Function driveSubagentChains

apps/sim/lib/copilot/request/lifecycle/run.ts:483–527

(
  continuation: AsyncContinuation,
  context: StreamingContext,
  execContext: ExecutionContext,
  options: CopilotLifecycleOptions,
  baseURL: string,
  workspaceId?: string
)

// caught (so Promise.all can't reject before its siblings unwind); we then
// rethrow the first REAL error, not the AbortErrors it triggered in the siblings.
async function driveSubagentChains(
  continuation: AsyncContinuation,
  context: StreamingContext,
  execContext: ExecutionContext,
  options: CopilotLifecycleOptions,
  baseURL: string,
  workspaceId?: string
): Promise<AsyncContinuation | null> {
  const frames = continuation.frames ?? []
  logger.info('Driving subagent checkpoint chains concurrently', {
    childCount: frames.length,
    checkpointIds: frames.map((f) => f.checkpointId),
  })

  const fanoutController = new AbortController()
  const parentSignal = options.abortSignal
  const onParentAbort = () => fanoutController.abort()
  if (parentSignal) {
    if (parentSignal.aborted) fanoutController.abort()
    else parentSignal.addEventListener('abort', onParentAbort, { once: true })
  }
  const legOptions: CopilotLifecycleOptions = { ...options, abortSignal: fanoutController.signal }

  let firstError: unknown
  try {
    const followOns = await Promise.all(
      frames.map((frame) =>
        driveOneChildChain(frame, context, execContext, legOptions, baseURL, workspaceId).catch(
          (error) => {
            // First real failure wins and cancels the siblings; their resulting
            // AbortErrors arrive later and don't overwrite it. Swallow here so
            // Promise.all doesn't reject before every leg has unwound.
            if (firstError === undefined) firstError = error
            fanoutController.abort()
            return null
          }
        )
      )
    )
    if (firstError !== undefined) throw firstError
    return followOns.find((c): c is AsyncContinuation => !!c) ?? null
  } finally {
    parentSignal?.removeEventListener('abort', onParentAbort)
  }
}

// ---------------------------------------------------------------------------
// Checkpoint loop – the core state machine