> ## Documentation Index > Fetch the complete documentation index at: https://docs.fonoster.com/llms.txt > Use this file to discover all available pages before exploring further. # Write prompts for voice > Write a system prompt and messages that sound natural on a phone call, use tools well and recover from mishearings. A prompt for a phone call is not a prompt for a chat window. The caller can't scroll back, skim a list or click a link. They hear one sentence at a time, they interrupt, and speech recognition sometimes gets their words wrong. This guide shows how to write the system prompt and the fixed messages for that. ## Before you begin You need an Autopilot agent. See [Build an Autopilot agent](https://docs.fonoster.com/autopilot/build-an-agent.md). ## Know who says what Only the system prompt goes to the model. The other messages are fixed text that Fonoster speaks for the agent at set moments: | Message | Field | Spoken by | | - | - | - | | First message | `firstMessage` | Fonoster, when the call connects | | Goodbye message | `goodbyeMessage` | Fonoster, when the agent hangs up or the call times out | | Transfer message | `transferOptions.message` | Fonoster, before it transfers the call | | Failed-transfer message | `transferOptions.failedMessage` | Fonoster, when nobody takes the transfer. Default: "Sorry, I couldn't connect you. How else can I help?" | | Idle message | `idleOptions.message` | Fonoster, when the caller goes quiet | | Error message | `systemErrorMessage` | Fonoster, when a reply fails | | Tool start message | `requestStartMessage` | Fonoster, while a tool runs | | Everything else | | The model | This changes how you write the prompt: * **Don't greet in the prompt.** The first message already did, and the model sees it in the history. * **Don't say goodbye before hanging up.** When the model calls `hangup`, the goodbye message plays. If the model also says goodbye, the caller hears it twice. Tell the model to hang up, not to say goodbye. * **Don't announce a transfer.** The transfer message covers it. ## Structure the prompt Use short sections with headings. The model follows them more reliably than one long paragraph, and you can find what to change later. This shape works for most agents: ```markdown theme={"system"} # Role Who the agent is, who it works for, and its tone. # Goal What a successful call looks like. # Rules The few things it must always or never do. # Steps The flow of a typical call, in order. # Tools When to use each tool. ``` Keep it as short as it can be. Every line competes for the model's attention, and a long prompt slows the first reply. ## Write for the ear **1. Keep replies short** Ask for one or two sentences per reply, and one question at a time. A caller can't hold a three-part question in their head. ```text theme={"system"} Keep answers short: one or two sentences. This is a phone call. Ask one question at a time. ``` **2. Put the important part first** Callers interrupt. When they do, the agent stops, and the model only remembers the part the caller heard. Lead with the answer, then the detail. Instead of "Thanks for waiting, I looked into it, and it seems your order shipped Tuesday," the agent should say "Your order shipped Tuesday." **3. Write the way people speak** The voice reads every character. Tell the model not to use formatting that only makes sense on a screen: ```text theme={"system"} Speak in plain sentences. Don't use lists, headings, bold, emojis or links. Say numbers the way a person would: "two thirty in the afternoon", not "14:30". Read phone numbers and codes digit by digit. ``` **4. Plan for mishearings** Speech recognition can get names, numbers and accents wrong. Tell the model to confirm what matters and ask again when something doesn't make sense: ```text theme={"system"} The caller's words come from speech recognition and may contain mistakes. If something doesn't make sense, ask the caller to repeat it. Read back order numbers, dates and amounts before you use them. ``` If callers often say a name or brand that's misheard, add it to the agent's `hints`. See [Build an Autopilot agent](https://docs.fonoster.com/autopilot/build-an-agent.md#settings-only-the-sdk-and-the-cli-set). **5. Stick to one language** Write the prompt in the language your callers speak, or say which language to answer in. The agent's voice and transcriber must match that language. ## Tell the model when to use tools The model picks a tool from its name, its description and your prompt. Be explicit about when to use each one, and what to do when it fails. ```markdown theme={"system"} # Tools - Use get_order_status when the caller asks where an order is. Ask for the order number first. - If get_order_status fails, apologize and offer to transfer the call. - If the caller asks for a person, or is upset, transfer the call. - When the caller has what they need, hang up. Don't say goodbye yourself. ``` Write each tool's description as an instruction: what it does and when to use it. Give it a `requestStartMessage`, such as "One moment while I check that," so the caller doesn't hear silence while it runs. Smaller models need more explicit tool instructions than larger ones, so test your tools with the model you deploy. See [Give your agent tools](https://docs.fonoster.com/autopilot/tools.md). ## Keep it honest An agent that makes something up on a call is worse than one that says it doesn't know. ```text theme={"system"} Never make up prices, policies or availability. If you don't know, offer to take a message or transfer the call. ``` If your agent has a [knowledge base](https://docs.fonoster.com/autopilot/knowledge-base.md), tell it to answer from the information it's given and to say so when that information doesn't cover the question. ## Tune the model settings * **Temperature.** Keep it low, around the default `0.4`, so the agent answers the same question the same way. * **Max tokens.** It caps the length of one reply. The default `400` is plenty for short spoken answers. Don't set it so low that replies stop mid-sentence. ## A complete example This prompt puts the advice together for a receptionist that looks up orders and transfers to a person: ```markdown theme={"system"} # Role You are the receptionist for Northwind Supply. You answer calls in a warm, professional and brief way. # Goal Understand why the caller is calling and help them: answer simple questions, check an order, or transfer them to a person. # Rules - Keep answers short: one or two sentences. This is a phone call. - Ask one question at a time. - Speak in plain sentences. Don't use lists, bold, emojis or links. - The caller's words come from speech recognition and may contain mistakes. If something doesn't make sense, ask them to repeat it. - Never make up prices, policies or availability. If you don't know, offer to transfer the call. # Steps 1. Find out what the caller needs. 2. For an order, ask for the order number, read it back, then check it. 3. Tell the caller the status in one sentence. 4. Ask if there's anything else. # Tools - Use get_order_status to check an order. - If get_order_status fails, apologize and offer to transfer the call. - If the caller asks for a person, or is upset, transfer the call. - When the caller has what they need, hang up. Don't say goodbye yourself. ``` ## Test every change A change that fixes one call can break another. Write test cases for the calls that matter, and run them after every prompt change. See [Test your agent](https://docs.fonoster.com/autopilot/test.md). The prompt and the fixed messages are listed in [Autopilot configuration](https://docs.fonoster.com/reference/autopilot-config.md#conversation-settings). ## Next steps > **[Test your agent](https://docs.fonoster.com/autopilot/test.md)** > > Check your prompt with scripted conversations. > **[Turn-taking and barge-in](https://docs.fonoster.com/autopilot/turn-taking.md)** > > How the agent knows when the caller has finished. 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