> ## Documentation Index
> Fetch the complete documentation index at: https://docs.heybreez.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Chat Model Nodes

Chat Model nodes determine which large-language model the agent uses for its reasoning, responses, and task execution.\
Each model node represents a different provider or model family, but they all serve the same purpose: powering the agent’s thought process during a session.

These nodes allow you to customize:

* **Model selection** — choose the specific LLM used for generation.
* **Advanced settings** — such as reasoning depth or provider-specific optimization.
* **Performance tradeoffs** — balancing accuracy, latency, and cost depending on your use case.

Chat models can be configured in two places:

* **Start Node** — sets the default model for the entire call.
* **Agent Nodes** — optionally override the Start Node’s model for specific tool paths.

Below are the supported model providers and their configuration details.

***

## OpenAI Brain

The **OpenAI Brain** node selects which OpenAI large-language model powers the agent’s reasoning and responses.\
This model is invoked whenever the agent “thinks” or generates text during a call.

### Main Settings

#### Model

Choose the OpenAI model used for this agent.

Models vary in speed, cost, and reasoning depth.\
The **o-series** models (o1, o3, o4) are optimized for advanced reasoning tasks, while the GPT-4.x and GPT-5.x families provide strong general performance for real-time voice agents.

The dropdown includes:

* GPT-5.1 and GPT-5.x variants
* GPT-4.1 and GPT-4.x variants
* o-series models (o1/o3/o4, Mini/Nano variants)

> Select the model that best fits your latency and reasoning requirements.

#### Reasoning Effort (GPT-5.1)

Controls how much internal reasoning GPT-5.1 performs. ***This setting only applies when using a GPT-5.1 model.*** Options:

* **None (Recommended for Voice)** Choosing None allows OpenAI to automatically route reasoning and is generally preferred for real-time conversational latency.
* **Low**
* **Medium**
* **High**

***

## OpenAI Realtime Brain

The OpenAI Realtime Brain is a multimodal realtime model that handles **hearing, thinking, and speaking** within a single node.\
When selected on the Start node or an Agent node, it automatically replaces the Hear, Think, and Speak connectors (unless you enable a separate TTS voice from the settings).

### Main Settings

#### Use Separate Voice

Controls whether or not the realtime model returns **audio**. Options:

* **No (Use OpenAI built-in voice)** — Use OpenAI's built-in realtime voice. Simpler but slightly more expensive than using a different Voice model.
* **Yes (Use separate TTS node)** — Run the realtime model in text-only mode and use a separate Voice model for audio. This gives you more control over voices.
  * A **Speak** connector becomes available so you can attach a separate TTS node.
  * **Voice** and **Speed** fields are hidden.

#### Voice

Voice used when the model returns built-in audio.\
Only shown when **Use Separate Voice = No**.

#### Model

Select the OpenAI Realtime model to use.

#### Speed

Controls playback speed of generated audio.\
Range: **0.25 – 1.5**\
Only shown when **Use Separate Voice = No**.