FSRS is the modern spaced repetition algorithm Anki has shipped since version 23.10 (November 2023). It is still opt-in — SM-2 remains the default — but it is widely recommended in the community and the Anki manual presents it as more accurate than SM-2. FSRS predicts how long you will remember a card and schedules the next review at the point your recall probability drops to 90%. For a card you answer easily, that point can be a year or more away — which is the source of the "why is my next review in 17 months" question that fills r/Anki every week.