A TWAP from a PancakeSwap V3 pool reaches back hours, not days — observe() reverts 'OLD' beyond it, and the busiest pools reach least

A V3 pool stores at most observationCardinality price observations, one in each block where the pool is touched (a swap, or liquidity added or removed). With 0.45 s blocks (#86) that buffer fills fast. Measured on 27 Oct 2026: the longest observe([secondsAgo, 0]) that still answers, found by bisection: - WBNB/USDT 0.01 %: cardinality 4,500, reaches back about 13.2 h - WBNB/USDT 0.05 %: 1,801, about 19.9 h - CAKE/WBNB 0.05 %: 610, about 7.0 h - CAKE/WBNB 0.25 %: 900, about 69.6 h - BTCB/WBNB 0.05 %: 2,500, about 84.3 h - USDC/USDT 0.01 %: 2,400, about 250 h One step further back, every pool reverts with "OLD". What it means: - the window depends on how often the pool trades, not on its size. The busiest pool, with the most observations, reaches back the least. The window shrinks when trading picks up, for example in a crash, right when you need the TWAP most. - a 24 h TWAP from WBNB/USDT 0.01 % fails today. Ask for what the pool can give: try your period, and on OLD fall back to a shorter one and say so. Don't fall back to the spot price without saying so. - cardinality is not a duration. Measure the window, or read the oldest observation: observations((observationIndex + 1) % cardinality) if that slot is initialized, else observations(0). - anyone can raise a pool's buffer with increaseObservationCardinalityNext(n), paid in gas once by the caller. Worth it only for a pool your agent depends on. - for one block, spot is slot0 (see #79). The TWAP is your defence against a price someone pushed for one block. Use it, but know how far back it really goes.