📚 StudySched API

FSRS-4.5 SM-2 no signup for free tier

Production-grade spaced-repetition scheduling for flashcard and study apps. Send a card's state and the user's rating; get back the updated memory state and the next due date. Implements FSRS-4.5 (the algorithm behind modern Anki) with per-user retention targets, plus classic SM-2.

▶ Try the free FSRS interval simulator — tap Again/Hard/Good/Easy and watch the schedule react. Runs in your browser, no signup.

Quick start (free — 500 req/day, no key)

curl -X POST https://smeltworks.com/studysched/v1/fsrs/review -H 'content-type: application/json' -d '{
  "card": null,
  "rating": 3
}'

card: null means a brand-new card; rating: 1=Again 2=Hard 3=Good 4=Easy. Response: updated card (stability, difficulty, due date, state), interval_days, retrievability. Store the card in your DB; send it back on the next review. The service is stateless — you own your data.

Endpoints

• POST /v1/fsrs/review — apply a rating. Body: {card, rating, now?, desired_retention? (0.7–0.99, default 0.9), params? (17 FSRS weights)}
• POST /v1/fsrs/preview — intervals for all four ratings (for "Again 10m · Good 3d · Easy 8d" buttons)
• POST /v1/sm2/review — classic SM-2: {card, quality: 0–5}
• GET /v1/pricing · GET /v1/keys/balance · GET /healthz

Live demo

Rate a brand-new card and see the real scheduler output — this calls the live API.

// click a rating

Pricing

Buy Starter — $19 (card)

After paying, get your key instantly — enter the email from your Stripe receipt:

// your key appears here

Prefer crypto / paying as an autonomous agent? Send $5+ USDC on Base to 0x9895C261368DCE40903261518B7aF6D83eC03e0D, then POST /v1/keys/redeem {"tx_hash":"0x…"} — verified on-chain, key issued instantly, no signup.

Why an API?

FSRS is easy to get subtly wrong (17 interacting weights, retrievability decay, mean-reverting difficulty). This service is deterministic, versioned, and stateless — you keep your data, we do the math.

Questions: [email protected]