Voice Agent

Voice Agents

A voice agent answers a phone call, holds a conversation, calls your code when it needs to do something, and hands off to a person when it should. It is the same agent object as a messaging agent, with a voice block on it.

Zavu runs the whole pipeline: speech recognition, the agent's model, speech synthesis, and interruption handling. You supply the prompt and the skills.

One thing to know before anything else: voice is the channel where tools actually run. On plain text the model is not offered tools at all (see ai-agent). If your agent has to do something, voice and flows are where that works today.

Senders vs accounts (one paragraph)

A Sender is the API handle you pass as Zavu-Sender; accounts (a WhatsApp Business Account, a Facebook Page, a Telegram bot, a phone number) are the connections it routes — and what bills. Senders are free. Connecting an account in the dashboard auto-creates its sender; find it with GET /v1/senders and trust its channels array for what it can send. See the channel-setup skill for the full model.

Fastest path: take a factory agent

Zavu ships working voice agents. Talking to one on zavu.dev and then owning its code is the same agent.

npx zavudev agents catalog                     # what's available
npx zavudev agents pull kepler --dir kepler    # scaffold it locally
# booking agents: add --calendar calcom for a working Cal.com client
cd kepler
npm install                             # types + local runs
npx zavudev senders list                       # find your sender id
npx zavudev fn secrets set SENDER_ID <senderId>
npx zavudev deploy
npx zavudev agents list                        # confirm what actually landed

An agent can answer on more than one number. Connect them by id:

npx zavudev agents senders connect --agent <agentId> --sender <senderId>

agents pull writes a real TypeScript project: index.ts with the agent and its skills, a tsconfig.json, and a local stand-in for @zavudev/functions so the project typechecks and runs on your machine. Edit it freely — the file is yours, and npx zavudev deploy reconciles whatever it declares.

Declaring a voice agent in code

import { defineAgent, defineTool } from "@zavudev/functions"

defineAgent({
  senderId: process.env.SENDER_ID!,
  name: "Kepler",
  provider: "zavu",
  model: "openai/gpt-4o-mini",
  channels: ["voice", "whatsapp"],
  voice: {
    enabled: true,
    model: "openai/gpt-4o",
    greeting: "Hi, I'm Kepler. What day would you like to book?",
    // Per-language greeting, used when the caller's language differs.
    greetings: { es: "Hola, soy Kepler. ¿Qué día querés reservar?" },
    // language: "es",           // omit to follow the caller's language
    interruptible: true,         // caller can talk over the agent
    maxCallDurationMinutes: 12,
    voiceSpeed: 1.15,
  },
  prompt: `# Personality
You book meetings. One idea per turn.

# Environment
You are on a phone call. They can hear you, they cannot see a screen.
Never read a URL or a code block aloud.`,
})

The voice block

channels and voice are separate: channels: ["voice"] routes calls to this agent, voice.enabled configures how it behaves on them. You need both. An agent routed for voice with no voice block will not answer properly.

Human handoff

The headline claim is "answer, resolve, and hand off to a person." The handoff is transferPhoneNumber: set it, and the agent is given a transfer tool it can decide to use. Say when to use it in the prompt.

voice: {
  enabled: true,
  transferPhoneNumber: "+14155551234",
},
prompt: `Transfer to a human when the caller asks for one, is upset,
or asks something about their account that you cannot answer.`,

Booking without writing a backend

A voice agent that books meetings needs no code at all: add check_availability and book_meeting from the dashboard (agent, Tools, Library) and connect a Cal.com or Google calendar in the same place. Zavu hosts both skills.

Use this instead of scaffolding a booking function when the only thing the agent has to do is read a calendar and put something on it. Reach for npx zavudev agents pull kepler --calendar calcom when the booking logic is yours: qualifying first, routing to different hosts, writing to your own system too.

Skills the agent can call

Tools run on voice. Declare them next to the agent; they execute in your function, in Zavu Cloud.

defineTool({
  name: "check_availability",
  description: "Find open meeting slots. Call before offering a time.",
  parameters: {
    type: "object",
    properties: {
      preferred_time: { type: "string", description: "What the caller asked for" },
    },
    required: ["preferred_time"],
  },
  handler: async (args, ctx) => {
    const res = await fetch(`https://api.example.com/slots?q=${args.preferred_time}`)
    if (!res.ok) return { ok: false, reason: "lookup_failed", slots: [] }
    return { ok: true, slots: (await res.json()).slots }
  },
})

Run one locally without deploying:

npx zavudev fn invoke --tool check_availability --args '{"preferred_time":"tomorrow 3pm"}'

Never return a fake success. A handler that answers { booked: true, confirmationCode: "ABC-123" } when it did nothing makes the agent tell a real caller their meeting exists. Return { ok: false, reason: "not_configured" } and let the prompt handle it — the agent can say it cannot book right now, which is recoverable. A false confirmation is not.

Placing and inspecting calls

npx zavudev calls create --to +14155551234        # outbound. COSTS MONEY.
npx zavudev calls list --status completed
npx zavudev calls get <callId>                    # includes the transcript
npx zavudev calls hangup <callId>

calls get prints the conversation turn by turn, including tool calls. It is the only record of what the agent actually said, and the first place to look when a call went wrong.

From the SDK (@zavudev/sdk 0.56.0+; earlier versions have no calls resource — call the REST endpoints directly):

const { call } = await zavu.calls.create({
  to: "+14155551234",
  greeting: "Hi, this is Acme calling about your appointment.",
  language: "es-ES",            // or "auto" to follow the caller
  metadata: { campaign: "reminders" },
})

Testing before you spend money

There is no way to hear the agent without a phone or a browser. But you can test everything upstream of the audio, for free:

# The agent's brain: prompt, model, knowledge base. Nothing is delivered.
npx zavudev agents test --agent <agentId> --message "do you have anything Tuesday?"

# A skill's handler, locally.
npx zavudev fn invoke --tool check_availability --args '{"preferred_time":"Tuesday"}'

agents test runs the text path, so it will not exercise tool calling even though voice would — it warns you when that applies. Use it for the prompt, and fn invoke for the handlers.

To actually hear it, you need a phone number and a real call. There is no way to listen to the agent for free: agents test is text-only, and it says so on every run. Everything before the call is verifiable (config, prompt, handlers); the audio itself is not. Budget for that before promising a delivery date.

Inbound requires owning a number:

npx zavudev phone-numbers search --country US

Do not decide from the capabilities column alone. It is carrier-reported and has been wrong in both directions: numbers that place and answer calls have listed ["sms"], and numbers listing voice have failed to complete a call. Every number sold through Zavu is provisioned for calls when you buy it. The check that actually answers the question is the sender's own channel list:

npx zavudev senders get <senderId>    # channels includes "voice" once it can call

Requirements and cost

• The sender's agent needs voice.enabled: true, or POST /v1/calls returns 400 "The sender's agent does not have voice enabled".
• Not available with test-mode API keys.
• Inbound needs a phone number with the voice capability.
• Billed per connected minute plus telephony, from your prepaid balance. A short estimate is reserved when the call starts; you are charged the real duration when it ends. 402 means insufficient balance.

Webhook events

Every voice event carries callId, direction, from, to, status, durationSeconds, endReason and transcriptAvailable.

npx zavudev senders update <senderId> \
  --webhook-url https://api.example.com/hooks/zavu \
  --webhook-events call.completed,call.failed

Getting it wrong

• Agent answers but says nothing useful. Check the knowledge base. A prompt that says "only state what retrieval returned" with no documents attached will invent answers instead of refusing. Verify with agents test — it reports how many knowledge chunks were used.

• The number rings and the agent does not pick up. Check the sender first:

  npx zavudev senders get <senderId>    # channels must be non-empty
  

  An empty channels array means the sender is not wired to anything and cannot send or receive on any channel, voice included. That is the cause people miss, because the agent, voice.enabled, and the number all look correct while it is true. Attach a number your project owns and turn voice on:

  npx zavudev phone-numbers update <phoneNumberId> --sender <senderId>
  npx zavudev senders update <senderId> --enable-voice
  

  Only once channels includes voice is it worth checking voice.enabled and that the agent itself is enabled.

• A field you set has no effect. voiceSpeed is only honoured by voices that support rate control. greetings needs a language tag that matches what the caller actually speaks.

• Deploy said it synced but nothing changed. Read the lines above the ✓, and do not trust the exit code. npx zavudev deploy prints its warnings before the success line, but it still exits 0 for cases that leave your agent unreachable — two agents on one sender, for instance, where only one answers. A green checkmark means the deploy ran, not that your agent will be reached. If you gate CI on this, gate it on the warning lines, not on the exit status.

Prompting for voice

Voice punishes prose. What matters, in order:

1. One idea per turn. Two sentences maximum. They cannot skim.
2. Lead with the answer, then offer detail.
3. Never read URLs, code, or JSON aloud. Say what it does and where it is.
4. Read numbers and codes one character at a time.
5. Expect the transcription to mangle names, including your own product's. Tell the agent what it might sound like and to assume it means you.
6. Say what is out of scope and what to do in one sentence when it comes up.