# Spaced Repetition Scheduler (FSRS)

> Which flashcards are due today? FSRS-5 or SM-2 from your review log, with a 30-day forecast. Replays a review log (date, card, rating again/hard/good/easy) through the FSRS-5 spaced-repetition model or the classic SM-2 algorithm and returns today’s queue (cards relearned today, due reviews, new cards up to your daily limit), the next interval for every answer, each card’s stability, difficulty and recall probability, a 30-day forecast and study statistics. The same scheduler runs Deckwright’s Study view.

Part of usefulweb.site (https://usefulweb.site/). Part of [Deckwright](https://deckwright.usefulweb.site/) (ownerApp: deckwright). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://deckwrightreviewschedule.usefulweb.site/ (interactive tool at https://deckwrightreviewschedule.usefulweb.site/tool)
- JSON API: POST https://deckwrightreviewschedule.usefulweb.site/api/v1/calculate with a JSON object of inputs (schema: https://deckwrightreviewschedule.usefulweb.site/api/v1/tool)
- OpenAPI: https://deckwrightreviewschedule.usefulweb.site/openapi.json
- MCP (Streamable HTTP): https://deckwrightreviewschedule.usefulweb.site/mcp - tool `deckwright_review_schedule`

## Inputs
- text (textarea): Cards - Markdown: “Q:” and “A:” lines, “term :: definition” (“:::” also makes the reverse card), {{c1::cloze}} gaps, # headings as tags. Or CSV/TSV rows (front, back, tag), an Anki plain-text export, or a slide deck.
- format (select): Format
- reverse (boolean): Also make reverse cards (answer → question)
- log (textarea): Review log (CSV: date, card, rating) - One review per line: 2026-11-02,hola,good — the card is its id or its question text; the rating is again, hard, good, easy or 1–4. Deckwright’s “Export review log” writes this format.
- today (date): Today - The day to plan for; empty = today (UTC).
- scheduler (select): Scheduler
- retention (number): Desired retention (FSRS) - Probability of remembering a card when it comes due; higher means shorter intervals and more reviews.
- newPerDay (integer): New cards per day
- maxReviews (integer): Maximum reviews per day

## Method
- Card states are replayed from the log in date order. FSRS-5: first rating G sets stability S = w[G−1] and difficulty D = w4 − e^(w5(G−1)) + 1; later reviews use the recall probability R = (1 + 19/81 · t/S)^−0.5 after t days to grow S (Hard × w15, Easy × w16) or, after Again, to set the post-lapse stability; reviews on the same day use S·e^(w17(G−3+w18)); D moves by −w6(G−3) with linear damping and mean reversion to D0(Easy).
- The next interval is the number of days until R falls to the desired retention: I = S/(19/81) · (r^(−2) − 1), rounded, at least 1 day. Again keeps the card in today’s queue until it is answered Hard or better.
- SM-2: grades Again = 1, Hard = 3, Good = 4, Easy = 5; EF changes by 0.1 − (5−q)(0.08 + (5−q)·0.02) (at least 1.3); intervals 1, 6, then the previous interval × EF rounded up; a grade below 3 restarts the repetitions; grades below 4 repeat the card the same day.
- Queue: cards answered Again today first, then due reviews (oldest due date first, up to the daily maximum), then new cards in their order in the set (up to the new-card limit minus the new cards already started today). True retention counts the first answer of each day for cards seen before, over the last 30 days.

## Limits
- FSRS runs with its published default parameters (no optimisation on your own history) and day-level steps without random fuzz. Recall probabilities are model estimates.