Control group is maintained throughout the experiment

An auto-winner mechanism that re-assigns control contacts to the leading variant partway through an experiment corrupts the experiment's results in two ways: it reduces the control group size mid-run (inflating variance) and introduces a survivor bias into the final comparison. Contacts who remained in the control group through the auto-switch may be systematically different from those who were switched. If the experiment hasn't reached required sample size when the auto-switch fires, you've now lost the ability to recover a clean result. You've shipped a variant based on preliminary data that may reverse at full sample — the classic peeking outcome — with no way to rerun the experiment cleanly.

Conclude experiments only through an explicit human action after significance is confirmed. Never auto-reassign historical contacts — apply the winner only to future sends:

async function concludeExperiment(
  experimentId: string,
  winnerId: string,
  concludedBy: string
) {
  const exp = await db.experiments.findUnique({ where: { id: experimentId } })
  if (exp.status !== 'active') throw new Error('Experiment is not active')

  await db.experiments.update({
    where: { id: experimentId },
    data: {
      status: 'concluded',
      winner_variant_id: winnerId,
      concluded_at: new Date(),
      concluded_by: concludedBy
    }
  })

  // Apply winner to future sends only — historical assignments are immutable
  await db.campaignDefaults.upsert({
    where:  { campaign_id: exp.campaign_id },
    update: { default_variant_id: winnerId },
    create: { campaign_id: exp.campaign_id, default_variant_id: winnerId }
  })
}

Remove any timer-based or threshold-based auto-conclusion logic from your experiment scheduler.