Streaming Events Schema

{
  "v": 1,
  "kind": "model.reconnecting",
  "severity": "warning",
  "attempt": 2,
  "retry_in_seconds": 1.46
}

const SYSTEM_TOPIC = "glot.system"

let systemState: SystemState = { model: null, attempt: null, retryInSeconds: null }

// A chain of its own, deliberately not the transcript's.
let systemReads = Promise.resolve()

room.registerTextStreamHandler(SYSTEM_TOPIC, (reader) => {
  systemReads = systemReads
    .then(async () => {
      const message = parseSystemMessage(await reader.readAll())
      if (message === null) return
      systemState = applySystemMessage(systemState, message)
      renderStatus(systemState)
    })
    .catch((err) => {
      // A rejection left on the chain would poison every read after it.
      console.error("Failed to read a system message", err)
    })
})

type TranscriptStatus = "streaming" | "stopped" | "final"

type TranscriptEntry = {
  segment: string
  /** Language translated into. */
  language: string
  /** Accumulated while streaming, replaced wholesale when confirmed. */
  text: string
  /** Known only once the utterance is confirmed. */
  sourceLanguage: string | null
  sourceText: string | null
  status: TranscriptStatus
}

export function applyTranscriptFrame(
  entries: TranscriptEntry[],
  frame: TranscriptFrame,
): TranscriptEntry[] {
  const index = entries.findIndex((entry) => entry.segment === frame.segment)

  if (frame.kind === "segment") {
    const confirmed: TranscriptEntry = {
      segment: frame.segment,
      language: frame.language,
      text: frame.text,
      sourceLanguage: frame.sourceLanguage,
      sourceText: frame.sourceText,
      status: "final",
    }
    // Appended when unseen rather than dropped: partials can be lost across a reconnect,
    // and the confirmed text is the half worth showing.
    if (index === -1) return [...entries, confirmed]
    return entries.map((entry, i) => (i === index ? confirmed : entry))
  }

  if (index === -1) {
    return [
      ...entries,
      {
        segment: frame.segment,
        language: frame.language,
        text: frame.text,
        sourceLanguage: null,
        sourceText: null,
        status: frame.last ? "stopped" : "streaming",
      },
    ]
  }

  const current = entries[index]
  // A partial that arrives after its utterance was confirmed is stale — the frames raced,
  // or the translator re-sent one. Appending it would tack a fragment onto a finished caption.
  if (current.status === "final") return entries

  return entries.map((entry, i) =>
    i === index
      ? { ...current, text: current.text + frame.text, status: frame.last ? "stopped" : "streaming" }
      : entry,
  )
}

type ModelStatus = "connecting" | "ready" | "reconnecting" | "unavailable" | "lost"

type SystemState = {
  /** `null` until the translator has said anything — including before it joins. */
  model: ModelStatus | null
  attempt: number | null
  retryInSeconds: number | null
}

const MODEL_STATUS_BY_KIND: Record<string, ModelStatus> = {
  "model.connecting": "connecting",
  "model.ready": "ready",
  "model.reconnecting": "reconnecting",
  "model.unavailable": "unavailable",
  "model.lost": "lost",
}

export function applySystemMessage(
  state: SystemState,
  message: SystemMessage,
): SystemState {
  // Recovery is not a status of its own — being back is what `ready` already means.
  if (message.kind === "model.recovered") {
    return { model: "ready", attempt: null, retryInSeconds: null }
  }

  const model = MODEL_STATUS_BY_KIND[message.kind]
  // A message outside the model's lifecycle. You may still want to surface it by
  // severity, but it does not move this state machine.
  if (model === undefined) return state

  // Only the retry carries these, so every other transition clears them — a stale
  // countdown under a status that has moved on reads as a bug.
  return {
    model,
    attempt: model === "reconnecting" ? message.attempt : null,
    retryInSeconds: model === "reconnecting" ? message.retryInSeconds : null,
  }
}

import { ParticipantKind, RoomEvent } from "livekit-client"

const syncTranslatorStatus = () => {
  for (const participant of room.remoteParticipants.values()) {
    if (participant.kind !== ParticipantKind.AGENT) continue
    if (participant.attributes.status !== "error") continue
    systemState = { model: "lost", attempt: null, retryInSeconds: null }
    renderStatus(systemState)
  }
}

room
  .on(RoomEvent.ParticipantAttributesChanged, syncTranslatorStatus)
  // A translator already in an error state when you join never fires a change event.
  .on(RoomEvent.ParticipantConnected, syncTranslatorStatus)