Reconciliation checks that the value an analytics tool gives each subscription equals what Stripe's own invoice bills for it.
For every counted subscription: monthly value in the tool = (latest regular invoice's subscription lines, without prorations, before discounts and tax) ÷ months in the period
An invoice is Stripe's own arithmetic, which makes it an independent witness: a wrong price, quantity, currency or billing interval shows as a difference.
A subscription whose plan changed since its last invoice cannot be compared until the next one. That should be counted as not comparable, never as agreement.
sumwerk runs this check after every sync and shows the result next to the MRR, with a trail of the last days.
A restricted, read-only key is all it takes. The first import runs in a few minutes; every number you see opens into the customers and Stripe references behind it.