Speech-to-speech models

A speech-to-speech model hears the caller and speaks its answer in one model, rather than through separate transcription, language model, and speech synthesis stages. Telnyx AI Assistants support two speech-to-speech models:

GPT Live, OpenAI’s speech-to-speech model, used with your OpenAI API key.
PersonaPlex (beta), NVIDIA’s PersonaPlex 7B, hosted by Telnyx.

Neither model calls the assistant’s tools itself. For lookups and actions, keep delegation enabled: the speaking model handles the conversation, and a backend model or a server you host performs the work and returns the result.

Requirements at a glance

	GPT Live	PersonaPlex (beta)
`model`	`openai/gpt-live-1`	`nvidia/personaplex-7b-v1`
Provider API key	An OpenAI API key stored as an integration secret, set in `llm_api_key_ref`	None
`voice_settings.voice`	An `OpenAILive.` voice, such as `OpenAILive.marin`	A `PersonaPlex.` voice, such as `PersonaPlex.NATF1`
Languages	Multilingual	English only
Tools	Through delegation	Through delegation
Conversation flows	Not supported	Not supported

In both cases the model and voice must be paired: a speech-to-speech model with a voice from another provider is rejected when the assistant is saved.

GPT Live

GPT Live runs on OpenAI with an OpenAI API key you store as an integration secret. It supports OpenAILive. voices and Telnyx avatars. The vision tool works through a delegation model that accepts images; this is currently available in the development environment. See GPT Live voice assistants for the full configuration, delegation behavior, and verification steps.

PersonaPlex

PersonaPlex is in beta.

PersonaPlex is NVIDIA’s PersonaPlex 7B speech-to-speech model, hosted by Telnyx. It needs no provider API key. It speaks English, in one of the PersonaPlex. voices.

Configure the speaking model

Set these fields on the assistant:

Field	Configuration
`model`	`nvidia/personaplex-7b-v1`
`voice_settings.voice`	A `PersonaPlex.` voice, such as `PersonaPlex.NATF1`. Required.
`enabled_features`	Include `"telephony"` to enable calls.
`greeting`	The opening message. PersonaPlex speaks it as written when the conversation starts, then listens.
`delegation_settings`	The backend configuration for lookups and actions. Set `enabled: true`, `mode: "telnyx"`, and an available backend `model`. If you omit `delegation_settings` when creating the assistant, a Telnyx-hosted backend model is set for you.

Send this body to Create an assistant, POST /v2/ai/assistants:

{
  "name": "PersonaPlex support assistant",
  "enabled_features": ["telephony"],
  "model": "nvidia/personaplex-7b-v1",
  "instructions": "Help callers with order questions. Ask for the order number before a lookup. Never invent an order status or claim an action succeeded without a result.",
  "greeting": "Hi, I'm your order support assistant. How can I help you today?",
  "voice_settings": {
    "voice": "PersonaPlex.NATF1"
  },
  "delegation_settings": {
    "enabled": true,
    "mode": "telnyx",
    "model": "zai-org/GLM-5.3-Flash",
    "instructions": "Use the configured order lookup tool to retrieve order status. If no lookup tool is available or the lookup fails, explain that the status cannot be verified.",
    "speak_results": true
  }
}

The example selects zai-org/GLM-5.3-Flash as the delegation backend. Confirm that this model is available to your account, or replace it with another supported backend model. Attach a shared lookup tool using the tools library before testing order lookups.

Voices

PersonaPlex has 18 voices: Natural voices, which sound more natural and conversational, and Varied voices, a more varied set. Every voice speaks English, the only language PersonaPlex supports. PersonaPlex.NATM1, PersonaPlex.VARF2, and PersonaPlex.VARM3 have British English accents; the other voices have American English accents.

Voices	IDs
Natural, female	`PersonaPlex.NATF0` to `PersonaPlex.NATF3`
Natural, male	`PersonaPlex.NATM0` to `PersonaPlex.NATM3`
Varied, female	`PersonaPlex.VARF0` to `PersonaPlex.VARF4`
Varied, male	`PersonaPlex.VARM0` to `PersonaPlex.VARM4`

List available voices returns them with provider: "personaplex".

Tools and delegation

PersonaPlex hands a request that needs a lookup or action to the delegation backend: it tells the caller it is checking, waits for the result, then speaks it. The backend runs the assistant’s tools. The delegation guide covers the settings, telnyx and client modes, and result handling. Post-conversation processing runs on the delegation backend, so it requires delegation enabled in telnyx mode with a backend model.

Limits

Language: English only.
Instructions: up to 1,030 tokens, post-conversation instructions included. That is about 4,100 characters of plain English, fewer with numbers or codes. The exact count is checked when the assistant is saved.
Conversation flows and fallback models: not supported.

Pricing

PersonaPlex conversations cost $0.04 USD per minute.

Verify the assistant

Use the assistant ID with the voice assistant calling setup or the realtime voice conversation WebSocket, then:

Confirm the assistant speaks its configured greeting, then ask a conversational question and verify the response uses the selected PersonaPlex. voice.
Ask for information that requires a configured tool. Confirm the backend performs the lookup and the assistant speaks the returned result without inventing data.

Next steps

GPT Live voice assistants — GPT Live configuration in full
Delegation — backend settings, spoken results, and the client event contract
Tools library — attach tools for delegated lookups and actions

Guides

Missions

AI Insights

Assistants API

Missions API

Conversations API

For AI Agents

Speech-to-speech models

Requirements at a glance

GPT Live

PersonaPlex

Configure the speaking model

Voices

Tools and delegation

Limits

Pricing

Verify the assistant

Next steps

​Requirements at a glance

​GPT Live

​PersonaPlex

​Configure the speaking model

​Voices

​Tools and delegation

​Limits

​Pricing

​Verify the assistant

​Next steps

Requirements at a glance

GPT Live

PersonaPlex

Configure the speaking model

Voices

Tools and delegation

Limits

Pricing

Verify the assistant

Next steps