Flux TTSStreaming (WebSocket)Control Messages

Client Messages

The messages you send to the Flux TTS /v2/speak WebSocket at Early Access — Speak, Flush, and Close.

All client-to-server traffic on /v2/speak is JSON text frames. You stream synthesis text with Speak, end a turn with Flush, and shut down with Close. The server replies on a parallel set of Server Messages.

A turn is one complete agent response, bounded by Flush. The server streams a turn’s audio as text arrives; only Flush completes the turn. For how these messages drive turn state, see The Speech Lifecycle.

Early Access surface. At EA the client messages are Speak, Flush, and Close. Interruption (Interrupt) and mid-stream configuration (Configure) are planned for GA.

Speak

Send text to be synthesized into the active turn. The Speak shape is unchanged from /v1/speak. The server tracks the active turn internally and assigns it a speech_id — a server-generated turn identifier, included in SpeechStarted (the start of a turn) and SpeechMetadata (the completion of the turn’s synthesis). Binary audio frames aren’t labeled with the speech_id, but all of a turn’s audio — and only that turn’s audio — arrives between those two messages. Clients do not specify one.

1{
2  "type": "Speak",
3  "text": "Sure, I can help you cancel your subscription."
4}
FieldTypeRequiredDescription
type"Speak"yesMessage type identifier.
textstringyesText to synthesize — see Text handling below.

Streaming LLM tokens

Streaming is just sending Speak messages as tokens arrive, then flushing at the end of the turn:

1{"type": "Speak", "text": "Sure, "}
2{"type": "Speak", "text": "I can "}
3{"type": "Speak", "text": "help you "}
4{"type": "Speak", "text": "cancel your "}
5{"type": "Speak", "text": "subscription."}
6{"type": "Flush"}

You may send Speak at any time. Text sent while the current turn is still generating appends to that turn. Once you Flush, though, the turn is closed — a later Speak starts a new turn, which the server queues as pending behind any turn still being synthesized. Only Flush ends a turn.

Text handling

Send plain text. The server applies text normalization (for example, expanding numbers and dates) before synthesis, but it does not reorder your content or insert or strip whitespace between successive Speak messages — so you can stream LLM tokens directly without coordinating chunk boundaries.

Insert whitespace between distinct generations. Because the server doesn’t add whitespace between Speak messages, sending "Hello world." immediately followed by "How are you?" is processed as "Hello world.How are you?", which can trigger sentence-boundary artifacts. When you concatenate separate LLM responses (a reply, a tool-call result, another reply), insert a single space — or the appropriate separator for non-whitespace languages — between them.

Markup isn’t interpreted. The model synthesizes plain text; SSML and competitor audio tags aren’t supported at EA. Automatic detection and stripping of known markup, with a Warning, is planned for GA — until then, send plain text.

Flush

End the active turn. The server drains the buffer, generates the remaining audio, and reports the turn. Flush is how you signal “the agent’s response is complete.”

1{"type": "Flush"}

On Flush, the server generates any remaining audio for the active turn and emits Flushed when the turn’s buffer has actually been flushed — not on receipt, and it can be held back behind earlier pending turns — followed by SpeechMetadata with the turn’s billing and timing. The next Speak begins a new turn with a new speech_id.

Flush is what ends a turn. The server may already be streaming a turn’s audio before you flush, but only Flush closes the turn and produces Flushed + SpeechMetadata. A Flush with no active turn is a no-op and produces a NO_ACTIVE_SPEECH warning.

Close

Gracefully close the connection. The server finishes draining all queued audio, emits a final SessionMetadata with cumulative totals, then closes the socket.

1{"type": "Close"}

Keeping a session alive

The server closes an idle session after 60 seconds with no inbound client message (NET-0004). If your agent may go quiet longer than that between turns, send a WebSocket Ping or Pong to reset the timer.