> For clean Markdown of any page, append .md to the page URL.
> For a complete documentation index, see https://docs.jambonz.org/tutorials/voice-ai-examples/assembly-ai-voice-agent/llms.txt.
> For AI client integration (Claude Code, Cursor, etc.), connect to the MCP server at https://docs.jambonz.org/_mcp/server.

# AssemblyAI Voice Agent

> **Note**
>
> The jambonz application referenced in this article can be
> found [here](https://github.com/jambonz/assemblyai-voice-agent-example).

This is an example jambonz application that connects to the [AssemblyAI Voice Agent API](https://www.assemblyai.com/docs/voice-agents/voice-agent-api) and illustrates how to build a voice-AI application using jambonz and AssemblyAI. The application uses an open-meteo REST API to enable the agent to answer callers' questions about the weather for specified locations.

## Authentication

You'll need an AssemblyAI API key with Voice Agent access. Configure it as a jambonz application environment variable in the portal (not via `process.env`):

| Variable             | Required | Description                                                                                            |
| -------------------- | -------- | ------------------------------------------------------------------------------------------------------ |
| `ASSEMBLYAI_API_KEY` | yes      | AssemblyAI API key (sent as `Authorization: Bearer …` on the voice-agent websocket). Mark as obscured. |

The example application declares this variable via the SDK's `envVars` option on `createEndpoint`, and reads it at call time from `session.data.env_vars.ASSEMBLYAI_API_KEY`. See [Application Environment Variables](/sdks/node-sdk#application-environment-variables) in the Node.js SDK guide for the declaration pattern.

## Configuring the Assistant

AssemblyAI's protocol requires a `session.update` message to be sent before the agent will accept audio. The jambonz `llm` verb sends this automatically using whatever you pass in `llmOptions` — system prompt, greeting, output voice, input biasing, turn-detection thresholds, and tools.

`llmOptions` is the AssemblyAI [`session.update.session` payload](https://www.assemblyai.com/docs/voice-agents/voice-agent-api/events-reference#sessionupdate) passed through verbatim:

```js
llmOptions: {
  system_prompt: 'You are a helpful voice agent. Help callers get the weather for a city they ask about.',
  greeting: 'Hello, how can I help you today?',
  output: { voice: 'ivy' },
  input: {
    keyterms: ['weather', 'temperature', 'celsius', 'fahrenheit'],
    turn_detection: {
      vad_threshold: 0.5,
      min_silence: 1000,
      max_silence: 3000,
      interrupt_response: true
    }
  },
  tools: [ /* ... */ ]
}
```

> **Note**
>
> Audio format is **not** configurable. AssemblyAI Voice Agent only supports `audio/pcm` at 24 kHz, which jambonz uses unconditionally. The `input.format` and `output.format` keys are overridden by jambonz before the message is sent to AssemblyAI. jambonz resamples to/from the channel's native rate automatically.

For the full list of `session` fields (voices, keyterm biasing, turn-detection knobs, etc.), refer to the [AssemblyAI events reference](https://www.assemblyai.com/docs/voice-agents/voice-agent-api/events-reference).

## Tool calls

The example application registers a `getWeather` tool that the agent can invoke to answer weather questions. Each tool entry must use AssemblyAI's flat format:

```js
{
  type: 'function',
  name: 'getWeather',
  description: 'Get current weather for a given city',
  parameters: {
    type: 'object',
    properties: {
      location: { type: 'string', description: 'City name' },
      scale: { type: 'string', enum: ['celsius', 'fahrenheit'] }
    },
    required: ['location']
  }
}
```

When the agent decides to invoke a tool, jambonz fires a `tool.call` event and routes it to the configured `toolHook`. The handler replies via `session.sendToolOutput(tool_call_id, {type: 'tool.result', tool_call_id, result})`. The `result` field should be a string the model can read — jambonz JSON-stringifies non-string values automatically.

See [AssemblyAI tool calling](https://www.assemblyai.com/docs/voice-agents/voice-agent-api/tool-calling) for the underlying protocol.

## actionHook properties

Like many jambonz verbs, the `llm` verb sends an `actionHook` with a final status when the verb completes. The payload includes a `completion_reason` property indicating why the session ended. Possible values are:

* `normal conversation end`
* `connection failure`
* `disconnect from remote end`
* `server error`
* `client error calling function`
* `client error calling mcp function`

## Resources

* [AssemblyAI Voice Agent API — product page](https://www.assemblyai.com/products/voice-agent-api)
* [AssemblyAI Voice Agent API — documentation](https://www.assemblyai.com/docs/voice-agents/voice-agent-api)

> Using jambonz to connect custom telephony to AssemblyAI's Voice Agent API