The Ultravox REST API is used to create calls but you must use one of the Ultravox client SDKs to join and end calls. This page primarily uses examples in JavaScript. The concepts are the same across all the different SDK implementations.

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Methods

The core of the SDK is the UltravoxSession. The session is used to join and leave calls. The UltravoxSession contains methods for:
  • Joining/leaving a call
  • Sending text messages to the agent
  • Changing the output medium for how the agent replies
  • Registering client tools
  • Muting the microphone/speaker

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joinCall()

Joins a call. 
joinCall(joinUrl: string, clientVersion?: string): void
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joinUrl
string
required
The joinUrl that was returned from the Create Call request.
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clientVersion
string
Optional string that can be used for application version tracking. Will be appended to the call and be available in the clientVersion field in the Get Call response.

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leaveCall()

Leaves the current call. Returns a promise (with no return value) that resolves when the call has successfully been left. 
async leaveCall(): Promise<void>

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sendText()

Sends a text message to the agent. If deferResponse is set, the agent will not respond (i.e. no LLM generation will be done). 
sendText(text: string, deferResponse?: boolean): void
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text
string
required
The message to send to the agent.
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deferResponse
boolean
Set to true to skip LLM generation (agent won’t reply). Can be used to provide additional guidance to the model.

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setOutputMedium()

Sets the agent’s output medium for future utterances. If the agent is currently speaking, this will take effect at the end of the agent’s utterance. 
setOutputMedium(medium: Medium): void
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medium
Medium
required
How replies are communicated. Must be either 'text' or 'voice'.
Also see muteSpeaker and unmuteSpeaker below.

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registerToolImplementation()

Registers a client tool implementation with the given name. If the call is started with a client-implemented tool, this implementation will be invoked when the model calls the tool. 
registerToolImplementation(name: string, implementation: ClientToolImplementation): void
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name
string
required
The name of the tool. Must match what is defined in selectedTools during Create Call.

If nameOverride is set then must match that name. Otherwise must match modelToolName.
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implementation
ClientToolImplementation
required
The function that implements the tool’s logic. This is a function that:Accepts parameters → An object containing key-value pairs for the tool’s parameters. The keys will be strings.Returns → Either a string result, or an object with a result string and a responseType, or a Promise that resolves to one of these.For example:
  const stock_price = (parameters) => {
    ...  // to be implemented
    return `Stock price is ${value}`;
  };

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registerToolImplementations()

Convenience batch wrapper for registerToolImplementation. 
registerToolImplementations(implementationMap: { [name: string]: ClientToolImplementation }): void
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implementationMap
Object
required
An object where each key (a string) represents the name of the tool and each value is a ClientToolImplementation function.

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isMicMuted()

Returns a boolen indicating if the end user’s microphone is muted. This is scoped to the Ultravox SDK and does not detect muting done by the user outside of your application. 
isMicMuted(): boolean

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isSpeakerMuted()

Returns a boolen indicating if the speaker (the agent’s voice output) is muted. This is scoped to the Ultravox SDK and does not detect muting done by the user outside of your application. 
isSpeakerMuted(): boolean

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muteMic()

Mutes the end user’s microphone. This is scoped to the Ultravox SDK. 
muteMic(): void

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unmuteMic()

Unmutes the end user’s microphone. This is scoped to the Ultravox SDK. 
unmuteMic(): void

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muteSpeaker()

Mutes the end user’s speaker (the agent’s voice output). This is scoped to the Ultravox SDK. 
muteSpeaker(): void

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unmuteSpeaker()

Unmutes the end user’s speaker (the agent’s voice output). This is scoped to the Ultravox SDK. 
unmuteSpeaker(): void

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Client Tools

Ultravox has robust support for tools. The SDK has support for client tools. Client tools will be invoked in your client code and enable you to add interactivity in your app that is driven by user interactions with your agent. For example, your agent could choose to invoke a tool that would trigger some UI change.
Message Size LimitMessages larger than 15-16KB may cause timeouts. Keep payload sizes within this limit.

Creating Client Tools Client tools are defined just like “server” tools with three exceptions:
1

'client' not 'http'

You don’t add the URL and HTTP method for client tools.Instead, you add "client": {} to the tool definition.
{
  "systemPrompt": ...
  "selectedTools": [
    "temporaryTool": {
      "modelToolName": "stock_price",
      "description": "Get the current stock price for a given symbol",
      "dynamicParameters": [
        {
          "name": "symbol",
          "location": "PARAMETER_LOCATION_BODY",
          "schema": {
            "type": "string",
            "description": "Stock symbol (e.g., AAPL for Apple Inc.)"
          },
          "required": true
        }
      ],
      "client": {}
    }
  ]
}
2

Register Tool with Client

Your client tool must be registered in your client code. Here’s a snippet that might be found in client code to register the client tool and implement the logic for the tool.See the SDK method registerToolImplementation() for more information.
Registering a Client Tool
// Start up our Ultravox Session
uvSession = new UltravoxSession();

// Register our client-side tool
uvSession.registerToolImplementation(
  "stock_price",
  stock_price
);

uvSession.joinCall(joinUrl);

// Function that implements tool logic
const stock_price = (parameters) => {
  ...  // to be implemented
  return `Stock price is ${value}`;
};
3

Only Body Parameters Allowed

Unlike server tools (which accept parameters passed by path, header, body, etc.), client tools only allow parameters to be passed in the body of the request. That means client tools will always have parameter location set like this:
"location": "PARAMETER_LOCATION_BODY"

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Session Status

The UltravoxSession exposes status. Based on the UltravoxSessionStatus enum, status can be one of the following:
statusdescription
disconnectedSession is not connected. This is the initial state prior to joinCall.
disconnectingSession is in the process of disconnecting.
connectingSession is establishing the connection.
idleSession is connected but not yet active.
listeningListening to the end user.
thinkingThe model is processing/thinking.
speakingThe model is speaking.
Status Events The status can be retrieved by adding an event listener to the session status.
Get Session Status Events
// Listen for status changing events
session.addEventListener('status', (event) => {
  console.log('Session status changed: ', session.status);
});

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Transcripts

Sometimes you may want to augment the audio with text transcripts (e.g. if you want to show the end user the model’s output in real-time). Transcripts can be retrieved by adding an event listener:
Get Transcripts
// Listen for transcripts changing events
session.addEventListener('transcripts', (event) => {
  console.log('Transcripts updated: ', session.transcripts);
});
Transcripts are returned as an array of transcript objects.
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transcript
Transcript Object

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Debug Messages

No GuaranteeDebug messages from Ultravox should be treated as debug logs. They can change regularly and don’t have a contract. Relying on the specific structure or content should be avoided.
The UltravoxSession object also provides debug messages. Debug messages must be enabled when creating a new session and then are available via an event listener similar to status and transcripts:
Get Debug Messages
// Start up our Ultravox Session with Debug Messages Enabled
session = new UltravoxSession({ experimentalMessages: ["debug"] });

// Listen for debug messages
session.addEventListener('experimental_message', (msg) => {
  console.log('Got a debug message: ', JSON.stringify(msg));
});
Debug Message: Tool Call When the agent invokes a tool, the message contains the function, all arguments, and an invocation ID: 
LLM response: Tool calls: [FunctionCall(name='createProfile', args='{"firstName":"Ron","lastName":"Burgandy","organization":"Fixie.ai","useCase":"creating a talking AI news reporter"}', invocation_id='call_D2qQVS8OQc998aMEw5PRa9cF')]
Debug Message: Tool Call Result When the tool call completes, the message contains an array of messages. Multiple tools can be invoked by the model. This message array will conatain all the calls followed by all the results. These messages are also available via List Call Messages. Here’s an example of what we might see from a single tool invocation: 
Tool call complete.

Result: [
  role: MESSAGE_ROLE_TOOL_CALL ordinal: 6 text: "{\"firstName\":\"Ron\",\"lastName\":\"Burgandy\",\"organization\":\"Fixie.ai\",\"useCase\":\"creating a talking AI news reporter\"}" tool_name: "createProfile" invocation_id: "call_D2qQVS8OQc998aMEw5PRa9cF" tool_id: "aa737e12-0989-4adb-9895-f387f40557d8" ,
  role: MESSAGE_ROLE_TOOL_RESULT ordinal: 7 text: "{\"firstName\":\"Ron\",\"lastName\":\"Burgandy\",\"emailAddress\":null,\"organization\":\"Fixie\",\"useCase\":\"creating a talking AI news reporter\"}" tool_name: "createProfile" invocation_id: "call_D2qQVS8OQc998aMEw5PRa9cF" tool_id: "aa737e12-0989-4adb-9895-f387f40557d8"
]

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SDK Implementations

There are currently five implementations of the SDK available:

JavaScript

npm install ultravox-client

Flutter

flutter add ultravox_client

Python

pip install ultravox-client

Kotlin (Android)

Find it on Maven Central

Swift (iOS)

Find it on Swift Package Index