Exceptions to recurring schedules override the rule in real time

A recurring schedule exception that does not override the calendar in real time is an invisible lie. The host marks next Monday as closed; the customer sees open slots for next Monday; the booking confirms; the host is unavailable. This ISO 25010 functional-correctness failure occurs when the exception record is stored correctly but the slot generation query does not join against the exceptions table before returning availability. The result is phantom slots: the system thinks it is correctly managing exceptions, but customers book into them anyway.

In your slot generation function (typically src/lib/availability.ts or your availability API route), query exceptions in the same request as the recurring rule and filter before returning results.

// src/lib/availability.ts
export async function getRecurringSlots(recurringId: string, start: Date, end: Date) {
  const [rec, exceptions] = await Promise.all([
    db.recurringAvailability.findUnique({ where: { id: recurringId } }),
    db.availabilityExceptions.findMany({
      where: { recurringId, exceptionDate: { gte: start, lte: end } },
    }),
  ])

  if (!rec) return []

  const blocked = new Set(exceptions.map(e => e.exceptionDate.toISOString().slice(0, 10)))

  const occurrences = rrulestr(rec.rrule, { dtstart: start }).between(start, end)
  return occurrences.filter(d => !blocked.has(d.toISOString().slice(0, 10)))
}

The exception filter must run synchronously in the slot generation path, not as a background job or deferred reconciliation task.