Where it came from
Many earlier spaced repetition tools, including early versions of Anki, used an algorithm called SM-2, which adjusts review intervals using a relatively simple ease factor per card. It works, but it treats memory stability somewhat crudely and can be slow to adapt when a card turns out to be much easier or harder than its early history suggested.
FSRS was developed as an open-source alternative that models memory using more of your review history at once, rather than leaning on a single running ease value. It has since been adopted or offered as an option in a number of spaced repetition tools beyond its original implementation.
What it actually models
At a basic level, FSRS estimates two things for each card: how likely you are to recall it right now, and how quickly that likelihood would decay without another review. It uses your actual answer history—not just whether you were right or wrong, but the pattern of intervals and results over time—to keep refining that estimate.
The practical effect is a scheduler that reacts more precisely to an individual card's real difficulty for you specifically, rather than applying the same multiplier logic to every card regardless of how it has actually behaved.
Why this produces different review intervals
Two cards added on the same day, both answered correctly the first time, can end up with meaningfully different next-review intervals under FSRS if their surrounding history suggests different stability—for example, one card you have also seen in a related context versus one that is entirely new.
In practice, this tends to mean fewer wasted reviews on material that is already solid, and earlier, more frequent attention on material that is genuinely still forming.
Why Cueable uses FSRS-based scheduling
Cueable's scheduling is FSRS-based specifically because it adapts to your real recall pattern rather than a fixed sequence of intervals. Combined with deadline-aware planning, this lets the app change the mix of what to review as an exam date approaches—not just when a card is 'due' in the abstract, but when reviewing it is actually worth your limited study time.