Session

Real-time WebSocket session for two-way communication within a Conversation.

Send messages and receive agent responses through a nested stream hierarchy. The SessionStream is the top-level entry point — events flow down through exchanges, messages, content parts, and tool calls.

Usage

Important: Always wait for onSessionStarted before calling startExchange. The session must be fully connected via WebSocket before exchanges can be sent — calling startExchange earlier may lose events or cause errors.

const session = conversation.startSession();

// Set up handlers for incoming assistant responses
session.onExchangeStart((exchange) => {
  exchange.onMessageStart((message) => {
    if (message.isAssistant) {
      message.onContentPartStart((part) => {
        if (part.isMarkdown) {
          part.onChunk((chunk) => {
            process.stdout.write(chunk.data ?? '');
          });
        }
      });
    }
  });
});

// Wait for the session to be ready, then send a message
session.onSessionStarted(() => {
  const exchange = session.startExchange();
  exchange.sendMessageWithContentPart({
    data: 'Hello!',
    role: MessageRole.User
  });
});

// End the session when done
conversation.endSession();

Related Streams

Stream	Description
ExchangeStream	A single request-response cycle within a session. Contains user and assistant messages.
MessageStream	A single message (user, assistant, or system). Contains content parts and tool calls.
ContentPartStream	A piece of streamed content (text, audio, image, transcript). Delivers data via onChunk.
ToolCallStream	An external tool invocation by the assistant. Has a start event (name, input) and end event (output).

Properties

Property	Modifier	Type	Description
conversationId	readonly	string	The conversation ID this session belongs to
ended	readonly	boolean	Whether this session has ended
exchanges	readonly	Iterable<ExchangeStream>	Iterator over all active exchanges in this session

Methods

getExchange()

getExchange(exchangeId: string): undefined | ExchangeStream

Retrieves an exchange by ID

Parameters

Parameter	Type	Description
exchangeId	string	The exchange ID to look up

Returns

undefined | ExchangeStream

The exchange stream, or undefined if not found

---

onErrorEnd()

onErrorEnd(cb: (error: { errorId: string; } & ErrorEndEvent) => void): () => void

Registers a handler for error end events at the session level

Parameters

Parameter	Type	Description
cb	(error: { errorId: string; } & ErrorEndEvent) => void	Callback receiving the error end event

Returns

Cleanup function to remove the handler

(): void

Returns

void

Example

session.onErrorEnd((error) => {
  console.log(`Error ${error.errorId} resolved`);
});

---

onErrorStart()

onErrorStart(cb: (error: { errorId: string; } & ErrorStartEvent) => void): () => void

Registers a handler for error start events at the session level

Parameters

Parameter	Type	Description
cb	(error: { errorId: string; } & ErrorStartEvent) => void	Callback receiving the error event

Returns

Cleanup function to remove the handler

(): void

Returns

void

Example

session.onErrorStart((error) => {
  console.error(`Session error [${error.errorId}]: ${error.message}`);
});

---

onExchangeStart()

onExchangeStart(cb: (exchange: ExchangeStream) => void): () => void

Registers a handler for exchange start events

This is the primary entry point for handling agent responses. Each exchange represents a request-response cycle containing user and assistant messages.

Parameters

Parameter	Type	Description
cb	(exchange: ExchangeStream) => void	Callback receiving each new exchange

Returns

Cleanup function to remove the handler

(): void

Returns

void

Examples

session.onExchangeStart((exchange) => {
  exchange.onMessageStart((message) => {
    if (message.isAssistant) {
      message.onContentPartStart((part) => {
        if (part.isMarkdown) {
          part.onChunk((chunk) => renderMarkdown(chunk.data ?? ''));
        } else if (part.isAudio) {
          part.onChunk((chunk) => audioPlayer.enqueue(chunk.data ?? ''));
        } else if (part.isImage) {
          part.onChunk((chunk) => imageBuffer.append(chunk.data ?? ''));
        } else if (part.isTranscript) {
          part.onChunk((chunk) => showTranscript(chunk.data ?? ''));
        }
      });
    }
  });
});

session.onExchangeStart((exchange) => {
  exchange.onMessageCompleted((completed) => {
    console.log(`Message ${completed.messageId} (role: ${completed.role})`);
    for (const part of completed.contentParts) {
      console.log(part.data);
    }
    for (const tool of completed.toolCalls) {
      console.log(`${tool.toolName} → ${tool.output}`);
    }
  });
});

session.onExchangeStart((exchange) => {
  exchange.onMessageStart((message) => {
    if (message.isAssistant) {
      // Stream tool call events
      message.onToolCallStart((toolCall) => {
        const { toolName, input } = toolCall.startEvent;
        console.log(`Calling ${toolName}:`, JSON.parse(input ?? '{}'));
        toolCall.onToolCallEnd((end) => {
          console.log(`Result:`, JSON.parse(end.output ?? '{}'));
        });
      });

      // Handle confirmation interrupts
      message.onInterruptStart(({ interruptId, startEvent }) => {
        if (startEvent.type === 'uipath_cas_tool_call_confirmation') {
          message.sendInterruptEnd(interruptId, { approved: true });
        }
      });
    }
  });
});

---

onLabelUpdated()

onLabelUpdated(cb: (event: LabelUpdatedEvent) => void): () => void

Registers a handler for conversation label updates

Fired when the conversation label changes, typically when the server auto-generates a title based on the first message.

Parameters

Parameter	Type	Description
cb	(event: LabelUpdatedEvent) => void	Callback receiving the LabelUpdatedEvent with the new label

Returns

Cleanup function to remove the handler

(): void

Returns

void

Example

session.onLabelUpdated((event) => {
  console.log(`New label: ${event.label} (auto: ${event.autogenerated})`);
  updateConversationTitle(event.label);
});

---

onSessionEnd()

onSessionEnd(cb: (event: SessionEndEvent) => void): () => void

Registers a handler for session end events

Fired when the session has fully closed.

Parameters

Parameter	Type	Description
cb	(event: SessionEndEvent) => void	Callback receiving the end event

Returns

Cleanup function to remove the handler

(): void

Returns

void

Example

session.onSessionEnd((event) => {
  console.log('Session ended');
});

---

onSessionEnding()

onSessionEnding(cb: (event: SessionEndingEvent) => void): () => void

Registers a handler for session ending events

Fired when the session is about to end. Use this for cleanup before the session fully closes.

Parameters

Parameter	Type	Description
cb	(event: SessionEndingEvent) => void	Callback receiving the ending event

Returns

Cleanup function to remove the handler

(): void

Returns

void

Example

session.onSessionEnding((event) => {
  console.log('Session is ending, performing cleanup...');
});

---

onSessionStarted()

onSessionStarted(cb: (event: SessionStartedEvent) => void): () => void

Registers a handler for session started events

Fired when the WebSocket connection is established and the session is ready to send and receive events.

Parameters

Parameter	Type	Description
cb	(event: SessionStartedEvent) => void	Callback receiving the started event

Returns

Cleanup function to remove the handler

(): void

Returns

void

Example

session.onSessionStarted(() => {
  console.log('Session is ready — now safe to start exchanges');

  const exchange = session.startExchange();
  exchange.sendMessageWithContentPart({
    data: 'Hello!',
    role: MessageRole.User
  });
});

---

startExchange()

startExchange(args?: { exchangeId?: string; } & ExchangeStartEvent): ExchangeStream

Starts a new exchange in this session

Each exchange is a request-response cycle. Use sendMessageWithContentPart on the returned ExchangeStream to send a user message, or startMessage for fine-grained control.

Parameters

Parameter	Type	Description
args?	{ exchangeId?: string; } & ExchangeStartEvent	Optional exchange start options

Returns

ExchangeStream

The exchange stream for sending messages

Example

const session = conversation.startSession();

// Listen for all assistant responses
session.onExchangeStart((exchange) => {
  exchange.onMessageCompleted((completed) => {
    if (completed.role === MessageRole.Assistant) {
      for (const part of completed.contentParts) {
        console.log('Assistant:', part.data);
      }
    }
  });
});

// Wait for session to be ready before starting exchanges
session.onSessionStarted(async () => {
  // Send first user message
  const exchange1 = session.startExchange();
  await exchange1.sendMessageWithContentPart({
    data: 'What is the weather today?',
    role: MessageRole.User
  });

  // Send follow-up in a new exchange
  const exchange2 = session.startExchange();
  await exchange2.sendMessageWithContentPart({
    data: 'And tomorrow?',
    role: MessageRole.User
  });
});