The meta reads by season, and says when a window crosses one

The card, deck and synergy meta tools answered a rolling 28 days, which on 17 September was 14% last season and 86% this one with nothing in the response saying so; balance changes land on the season roll, so those are two populations. The record now holds each season as a row, first Monday 10:00 UTC to first Monday 10:00 UTC under the month the API names it by, with the river race season number derived from the month and confirmed against every war log entry it admits. battles_meta_decks, battles_meta_cards and cards_synergy default to the current season to date and take season: 'current', 'previous', '2026-08' or 135; battles_trends takes it too. Every such response says which season the window starts in, lists each season roll it crosses (empty when clean) and how old the season is, and a thin new season points at 'previous' rather than quietly widening. What is deliberately not there: balance changes themselves, and the in-game Pass season, neither of which the API exposes. Contract 3.10.0.