Under lastCharge anchoring, an everyNMonths cost stepped from the
confirmation's exact date even when that date fell earlier in the same
month as the slot it claims (e.g. rent for 2026-09 paid 2026-08-28). Since
stepFrom normalises through monthOf, that produced the same month as the
cursor, which the walk read as "did not advance" and threw — for a
completely ordinary early payment.
The month-granular step now runs from whichever is later, the cursor or
the confirmation's month, so a payment within the same month simply does
not rebase. A payment recorded in a LATER month still rebases the series
forward, symmetric with the day-granular cap-hit-early behaviour.
Also corrects the termination error's message, which claimed a purchase
was dated "more than one step earlier" regardless of how early it
actually was.
A walk rather than a per-month query: under lastCharge anchoring, where a
series sits in a given month depends on every confirmation before it, so the
projection starts at startDate and steps forward. Bounded by upToMonth, which
the caller supplies — the domain still reads no clock.
The data-cap behaviour is one line. `from` is the confirmation's own date
under lastCharge, so confirming a slot early moves every later occurrence with
it. Confirm the 2026-01-31 slot with a purchase dated the 12th and the next
lands on 2026-02-11, not 2026-03-02.
The walk throws if a step fails to advance. n >= 1 is enforced at both write
boundaries so this should be unreachable, but a hang is worse than a throw and
dropping a bill silently is worse than both.
Rewrote the "deleting the confirming purchase" test per controller ruling: the
brief's version was byte-identical to the unconfirmed-series test above it and
asserted nothing new. It now walks the phone cost with the capHit
confirmations, then walks the same cost with an empty map, and asserts the
dates differ and match the unconfirmed series — which is what "deleting the
purchase restores the series" actually means.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Lucas Winther