> ## Documentation Index > Fetch the complete documentation index at: https://www.bolna.ai/docs/llms.txt > Use this file to discover all available pages before exploring further. # OpenAI GPT Models for Bolna Voice Agents > Configure GPT-5.6 (Sol, Terra, Luna), GPT-5.5, GPT-5.4, or GPT-5.4-mini as the LLM for your Bolna voice agent. Covers model selection, streaming config, prompt tips, and function calling. [OpenAI's](https://openai.com/) GPT-5 family is the current generation, with **GPT-5.6** (Sol, Terra, Luna) the newest release. `gpt-5.4-mini` remains the default recommendation for most voice agents: it has low time-to-first-token and strong instruction following at a fraction of the cost of the full models. *** ## Quick config ```json theme={"system"} "llm_agent": { "agent_type": "simple_llm_agent", "agent_flow_type": "streaming", "llm_config": { "provider": "openai", "model": "gpt-5.4-mini", "max_tokens": 150, "temperature": 0.2 } } ``` To use your own OpenAI API key, connect it at [platform.bolna.ai/auth/openai](https://platform.bolna.ai/auth/openai). *** ## Supported models | Model | Context | Best for | Notes | | --------------- | ----------- | -------------------------------------------------------- | --------------------------------------------------------------- | | `gpt-5.6-sol` | 1M tokens | Hardest tasks: complex coding, deep multi-step reasoning | Newest flagship; highest cost | | `gpt-5.6-terra` | 1M tokens | High-volume production agents; balanced quality and cost | Newest; about 2x cheaper than Sol | | `gpt-5.6-luna` | 1M tokens | Fast, low-cost everyday voice agents | Newest low-cost tier; strong quality per dollar | | `gpt-5.5` | 1M tokens | Most demanding reasoning and quality | Flagship 5.5 line; high cost | | `gpt-5.5-pro` | 1M tokens | Maximum quality on complex reasoning | Highest cost and latency; not ideal for latency-sensitive voice | | `gpt-5.4` | 1M tokens | General-purpose production agents | Strong reasoning, lower cost than 5.5 | | `gpt-5.4-mini` | 400K tokens | Most voice agents | **Recommended**: fastest TTFT, lowest cost | | `gpt-4.1` | 1M tokens | Previous-gen; still available | Use if already deployed | | `gpt-4.1-mini` | 1M tokens | Previous-gen; still available | Use if already deployed | | `gpt-4o` | 128K tokens | Previous-gen; still available | Use if already deployed | **Recommendation:** Start with `gpt-5.4-mini`. Step up to `gpt-5.6-terra` or `gpt-5.6-luna` for newest-generation quality at moderate cost, or `gpt-5.6-sol` / `gpt-5.5` when you need the strongest multi-step reasoning and highest output quality (financial, medical, nuanced escalation). *** ## Key settings | Setting | Type | Recommended | Description | | ----------------- | ------- | ---------------- | --------------------------------------------- | | `provider` | string | `"openai"` | Provider name | | `model` | string | `"gpt-4.1-mini"` | Model to use | | `max_tokens` | integer | `150` | Cap on response length — keep short for voice | | `temperature` | float | `0.2` | Lower = more deterministic; good for scripts | | `agent_flow_type` | string | `"streaming"` | Always `"streaming"` for voice | ### Keep max\_tokens short Voice responses should be 1–3 sentences. `max_tokens: 150` (about 100–120 spoken words) is appropriate for most turns. A higher cap doesn't hurt quality but increases tail latency on long responses. *** ## Writing prompts for voice Prompts for voice agents differ from chat prompts: * **Use imperative sentences**: "Keep all responses under 3 sentences." * **Specify spoken format**: "Never use bullet points or markdown — speak in complete sentences." * **Define handling for off-topic questions**: "If asked something outside your scope, say: 'I can only help with appointment scheduling today.'" * **Include the welcome message in the prompt or agent config**, not as part of the system prompt instructions. See [Prompting Guide](/docs/guides/prompting/prompting-guide) for full guidance. *** ## Function calling GPT-4.1 and gpt-4.1-mini both support function calling. In Bolna, functions are defined in the [Tools Tab](/docs/agent-setup/tools-tab) and called automatically by the LLM during conversation. See [Custom Function Calls](/docs/tool-calling/custom-function-calls) for configuration. *** ## FAQ Use `gpt-5.4-mini` for most agents: lowest time-to-first-token and significantly lower cost per call. Step up to `gpt-5.6-terra` or `gpt-5.6-luna` for newest-generation quality at moderate cost, or `gpt-5.6-sol` / `gpt-5.5` for the most demanding tasks (complex financial/medical, long multi-step tool chains) where quality is the top priority. Avoid `gpt-5.5-pro` for live voice; its latency is too high for real-time calls. Lower temperature makes the model more consistent and predictable — important for scripted outbound calls where you want the agent to follow instructions reliably. For more conversational or exploratory agents, 0.5–0.7 is fine. Lower `max_tokens`, use `gpt-4.1-mini` instead of larger models, and write shorter system prompts (large prompts increase prefill time). See [Latency](/docs/concepts/latency) for a full breakdown. Yes. Connect your OpenAI account at [platform.bolna.ai/auth/openai](https://platform.bolna.ai/auth/openai). Costs will be charged to your OpenAI account, not Bolna's platform wallet (for the LLM component). *** ## Related * [LLM Tab](/docs/agent-setup/llm-tab) — configure LLM in the dashboard * [Anthropic Claude](/docs/providers/llm-model/anthropic) — alternative LLM * [Azure OpenAI](/docs/providers/llm-model/azure-openai) — OpenAI models with enterprise data residency * [Prompting Guide](/docs/guides/prompting/prompting-guide) — write effective prompts for voice * [Custom Function Calls](/docs/tool-calling/custom-function-calls) — add tools to your agent