Events & Callbacks

How to respond to bot and session events in Pipecat client applications.

The Pipecat client emits events throughout the session lifecycle — when the bot connects, when the user speaks, when a transcript arrives, and more.

Subscribing to events

Callbacks

Event listeners

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Event reference

Session and connectivity

These events track the connection state of the client and bot. See Session Lifecycle for the full state progression.

Event	Callback	When it fires
Connected	onConnected	Client transport connection established
Disconnected	onDisconnected	Client disconnected (intentional or error)
TransportStateChanged	onTransportStateChanged	Any transport state change; receives the new TransportState string
BotConnected	onBotConnected	Bot joined the transport; pipeline may still be initializing
BotReady	onBotReady	Bot pipeline is ready; safe to send messages and expect audio
BotDisconnected	onBotDisconnected	Bot left the session; client will also disconnect unless disconnectOnBotDisconnect: false
ParticipantConnected	onParticipantJoined	Any participant joined (bot, local, or other)
ParticipantLeft	onParticipantLeft	Any participant left (bot, local, or other)

BotReady receives a BotReadyData object with a version field — the RTVI version the bot is running. You can use this to check compatibility if your client and server may be on different versions.

Voice activity

These events are driven by the bot’s VAD (voice activity detection) model. VAD is smarter than tracking raw audio levels — it understands turn-taking, so it can distinguish between a user who has finished speaking and one who has simply paused or is speaking slowly.

Event	Callback	When it fires
UserStartedSpeaking	onUserStartedSpeaking	VAD detected the user started speaking
UserStoppedSpeaking	onUserStoppedSpeaking	VAD detected the user stopped speaking
BotStartedSpeaking	onBotStartedSpeaking	Bot started sending audio
BotStoppedSpeaking	onBotStoppedSpeaking	Bot stopped sending audio
LocalAudioLevel	onLocalAudioLevel	Local audio gain level (0–1); fires continuously
RemoteAudioLevel	onRemoteAudioLevel	Remote audio gain level (0–1); fires continuously
UserMuteStarted	onUserMuteStarted	Server started ignoring client audio (server-side mute)
UserMuteStopped	onUserMuteStopped	Server resumed processing client audio

UserMuteStarted/UserMuteStopped reflect server-side muting — the client continues sending audio, but the bot is ignoring it. Use these to update your UI (e.g., show a muted indicator) without actually stopping the local mic.

Transcription and bot output

Event	Callback	Data	When it fires
UserTranscript	onUserTranscript	TranscriptData	User speech transcribed; fires for both partial (final: false) and final results
BotOutput	onBotOutput	BotOutputData	Bot text output, typically aggregated by sentence or word during TTS synthesis
BotLlmText	onBotLlmText	BotLLMTextData	Raw LLM token stream
BotLlmStarted	onBotLlmStarted	—	LLM inference started
BotLlmStopped	onBotLlmStopped	—	LLM inference finished
BotTtsText	onBotTtsText	BotTTSTextData	Words from TTS as they are synthesized (streaming TTS only)
BotTtsStarted	onBotTtsStarted	—	TTS synthesis started
BotTtsStopped	onBotTtsStopped	—	TTS synthesis finished

UserTranscript fires continuously as speech is recognized. Check data.final to distinguish committed transcripts from work-in-progress partials:

BotOutput is the recommended way to display the bot’s response text. It provides the best possible representation of what the bot is saying — supporting interruptions and unspoken responses. By default, Pipecat aggregates output by sentences and words (assuming your TTS supports streaming), but custom aggregation strategies are supported too - like breaking out code snippets or other structured content:

Errors

Event	Callback	When it fires
Error	onError	Bot signalled an error; data.fatal is true if the session is unrecoverable
MessageError	onMessageError	A client message failed or got an error response

Always handle Error. If data.fatal is true, the bot has already disconnected — update your UI accordingly:

Devices and tracks

Event	Callback	When it fires
AvailableMicsUpdated	onAvailableMicsUpdated	Mic list changed or initDevices() called
AvailableCamsUpdated	onAvailableCamsUpdated	Camera list changed or initDevices() called
AvailableSpeakersUpdated	onAvailableSpeakersUpdated	Speaker list changed or initDevices() called
MicUpdated	onMicUpdated	Active microphone changed
CamUpdated	onCamUpdated	Active camera changed
SpeakerUpdated	onSpeakerUpdated	Active speaker changed
DeviceError	onDeviceError	Mic, camera, or permission error
TrackStarted	onTrackStarted	A media track (audio or video) became playable
TrackStopped	onTrackStopped	A media track stopped

Function calling

Other

Event	Callback	When it fires
ServerMessage	onServerMessage	Custom message sent from the bot to the client
Metrics	onMetrics	Pipeline performance metrics from Pipecat

For custom server<->client messaging, see Custom Messaging.

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API reference