Function startBackgroundSession

src/tasks/LocalMainSessionTask.ts:341–493

({
  messages,
  queryParams,
  description,
  setAppState,
  agentDefinition,
}: {
  messages: Message[]
  queryParams: Omit<QueryParams, 'messages'>
  description: string
  setAppState: SetAppState
  agentDefinition?: AgentDefinition
})

 * as a background task. The caller's foreground query continues running normally.
 */
export function startBackgroundSession({
  messages,
  queryParams,
  description,
  setAppState,
  agentDefinition,
}: {
  messages: Message[]
  queryParams: Omit<QueryParams, 'messages'>
  description: string
  setAppState: SetAppState
  agentDefinition?: AgentDefinition
}): string {
  const { taskId, abortSignal } = registerMainSessionTask(
    description,
    setAppState,
    agentDefinition,
  )

  // Persist the pre-backgrounding conversation to the task's isolated
  // transcript so TaskOutput shows context immediately. Subsequent messages
  // are written incrementally below.
  void recordSidechainTranscript(messages, taskId).catch(err =>
    logForDebugging(`bg-session initial transcript write failed: ${err}`),
  )

  // Wrap in agent context so skill invocations scope to this task's agentId
  // (not null). This lets clearInvokedSkills(preservedAgentIds) selectively
  // preserve this task's skills across /clear. AsyncLocalStorage isolates
  // concurrent async chains — this wrapper doesn't affect the foreground.
  const agentContext: SubagentContext = {
    agentId: taskId,
    agentType: 'subagent',
    subagentName: 'main-session',
    isBuiltIn: true,
  }

  void runWithAgentContext(agentContext, async () => {
    try {
      const bgMessages: Message[] = [...messages]
      const recentActivities: ToolActivity[] = []
      let toolCount = 0
      let tokenCount = 0
      let lastRecordedUuid: UUID | null = messages.at(-1)?.uuid ?? null

      for await (const event of query({
        messages: bgMessages,
        ...queryParams,
      })) {
        if (abortSignal.aborted) {
          // Aborted mid-stream — completeMainSessionTask won't be reached.
          // chat:killAgents path already marked notified + emitted; stopTask path did not.
          let alreadyNotified = false
          updateTaskState<LocalMainSessionTaskState>(
            taskId,
            setAppState,
            task => {
              alreadyNotified = task.notified === true