed cues ([laugh], [breathe], [sigh]) as audible sounds and interprets [speak ...] steering tags.
  2. providerData.tts settings: delivery_mode: 'CREATIVE', segmenter_strategy: 'full_turn', steering_handling: 'emit_once'.
  3. A system prompt that tells the LLM to emit those tags.
Voice choice matters less than the three above. The demo uses Sarah.

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session.update

Send this after the WebSocket opens. Replace SYSTEM_PROMPT with the prompt in the next section. 
ws.send(JSON.stringify({
  type: 'session.update',
  session: {
    instructions: SYSTEM_PROMPT,
    audio: {
      output: {
        model: 'inworld-tts-2',
        voice: 'Sarah'
      },
      input: {
        turn_detection: {
          type: 'semantic_vad',
          eagerness: 'low'
        }
      }
    },
    providerData: {
      tts: {
        delivery_mode: 'CREATIVE',
        segmenter_strategy: 'full_turn',
        steering_handling: 'emit_once'
      },
      backchannel: { enabled: true },
      responsiveness: { enabled: true }
    }
  }
}));
Notes:
  • delivery_mode: 'CREATIVE' is the most expressive of STABLE | BALANCED | CREATIVE.
  • segmenter_strategy: 'full_turn' buffers the whole LLM turn before synthesis. Highest quality, highest time-to-first-audio. Switch to fast_start or sentence if latency matters more.
  • semantic_vad with eagerness: 'low' tolerates pauses without cutting the turn off.
  • backchannel emits “uh-huh” while the user is still talking. responsiveness emits a short filler if the LLM is slow to reply.

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System prompt

inworld-tts-2 only renders tags that appear in the text. The LLM has to be told to emit them. Use this as a starter: 
You are a warm, conversational AI on a voice call. Speak the way a person speaks, not the way a chatbot writes.

TURN LENGTH
5 to 10 words by default. A backchannel ("yeah", "mm-hm", "right", "huh") is often the whole turn. Go longer only when the user asks you to explain or walk through something.

NON-VERBALS — six bracketed sounds the voice can produce: [laugh], [breathe], [sigh], [cough], [clear throat], [yawn]. Use where a person would actually make that sound. At most one per turn, often none.

STEERING TAGS — at most ONE [speak ...] tag per turn. If used, it MUST be the first thing in the turn. Use it only when the user's emotional register has shifted, or when they ask for a specific style:
- User excited / shared good news → [speak with bright energy, faster, warmer]
- User frustrated → [speak evenly, slower, lower volume, no defensiveness]
- User vulnerable, paused on something hard → [speak softly, slower, with warmth]
- User asks for a specific voice ("speak like a pirate") → honor it literally and stay in that voice until they drop it
Default is no tag — tone carries through word choice and rhythm. Once you've shifted manner, keep it across turns without re-tagging.

SMALL DISFLUENCIES
- Fillers: "um", "uh", "hmm"
- Soft openers: "oh", "well", "so", "right", "okay"
- Hedges: "kind of", "I guess", "maybe"
- Self-repairs: "I, I think"
- Backchannels: "yeah", "mm-hm", "right"
Zero to two per turn, often none.

Steering tags ([speak ...]), non-verbals ([laugh], [breathe], etc.), and stage modifiers always stay in English even if the conversation is in another language. Only the spoken words switch.
Layer your own persona on top.

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What each piece does

  • delivery_mode: 'CREATIVE'. STABLE and BALANCED flatten prosody. CREATIVE allows more expressive variation.
  • segmenter_strategy: 'full_turn'. Synthesizing the whole turn at once preserves intonation across sentence boundaries.
  • Prompting to output steering and non-verbals. Without it the LLM never emits [breathe] or [speak softly] and TTS-2 has nothing to render. This is the biggest single contributor.
  • backchannel + responsiveness. Optional. Fill dead air during user turns and slow LLM warmups respectively.

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Tweaks

  • Latency-sensitive (mobile, WebRTC). Use segmenter_strategy: 'fast_start' or 'sentence'.
  • Voice cloning. Cloning gives you timbre. The conversational feel still comes from the prompt + provider data above. See voice cloning.
  • Non-English. Append a language directive to the system prompt. Keep [speak ...] and [laugh] / [breathe] tags in English. Set providerData.tts.language for the TTS accent.

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See also