diff --git a/apps/api/src/db/connection.ts b/apps/api/src/db/connection.ts
new file mode 100644
index 0000000..86d0b3f
--- /dev/null
+++ b/apps/api/src/db/connection.ts
@@ -0,0 +1,28 @@
+import { drizzle, type PostgresJsDatabase } from 'drizzle-orm/postgres-js';
+import postgres from 'postgres';
+
+import * as schema from './schema/index.ts';
+
+export type CardsDb = PostgresJsDatabase<typeof schema>;
+
+let _db: CardsDb | null = null;
+let _client: postgres.Sql<{}> | null = null;
+
+export function getDb(): CardsDb {
+	if (_db) return _db;
+	const url = process.env.DATABASE_URL;
+	if (!url) {
+		throw new Error('DATABASE_URL not set — Cards-API kann nicht ohne Postgres laufen');
+	}
+	_client = postgres(url, { max: 10 });
+	_db = drizzle(_client, { schema });
+	return _db;
+}
+
+export async function closeDb(): Promise<void> {
+	if (_client) {
+		await _client.end();
+		_client = null;
+		_db = null;
+	}
+}
diff --git a/apps/api/src/db/schema/_schema.ts b/apps/api/src/db/schema/_schema.ts
new file mode 100644
index 0000000..711b782
--- /dev/null
+++ b/apps/api/src/db/schema/_schema.ts
@@ -0,0 +1,7 @@
+// Single Source of Truth für die pgSchema-Deklaration.
+// Wird von allen Tabellen-Files importiert, vermeidet Zirkular-Imports
+// (siehe Lesson aus mana-share/-events während F-0).
+
+import { pgSchema } from 'drizzle-orm/pg-core';
+
+export const cardsSchema = pgSchema('cards');
diff --git a/apps/api/src/db/schema/cards.ts b/apps/api/src/db/schema/cards.ts
new file mode 100644
index 0000000..84be8cb
--- /dev/null
+++ b/apps/api/src/db/schema/cards.ts
@@ -0,0 +1,65 @@
+import { index, jsonb, primaryKey, text, timestamp } from 'drizzle-orm/pg-core';
+
+import { cardsSchema } from './_schema.ts';
+import { decks } from './decks.ts';
+
+/**
+ * Karten. `fields` ist ein generischer JSONB-Slot, der je nach `type`
+ * unterschiedliche Felder enthält:
+ *
+ * - basic / basic-reverse: { front, back }
+ * - cloze:                 { text, extra? }
+ * - type-in:               { question, expected }
+ * - image-occlusion:       { image_ref, mask_regions: [...] }
+ *
+ * MVP unterstützt nur `basic` und `basic-reverse`.
+ *
+ * `tags` ist ein eigener Tag-Mapping-Layer (siehe `tags.ts` + `cardTags`),
+ * NICHT in dieser Tabelle gespeichert.
+ */
+export const cards = cardsSchema.table(
+	'cards',
+	{
+		id: text('id').primaryKey(),
+		deckId: text('deck_id')
+			.notNull()
+			.references(() => decks.id, { onDelete: 'cascade' }),
+		userId: text('user_id').notNull(),
+		type: text('type').notNull(),
+		fields: jsonb('fields').notNull(),
+		mediaRefs: jsonb('media_refs').notNull().$type<string[]>().default([]),
+		contentHash: text('content_hash'),
+		createdAt: timestamp('created_at', { withTimezone: true, mode: 'date' })
+			.notNull()
+			.defaultNow(),
+		updatedAt: timestamp('updated_at', { withTimezone: true, mode: 'date' })
+			.notNull()
+			.defaultNow(),
+	},
+	(t) => ({
+		deckIdx: index('cards_deck_idx').on(t.deckId),
+		userIdx: index('cards_user_idx').on(t.userId),
+	})
+);
+
+export type CardRow = typeof cards.$inferSelect;
+export type CardInsert = typeof cards.$inferInsert;
+
+/**
+ * Card↔Tag-Mapping. PK ist (card_id, tag_id) — kein eigenes id-Feld nötig.
+ */
+export const cardTags = cardsSchema.table(
+	'card_tags',
+	{
+		cardId: text('card_id')
+			.notNull()
+			.references(() => cards.id, { onDelete: 'cascade' }),
+		tagId: text('tag_id').notNull(),
+	},
+	(t) => ({
+		pk: primaryKey({ columns: [t.cardId, t.tagId] }),
+	})
+);
+
+export type CardTagRow = typeof cardTags.$inferSelect;
+export type CardTagInsert = typeof cardTags.$inferInsert;
diff --git a/apps/api/src/db/schema/decks.ts b/apps/api/src/db/schema/decks.ts
new file mode 100644
index 0000000..06e65e4
--- /dev/null
+++ b/apps/api/src/db/schema/decks.ts
@@ -0,0 +1,37 @@
+import { sql } from 'drizzle-orm';
+import { index, jsonb, text, timestamp } from 'drizzle-orm/pg-core';
+
+import { cardsSchema } from './_schema.ts';
+
+/**
+ * Decks — Sammlungen von Karten. Eine Karte gehört zu genau einem Deck.
+ * `fsrs_settings` ist ein JSONB-Slot für per-Deck-Overrides der globalen
+ * FSRS-Defaults (siehe @cards/domain `FsrsSettings`).
+ */
+export const decks = cardsSchema.table(
+	'decks',
+	{
+		id: text('id').primaryKey(),
+		userId: text('user_id').notNull(),
+		name: text('name').notNull(),
+		description: text('description'),
+		color: text('color'),
+		visibility: text('visibility', { enum: ['private', 'space', 'public'] })
+			.notNull()
+			.default('private'),
+		fsrsSettings: jsonb('fsrs_settings').notNull().default(sql`'{}'::jsonb`),
+		contentHash: text('content_hash'),
+		createdAt: timestamp('created_at', { withTimezone: true, mode: 'date' })
+			.notNull()
+			.defaultNow(),
+		updatedAt: timestamp('updated_at', { withTimezone: true, mode: 'date' })
+			.notNull()
+			.defaultNow(),
+	},
+	(t) => ({
+		userIdx: index('decks_user_idx').on(t.userId),
+	})
+);
+
+export type DeckRow = typeof decks.$inferSelect;
+export type DeckInsert = typeof decks.$inferInsert;
diff --git a/apps/api/src/db/schema/imports.ts b/apps/api/src/db/schema/imports.ts
new file mode 100644
index 0000000..3f55cd0
--- /dev/null
+++ b/apps/api/src/db/schema/imports.ts
@@ -0,0 +1,34 @@
+import { index, jsonb, text, timestamp } from 'drizzle-orm/pg-core';
+
+import { cardsSchema } from './_schema.ts';
+
+/**
+ * Import-Jobs für Bulk-Vorgänge (Anki-.apkg, CSV-Upload, etc.).
+ * Der Job-Status durchläuft `queued` → `processing` → `done | failed`.
+ * `meta` ist ein freier JSONB-Slot für Source-spezifische Infos
+ * (Datei-Name, Mapping-Tabellen, Fortschritt).
+ */
+export const importJobs = cardsSchema.table(
+	'import_jobs',
+	{
+		id: text('id').primaryKey(),
+		userId: text('user_id').notNull(),
+		source: text('source', { enum: ['anki', 'csv', 'json'] }).notNull(),
+		state: text('state', { enum: ['queued', 'processing', 'done', 'failed'] })
+			.notNull()
+			.default('queued'),
+		meta: jsonb('meta'),
+		error: text('error'),
+		createdAt: timestamp('created_at', { withTimezone: true, mode: 'date' })
+			.notNull()
+			.defaultNow(),
+		finishedAt: timestamp('finished_at', { withTimezone: true, mode: 'date' }),
+	},
+	(t) => ({
+		userIdx: index('imports_user_idx').on(t.userId),
+		stateIdx: index('imports_state_idx').on(t.state),
+	})
+);
+
+export type ImportJobRow = typeof importJobs.$inferSelect;
+export type ImportJobInsert = typeof importJobs.$inferInsert;
diff --git a/apps/api/src/db/schema/index.ts b/apps/api/src/db/schema/index.ts
index e84f525..a3dd7be 100644
--- a/apps/api/src/db/schema/index.ts
+++ b/apps/api/src/db/schema/index.ts
@@ -1,13 +1,29 @@
-// Drizzle-Schemas für die `cards`-Datenbank.
+// Public Re-exports für Drizzle-Schemas.
 //
-// Phase-3-Aufgabe (siehe CARDS_GREENFIELD.md): hier landen
-// `decks`, `cards`, `reviews`, `study_sessions`, `tags`, `media_refs`,
-// `import_jobs` als pgSchema('cards').table(...) Definitionen.
-//
-// Schema-Skizze in mana/docs/playbooks/CARDS_GREENFIELD.md §"Drizzle-Schema-Skizze".
-// Card-Type-Granularität (subIndex pro Karte) aus
-// docs/LESSONS_FROM_MANA_MONOREPO.md mitnehmen.
+// Konvention: Tabellen-Files importieren `cardsSchema` aus `_schema.ts`
+// (nie aus `index.ts`), damit es keine Zirkular-Imports gibt.
 
-import { pgSchema } from 'drizzle-orm/pg-core';
+export { cardsSchema } from './_schema.ts';
 
-export const cardsSchema = pgSchema('cards');
+export { decks } from './decks.ts';
+export type { DeckRow, DeckInsert } from './decks.ts';
+
+export { cards, cardTags } from './cards.ts';
+export type { CardRow, CardInsert, CardTagRow, CardTagInsert } from './cards.ts';
+
+export { reviews, studySessions } from './reviews.ts';
+export type {
+	ReviewRow,
+	ReviewInsert,
+	StudySessionRow,
+	StudySessionInsert,
+} from './reviews.ts';
+
+export { tags } from './tags.ts';
+export type { TagRow, TagInsert } from './tags.ts';
+
+export { mediaRefs } from './media.ts';
+export type { MediaRefRow, MediaRefInsert } from './media.ts';
+
+export { importJobs } from './imports.ts';
+export type { ImportJobRow, ImportJobInsert } from './imports.ts';
diff --git a/apps/api/src/db/schema/media.ts b/apps/api/src/db/schema/media.ts
new file mode 100644
index 0000000..b23c4db
--- /dev/null
+++ b/apps/api/src/db/schema/media.ts
@@ -0,0 +1,32 @@
+import { index, integer, text, timestamp } from 'drizzle-orm/pg-core';
+
+import { cardsSchema } from './_schema.ts';
+import { cards } from './cards.ts';
+
+/**
+ * Media-Verweise auf Object-IDs in mana-media. Die eigentlichen Files
+ * (Bilder, Audio, Video) liegen in MinIO via mana-media; diese Tabelle
+ * hält nur den Verweis + Sortier-Order pro Karte.
+ */
+export const mediaRefs = cardsSchema.table(
+	'media_refs',
+	{
+		id: text('id').primaryKey(),
+		cardId: text('card_id')
+			.notNull()
+			.references(() => cards.id, { onDelete: 'cascade' }),
+		userId: text('user_id').notNull(),
+		manaMediaObjectId: text('mana_media_object_id').notNull(),
+		kind: text('kind', { enum: ['image', 'audio', 'video'] }).notNull(),
+		ord: integer('ord').notNull().default(0),
+		createdAt: timestamp('created_at', { withTimezone: true, mode: 'date' })
+			.notNull()
+			.defaultNow(),
+	},
+	(t) => ({
+		cardIdx: index('media_card_idx').on(t.cardId),
+	})
+);
+
+export type MediaRefRow = typeof mediaRefs.$inferSelect;
+export type MediaRefInsert = typeof mediaRefs.$inferInsert;
diff --git a/apps/api/src/db/schema/reviews.ts b/apps/api/src/db/schema/reviews.ts
new file mode 100644
index 0000000..bdb5616
--- /dev/null
+++ b/apps/api/src/db/schema/reviews.ts
@@ -0,0 +1,76 @@
+import { index, integer, primaryKey, real, text, timestamp } from 'drizzle-orm/pg-core';
+
+import { cardsSchema } from './_schema.ts';
+import { cards } from './cards.ts';
+import { decks } from './decks.ts';
+
+/**
+ * FSRS-Review-State pro `(card, sub_index)`.
+ *
+ * `sub_index` granular:
+ * - basic:         1 Review (sub_index = 0)
+ * - basic-reverse: 2 Reviews (0 = front→back, 1 = back→front)
+ * - cloze:         1 Review pro Cluster-Index ({{c1::…}} → sub_index = 1)
+ *
+ * **Bewusst PLAINTEXT** (siehe `docs/LESSONS_FROM_MANA_MONOREPO.md` §3):
+ * Der Scheduler quert täglich `due <= now` — Encryption müsste das
+ * jedes Mal entschlüsseln. mana-monorepo hat das gleiche Pattern:
+ * cardReviews ist plaintext-allowlisted.
+ */
+export const reviews = cardsSchema.table(
+	'reviews',
+	{
+		cardId: text('card_id')
+			.notNull()
+			.references(() => cards.id, { onDelete: 'cascade' }),
+		subIndex: integer('sub_index').notNull().default(0),
+		userId: text('user_id').notNull(),
+		due: timestamp('due', { withTimezone: true, mode: 'date' }).notNull(),
+		stability: real('stability').notNull(),
+		difficulty: real('difficulty').notNull(),
+		elapsedDays: real('elapsed_days').notNull().default(0),
+		scheduledDays: real('scheduled_days').notNull().default(0),
+		reps: integer('reps').notNull().default(0),
+		lapses: integer('lapses').notNull().default(0),
+		state: text('state', { enum: ['new', 'learning', 'review', 'relearning'] })
+			.notNull()
+			.default('new'),
+		lastReview: timestamp('last_review', { withTimezone: true, mode: 'date' }),
+	},
+	(t) => ({
+		pk: primaryKey({ columns: [t.cardId, t.subIndex] }),
+		// Hot Path: Scheduler quert täglich `due <= now` für einen User.
+		userDueIdx: index('reviews_user_due_idx').on(t.userId, t.due),
+	})
+);
+
+export type ReviewRow = typeof reviews.$inferSelect;
+export type ReviewInsert = typeof reviews.$inferInsert;
+
+/**
+ * Study-Sessions als Statistik-Layer. Eine Session läuft pro
+ * `(user, deck)`-Studieren-Lauf, wird beim Start angelegt und beim
+ * Ende mit Total-Counts geupdatet.
+ */
+export const studySessions = cardsSchema.table(
+	'study_sessions',
+	{
+		id: text('id').primaryKey(),
+		userId: text('user_id').notNull(),
+		deckId: text('deck_id')
+			.notNull()
+			.references(() => decks.id, { onDelete: 'cascade' }),
+		startedAt: timestamp('started_at', { withTimezone: true, mode: 'date' })
+			.notNull()
+			.defaultNow(),
+		finishedAt: timestamp('finished_at', { withTimezone: true, mode: 'date' }),
+		cardsReviewed: integer('cards_reviewed').notNull().default(0),
+		cardsCorrect: integer('cards_correct').notNull().default(0),
+	},
+	(t) => ({
+		userStartedIdx: index('sessions_user_started_idx').on(t.userId, t.startedAt),
+	})
+);
+
+export type StudySessionRow = typeof studySessions.$inferSelect;
+export type StudySessionInsert = typeof studySessions.$inferInsert;
diff --git a/apps/api/src/db/schema/tags.ts b/apps/api/src/db/schema/tags.ts
new file mode 100644
index 0000000..d3affa2
--- /dev/null
+++ b/apps/api/src/db/schema/tags.ts
@@ -0,0 +1,30 @@
+import { index, text, timestamp, uniqueIndex } from 'drizzle-orm/pg-core';
+
+import { cardsSchema } from './_schema.ts';
+import { decks } from './decks.ts';
+
+/**
+ * Tags sind deck-skopiert (laut mana-monorepo-Pattern). Ein Tag-Name
+ * kann pro Deck nur einmal vorkommen.
+ */
+export const tags = cardsSchema.table(
+	'tags',
+	{
+		id: text('id').primaryKey(),
+		deckId: text('deck_id')
+			.notNull()
+			.references(() => decks.id, { onDelete: 'cascade' }),
+		userId: text('user_id').notNull(),
+		name: text('name').notNull(),
+		createdAt: timestamp('created_at', { withTimezone: true, mode: 'date' })
+			.notNull()
+			.defaultNow(),
+	},
+	(t) => ({
+		deckIdx: index('tags_deck_idx').on(t.deckId),
+		uniqueByDeckName: uniqueIndex('tags_deck_name_uniq').on(t.deckId, t.name),
+	})
+);
+
+export type TagRow = typeof tags.$inferSelect;
+export type TagInsert = typeof tags.$inferInsert;
diff --git a/apps/api/src/index.ts b/apps/api/src/index.ts
index 88bd064..85729ef 100644
--- a/apps/api/src/index.ts
+++ b/apps/api/src/index.ts
@@ -2,11 +2,17 @@ import { Hono } from 'hono';
 
 import { manifestRoute } from './routes/manifest.ts';
 import { healthRoute } from './routes/health.ts';
+import { decksRouter } from './routes/decks.ts';
+import { cardsRouter } from './routes/cards.ts';
+import { reviewsRouter } from './routes/reviews.ts';
 
 const app = new Hono();
 
 app.route('/', healthRoute);
 app.route('/.well-known/mana-app.json', manifestRoute);
+app.route('/api/v1/decks', decksRouter());
+app.route('/api/v1/cards', cardsRouter());
+app.route('/api/v1/reviews', reviewsRouter());
 
 app.get('/', (c) =>
 	c.json({
diff --git a/apps/api/src/lib/ulid.ts b/apps/api/src/lib/ulid.ts
new file mode 100644
index 0000000..6cb577b
--- /dev/null
+++ b/apps/api/src/lib/ulid.ts
@@ -0,0 +1,46 @@
+/**
+ * ULID-Generator (Crockford-Base-32).
+ * Bewusst lokal statt ulid-Lib, damit keine zusätzliche Abhängigkeit.
+ *
+ * 26 Zeichen: 10 Time + 16 Random.
+ * Lexikografisch sortierbar, monoton steigend.
+ */
+
+const CROCKFORD = '0123456789ABCDEFGHJKMNPQRSTVWXYZ';
+
+function randomBytes(n: number): Uint8Array {
+	const buf = new Uint8Array(n);
+	crypto.getRandomValues(buf);
+	return buf;
+}
+
+function encodeTime(now: number): string {
+	let s = '';
+	let n = now;
+	for (let i = 0; i < 10; i++) {
+		const r = n % 32;
+		s = CROCKFORD[r] + s;
+		n = Math.floor(n / 32);
+	}
+	return s;
+}
+
+function encodeRandom(): string {
+	const bytes = randomBytes(10);
+	let bits = 0;
+	let acc = 0;
+	let s = '';
+	for (const b of bytes) {
+		acc = (acc << 8) | b;
+		bits += 8;
+		while (bits >= 5) {
+			bits -= 5;
+			s += CROCKFORD[(acc >> bits) & 0x1f];
+		}
+	}
+	return s;
+}
+
+export function ulid(now: number = Date.now()): string {
+	return encodeTime(now) + encodeRandom();
+}
diff --git a/apps/api/src/middleware/auth.ts b/apps/api/src/middleware/auth.ts
new file mode 100644
index 0000000..94467c9
--- /dev/null
+++ b/apps/api/src/middleware/auth.ts
@@ -0,0 +1,27 @@
+import type { Context, MiddlewareHandler } from 'hono';
+
+/**
+ * Auth-Middleware-Stub für Phase 3.
+ *
+ * Heute (Dev): liest `X-User-Id`-Header.
+ * Phase 2 (echt): validiert User-JWT gegen mana-auth JWKS und extrahiert
+ * `sub`-Claim als userId.
+ *
+ * Implementations-Notiz: Phase 2 schwenkt auf `@mana/shared-hono`'s
+ * `authMiddleware()` um, das den JWKS-Cache verwaltet.
+ */
+export type AuthVars = { userId: string };
+
+export const authMiddleware: MiddlewareHandler<{ Variables: AuthVars }> = async (c, next) => {
+	const userId = c.req.header('X-User-Id');
+	if (!userId) {
+		return c.json({ error: 'unauthenticated', detail: 'X-User-Id header missing (dev stub)' }, 401);
+	}
+	c.set('userId', userId);
+	await next();
+};
+
+/** Helper zum Auslesen des userId aus dem Context (typed). */
+export function getUserId(c: Context<{ Variables: AuthVars }>): string {
+	return c.get('userId');
+}
diff --git a/apps/api/src/routes/cards.ts b/apps/api/src/routes/cards.ts
new file mode 100644
index 0000000..e38d9ba
--- /dev/null
+++ b/apps/api/src/routes/cards.ts
@@ -0,0 +1,163 @@
+import { and, eq } from 'drizzle-orm';
+import { Hono } from 'hono';
+
+import { CardCreateSchema, CardUpdateSchema, newReview, subIndexCount } from '@cards/domain';
+
+import { getDb, type CardsDb } from '../db/connection.ts';
+import { cards, decks, reviews } from '../db/schema/index.ts';
+import { authMiddleware, type AuthVars } from '../middleware/auth.ts';
+import { ulid } from '../lib/ulid.ts';
+
+export type CardsDeps = { db?: CardsDb };
+
+export function cardsRouter(deps: CardsDeps = {}): Hono<{ Variables: AuthVars }> {
+	const r = new Hono<{ Variables: AuthVars }>();
+	const dbOf = () => deps.db ?? getDb();
+
+	r.use('*', authMiddleware);
+
+	/**
+	 * Karte erstellen + automatisch initiale Reviews anlegen.
+	 *
+	 * Pro Card-Type werden N `(card_id, sub_index)`-Reviews angelegt
+	 * (basic = 1, basic-reverse = 2). Alles in einer Transaktion.
+	 */
+	r.post('/', async (c) => {
+		const body = await c.req.json().catch(() => null);
+		const parsed = CardCreateSchema.safeParse(body);
+		if (!parsed.success) {
+			return c.json(
+				{ error: 'invalid_input', issues: parsed.error.issues.map((i) => i.message) },
+				422
+			);
+		}
+		const userId = c.get('userId');
+
+		const [deck] = await dbOf()
+			.select({ id: decks.id, userId: decks.userId })
+			.from(decks)
+			.where(eq(decks.id, parsed.data.deck_id))
+			.limit(1);
+		if (!deck) return c.json({ error: 'deck_not_found' }, 404);
+		if (deck.userId !== userId) return c.json({ error: 'deck_not_owned' }, 403);
+
+		const cardId = ulid();
+		const now = new Date();
+		const subIndices = Array.from({ length: subIndexCount(parsed.data.type) }, (_, i) => i);
+
+		const [cardRow] = await dbOf().transaction(async (tx) => {
+			const [card] = await tx
+				.insert(cards)
+				.values({
+					id: cardId,
+					deckId: parsed.data.deck_id,
+					userId,
+					type: parsed.data.type,
+					fields: parsed.data.fields,
+					mediaRefs: parsed.data.media_refs ?? [],
+					createdAt: now,
+					updatedAt: now,
+				})
+				.returning();
+
+			const initialReviews = subIndices.map((subIndex) => {
+				const r = newReview({ userId, cardId, subIndex, now });
+				return {
+					cardId: r.card_id,
+					subIndex: r.sub_index,
+					userId: r.user_id,
+					due: new Date(r.due),
+					stability: r.stability,
+					difficulty: r.difficulty,
+					elapsedDays: r.elapsed_days,
+					scheduledDays: r.scheduled_days,
+					reps: r.reps,
+					lapses: r.lapses,
+					state: r.state,
+					lastReview: r.last_review ? new Date(r.last_review) : null,
+				};
+			});
+			if (initialReviews.length > 0) {
+				await tx.insert(reviews).values(initialReviews);
+			}
+
+			return [card];
+		});
+
+		return c.json(toCardDto(cardRow), 201);
+	});
+
+	r.get('/', async (c) => {
+		const userId = c.get('userId');
+		const deckId = c.req.query('deck_id');
+		const conditions = deckId
+			? and(eq(cards.userId, userId), eq(cards.deckId, deckId))
+			: eq(cards.userId, userId);
+		const rows = await dbOf().select().from(cards).where(conditions);
+		return c.json({ cards: rows.map(toCardDto), total: rows.length });
+	});
+
+	r.get('/:id', async (c) => {
+		const userId = c.get('userId');
+		const id = c.req.param('id');
+		const [row] = await dbOf()
+			.select()
+			.from(cards)
+			.where(and(eq(cards.id, id), eq(cards.userId, userId)))
+			.limit(1);
+		if (!row) return c.json({ error: 'not_found' }, 404);
+		return c.json(toCardDto(row));
+	});
+
+	r.patch('/:id', async (c) => {
+		const userId = c.get('userId');
+		const id = c.req.param('id');
+		const body = await c.req.json().catch(() => null);
+		const parsed = CardUpdateSchema.safeParse(body);
+		if (!parsed.success) {
+			return c.json(
+				{ error: 'invalid_input', issues: parsed.error.issues.map((i) => i.message) },
+				422
+			);
+		}
+		const [row] = await dbOf()
+			.update(cards)
+			.set({
+				...(parsed.data.fields !== undefined && { fields: parsed.data.fields }),
+				...(parsed.data.media_refs !== undefined && { mediaRefs: parsed.data.media_refs }),
+				updatedAt: new Date(),
+			})
+			.where(and(eq(cards.id, id), eq(cards.userId, userId)))
+			.returning();
+		if (!row) return c.json({ error: 'not_found' }, 404);
+		return c.json(toCardDto(row));
+	});
+
+	r.delete('/:id', async (c) => {
+		const userId = c.get('userId');
+		const id = c.req.param('id');
+		const result = await dbOf()
+			.delete(cards)
+			.where(and(eq(cards.id, id), eq(cards.userId, userId)))
+			.returning({ id: cards.id });
+		if (result.length === 0) return c.json({ error: 'not_found' }, 404);
+		// reviews kaskadiert per onDelete: 'cascade' in der Schema-Definition.
+		return c.json({ deleted: id });
+	});
+
+	return r;
+}
+
+function toCardDto(row: typeof cards.$inferSelect) {
+	return {
+		id: row.id,
+		deck_id: row.deckId,
+		user_id: row.userId,
+		type: row.type,
+		fields: row.fields,
+		media_refs: row.mediaRefs ?? [],
+		content_hash: row.contentHash,
+		created_at: row.createdAt.toISOString(),
+		updated_at: row.updatedAt.toISOString(),
+	};
+}
diff --git a/apps/api/src/routes/decks.ts b/apps/api/src/routes/decks.ts
new file mode 100644
index 0000000..ad83023
--- /dev/null
+++ b/apps/api/src/routes/decks.ts
@@ -0,0 +1,123 @@
+import { and, eq } from 'drizzle-orm';
+import { Hono } from 'hono';
+
+import { DeckCreateSchema, DeckUpdateSchema } from '@cards/domain';
+
+import { getDb, type CardsDb } from '../db/connection.ts';
+import { decks } from '../db/schema/index.ts';
+import { authMiddleware, type AuthVars } from '../middleware/auth.ts';
+import { ulid } from '../lib/ulid.ts';
+
+/** Optional injectable DB für Tests. */
+export type DecksDeps = { db?: CardsDb };
+
+export function decksRouter(deps: DecksDeps = {}): Hono<{ Variables: AuthVars }> {
+	const r = new Hono<{ Variables: AuthVars }>();
+	const dbOf = () => deps.db ?? getDb();
+
+	r.use('*', authMiddleware);
+
+	r.post('/', async (c) => {
+		const body = await c.req.json().catch(() => null);
+		const parsed = DeckCreateSchema.safeParse(body);
+		if (!parsed.success) {
+			return c.json(
+				{ error: 'invalid_input', issues: parsed.error.issues.map((i) => i.message) },
+				422
+			);
+		}
+		const userId = c.get('userId');
+		const id = ulid();
+		const now = new Date();
+		const [row] = await dbOf()
+			.insert(decks)
+			.values({
+				id,
+				userId,
+				name: parsed.data.name,
+				description: parsed.data.description,
+				color: parsed.data.color,
+				visibility: parsed.data.visibility ?? 'private',
+				fsrsSettings: parsed.data.fsrs_settings ?? {},
+				createdAt: now,
+				updatedAt: now,
+			})
+			.returning();
+		return c.json(toDeckDto(row), 201);
+	});
+
+	r.get('/', async (c) => {
+		const userId = c.get('userId');
+		const rows = await dbOf().select().from(decks).where(eq(decks.userId, userId));
+		return c.json({ decks: rows.map(toDeckDto), total: rows.length });
+	});
+
+	r.get('/:id', async (c) => {
+		const userId = c.get('userId');
+		const id = c.req.param('id');
+		const [row] = await dbOf()
+			.select()
+			.from(decks)
+			.where(and(eq(decks.id, id), eq(decks.userId, userId)))
+			.limit(1);
+		if (!row) return c.json({ error: 'not_found' }, 404);
+		return c.json(toDeckDto(row));
+	});
+
+	r.patch('/:id', async (c) => {
+		const userId = c.get('userId');
+		const id = c.req.param('id');
+		const body = await c.req.json().catch(() => null);
+		const parsed = DeckUpdateSchema.safeParse(body);
+		if (!parsed.success) {
+			return c.json(
+				{ error: 'invalid_input', issues: parsed.error.issues.map((i) => i.message) },
+				422
+			);
+		}
+		const [row] = await dbOf()
+			.update(decks)
+			.set({
+				...(parsed.data.name !== undefined && { name: parsed.data.name }),
+				...(parsed.data.description !== undefined && { description: parsed.data.description }),
+				...(parsed.data.color !== undefined && { color: parsed.data.color }),
+				...(parsed.data.visibility !== undefined && { visibility: parsed.data.visibility }),
+				...(parsed.data.fsrs_settings !== undefined && {
+					fsrsSettings: parsed.data.fsrs_settings,
+				}),
+				updatedAt: new Date(),
+			})
+			.where(and(eq(decks.id, id), eq(decks.userId, userId)))
+			.returning();
+		if (!row) return c.json({ error: 'not_found' }, 404);
+		return c.json(toDeckDto(row));
+	});
+
+	r.delete('/:id', async (c) => {
+		const userId = c.get('userId');
+		const id = c.req.param('id');
+		const result = await dbOf()
+			.delete(decks)
+			.where(and(eq(decks.id, id), eq(decks.userId, userId)))
+			.returning({ id: decks.id });
+		if (result.length === 0) return c.json({ error: 'not_found' }, 404);
+		return c.json({ deleted: id });
+	});
+
+	return r;
+}
+
+function toDeckDto(row: typeof decks.$inferSelect) {
+	return {
+		id: row.id,
+		user_id: row.userId,
+		name: row.name,
+		description: row.description,
+		color: row.color,
+		visibility: row.visibility,
+		fsrs_settings: row.fsrsSettings,
+		content_hash: row.contentHash,
+		created_at: row.createdAt.toISOString(),
+		updated_at: row.updatedAt.toISOString(),
+	};
+}
diff --git a/apps/api/src/routes/reviews.ts b/apps/api/src/routes/reviews.ts
new file mode 100644
index 0000000..2c8fa1c
--- /dev/null
+++ b/apps/api/src/routes/reviews.ts
@@ -0,0 +1,160 @@
+import { and, asc, eq, lte } from 'drizzle-orm';
+import { Hono } from 'hono';
+
+import {
+	GradeReviewInputSchema,
+	gradeReview,
+	type Review as DomainReview,
+} from '@cards/domain';
+
+import { getDb, type CardsDb } from '../db/connection.ts';
+import { cards, decks, reviews } from '../db/schema/index.ts';
+import { authMiddleware, type AuthVars } from '../middleware/auth.ts';
+
+export type ReviewsDeps = { db?: CardsDb };
+
+export function reviewsRouter(deps: ReviewsDeps = {}): Hono<{ Variables: AuthVars }> {
+	const r = new Hono<{ Variables: AuthVars }>();
+	const dbOf = () => deps.db ?? getDb();
+
+	r.use('*', authMiddleware);
+
+	/**
+	 * Hot Path: alle Reviews holen, deren due <= now ist.
+	 * Optional auf ein Deck eingeschränkt. Default-Limit: 100.
+	 *
+	 * Index: `reviews_user_due_idx` deckt diese Query ab.
+	 */
+	r.get('/due', async (c) => {
+		const userId = c.get('userId');
+		const deckId = c.req.query('deck_id');
+		const limit = Math.min(Number(c.req.query('limit') ?? 100), 500);
+		const now = new Date();
+
+		const conditions = [eq(reviews.userId, userId), lte(reviews.due, now)];
+
+		if (deckId) {
+			// Wenn deck_id angegeben, joinen wir auf cards.deck_id.
+			const rows = await dbOf()
+				.select({
+					review: reviews,
+					card: { id: cards.id, deckId: cards.deckId, type: cards.type, fields: cards.fields },
+				})
+				.from(reviews)
+				.innerJoin(cards, eq(cards.id, reviews.cardId))
+				.where(and(...conditions, eq(cards.deckId, deckId)))
+				.orderBy(asc(reviews.due))
+				.limit(limit);
+			return c.json({
+				reviews: rows.map((r) => ({ ...toReviewDto(r.review), card: r.card })),
+				total: rows.length,
+			});
+		}
+
+		const rows = await dbOf()
+			.select()
+			.from(reviews)
+			.where(and(...conditions))
+			.orderBy(asc(reviews.due))
+			.limit(limit);
+		return c.json({ reviews: rows.map(toReviewDto), total: rows.length });
+	});
+
+	/**
+	 * User bewertet eine Karte. FSRS rechnet nächste due-time aus,
+	 * wir schreiben das Update zurück.
+	 */
+	r.post('/:cardId/:subIndex/grade', async (c) => {
+		const userId = c.get('userId');
+		const cardId = c.req.param('cardId');
+		const subIndex = Number(c.req.param('subIndex'));
+		if (!Number.isInteger(subIndex) || subIndex < 0) {
+			return c.json({ error: 'invalid_sub_index' }, 422);
+		}
+
+		const body = await c.req.json().catch(() => null);
+		const parsed = GradeReviewInputSchema.safeParse({
+			...((body as object) ?? {}),
+			card_id: cardId,
+			sub_index: subIndex,
+		});
+		if (!parsed.success) {
+			return c.json(
+				{ error: 'invalid_input', issues: parsed.error.issues.map((i) => i.message) },
+				422
+			);
+		}
+
+		// Aktuellen Review-State holen + Deck-FSRS-Settings für den Scheduler.
+		const [hit] = await dbOf()
+			.select({
+				review: reviews,
+				deck: { fsrsSettings: decks.fsrsSettings },
+			})
+			.from(reviews)
+			.innerJoin(cards, eq(cards.id, reviews.cardId))
+			.innerJoin(decks, eq(decks.id, cards.deckId))
+			.where(
+				and(
+					eq(reviews.cardId, cardId),
+					eq(reviews.subIndex, subIndex),
+					eq(reviews.userId, userId)
+				)
+			)
+			.limit(1);
+
+		if (!hit) return c.json({ error: 'not_found' }, 404);
+
+		const reviewedAt = parsed.data.reviewed_at ? new Date(parsed.data.reviewed_at) : new Date();
+		const currentDomain: DomainReview = toReviewDto(hit.review);
+		const next = gradeReview(
+			currentDomain,
+			parsed.data.rating,
+			reviewedAt,
+			(hit.deck.fsrsSettings as object) ?? {}
+		);
+
+		const [updated] = await dbOf()
+			.update(reviews)
+			.set({
+				due: new Date(next.due),
+				stability: next.stability,
+				difficulty: next.difficulty,
+				elapsedDays: next.elapsed_days,
+				scheduledDays: next.scheduled_days,
+				reps: next.reps,
+				lapses: next.lapses,
+				state: next.state,
+				lastReview: next.last_review ? new Date(next.last_review) : null,
+			})
+			.where(
+				and(
+					eq(reviews.cardId, cardId),
+					eq(reviews.subIndex, subIndex),
+					eq(reviews.userId, userId)
+				)
+			)
+			.returning();
+
+		return c.json(toReviewDto(updated));
+	});
+
+	return r;
+}
+
+function toReviewDto(row: typeof reviews.$inferSelect): DomainReview {
+	return {
+		card_id: row.cardId,
+		sub_index: row.subIndex,
+		user_id: row.userId,
+		due: row.due.toISOString(),
+		stability: row.stability,
+		difficulty: row.difficulty,
+		elapsed_days: row.elapsedDays,
+		scheduled_days: row.scheduledDays,
+		reps: row.reps,
+		lapses: row.lapses,
+		state: row.state,
+		last_review: row.lastReview ? row.lastReview.toISOString() : null,
+	};
+}
diff --git a/apps/api/tests/cards.test.ts b/apps/api/tests/cards.test.ts
new file mode 100644
index 0000000..d8bce5f
--- /dev/null
+++ b/apps/api/tests/cards.test.ts
@@ -0,0 +1,98 @@
+import { describe, it, expect } from 'vitest';
+import { Hono } from 'hono';
+
+import { cardsRouter } from '../src/routes/cards.ts';
+import type { CardsDb } from '../src/db/connection.ts';
+
+/**
+ * Routen-Tests ohne echte DB. Drizzle-Aufrufe werden durch eine
+ * minimale Stub-DB ersetzt, die nur die Validations-Pfade abdeckt.
+ */
+
+function buildApp() {
+	const app = new Hono();
+	const stub = {
+		select: () => ({
+			from: () => ({
+				where: () => ({ limit: () => [] }),
+			}),
+		}),
+	};
+	app.route('/api/v1/cards', cardsRouter({ db: stub as unknown as CardsDb }));
+	return { app };
+}
+
+describe('cardsRouter — auth-gate', () => {
+	it('GET ohne X-User-Id ist 401', async () => {
+		const { app } = buildApp();
+		const res = await app.request('/api/v1/cards');
+		expect(res.status).toBe(401);
+	});
+});
+
+describe('cardsRouter — Input-Validation', () => {
+	it('POST mit leerem Body ist 422', async () => {
+		const { app } = buildApp();
+		const res = await app.request('/api/v1/cards', {
+			method: 'POST',
+			headers: { 'X-User-Id': 'u-1', 'Content-Type': 'application/json' },
+			body: '{}',
+		});
+		expect(res.status).toBe(422);
+	});
+
+	it('POST mit basic-Card ohne back-Feld ist 422', async () => {
+		const { app } = buildApp();
+		const res = await app.request('/api/v1/cards', {
+			method: 'POST',
+			headers: { 'X-User-Id': 'u-1', 'Content-Type': 'application/json' },
+			body: JSON.stringify({
+				deck_id: 'd-1',
+				type: 'basic',
+				fields: { front: 'Q' },
+			}),
+		});
+		expect(res.status).toBe(422);
+	});
+
+	it('POST mit unknown CardType ist 422', async () => {
+		const { app } = buildApp();
+		const res = await app.request('/api/v1/cards', {
+			method: 'POST',
+			headers: { 'X-User-Id': 'u-1', 'Content-Type': 'application/json' },
+			body: JSON.stringify({
+				deck_id: 'd-1',
+				type: 'cloze',
+				fields: { text: 'x' },
+			}),
+		});
+		expect(res.status).toBe(422);
+	});
+
+	it('PATCH mit extra prop ist 422', async () => {
+		const { app } = buildApp();
+		const res = await app.request('/api/v1/cards/c-1', {
+			method: 'PATCH',
+			headers: { 'X-User-Id': 'u-1', 'Content-Type': 'application/json' },
+			body: JSON.stringify({ fields: { front: 'X' }, leak: 'bad' }),
+		});
+		expect(res.status).toBe(422);
+	});
+
+	it('POST mit gültigem basic-Card erreicht Deck-Lookup (404 bei stub)', async () => {
+		const { app } = buildApp();
+		const res = await app.request('/api/v1/cards', {
+			method: 'POST',
+			headers: { 'X-User-Id': 'u-1', 'Content-Type': 'application/json' },
+			body: JSON.stringify({
+				deck_id: 'd-1',
+				type: 'basic',
+				fields: { front: 'Q', back: 'A' },
+			}),
+		});
+		// Stub-DB gibt empty array → Deck-Not-Found-Pfad
+		expect(res.status).toBe(404);
+		const body = (await res.json()) as { error: string };
+		expect(body.error).toBe('deck_not_found');
+	});
+});
diff --git a/apps/api/tests/decks.test.ts b/apps/api/tests/decks.test.ts
new file mode 100644
index 0000000..65b82f8
--- /dev/null
+++ b/apps/api/tests/decks.test.ts
@@ -0,0 +1,164 @@
+import { describe, it, expect } from 'vitest';
+import { Hono } from 'hono';
+
+import { decksRouter } from '../src/routes/decks.ts';
+import type { CardsDb } from '../src/db/connection.ts';
+
+/**
+ * Routen-Tests ohne echte DB. Wir mocken die paar Drizzle-Methoden, die
+ * der Decks-Router nutzt, mit einem winzigen In-Memory-Store.
+ *
+ * Echte Integrations-Tests (gegen postgres/pg-mem) folgen in einer späteren
+ * Phase, wenn die Test-Infra steht.
+ */
+
+type Row = {
+	id: string;
+	userId: string;
+	name: string;
+	description: string | null;
+	color: string | null;
+	visibility: 'private' | 'space' | 'public';
+	fsrsSettings: unknown;
+	contentHash: string | null;
+	createdAt: Date;
+	updatedAt: Date;
+};
+
+function makeFakeDb() {
+	const store = new Map<string, Row>();
+
+	const fakeDb = {
+		insert: (_table: unknown) => ({
+			values: (vals: Partial<Row> & { id: string; userId: string }) => ({
+				returning: async () => {
+					const row: Row = {
+						id: vals.id,
+						userId: vals.userId,
+						name: vals.name ?? '',
+						description: vals.description ?? null,
+						color: vals.color ?? null,
+						visibility: vals.visibility ?? 'private',
+						fsrsSettings: vals.fsrsSettings ?? {},
+						contentHash: vals.contentHash ?? null,
+						createdAt: vals.createdAt ?? new Date(),
+						updatedAt: vals.updatedAt ?? new Date(),
+					};
+					store.set(row.id, row);
+					return [row];
+				},
+			}),
+		}),
+		select: () => ({
+			from: (_table: unknown) => ({
+				where: (filter: { userId?: string; id?: string }) => {
+					const items = Array.from(store.values()).filter((r) => {
+						if (filter.userId && r.userId !== filter.userId) return false;
+						if (filter.id && r.id !== filter.id) return false;
+						return true;
+					});
+					return Object.assign(items as Row[] | Promise<Row[]>, {
+						limit: (_n: number) => items.slice(0, _n),
+					});
+				},
+			}),
+		}),
+		update: (_table: unknown) => ({
+			set: (patch: Partial<Row>) => ({
+				where: (filter: { userId: string; id: string }) => ({
+					returning: async () => {
+						const existing = store.get(filter.id);
+						if (!existing || existing.userId !== filter.userId) return [];
+						const updated = { ...existing, ...patch, updatedAt: new Date() };
+						store.set(updated.id, updated);
+						return [updated];
+					},
+				}),
+			}),
+		}),
+		delete: (_table: unknown) => ({
+			where: (filter: { userId: string; id: string }) => ({
+				returning: async () => {
+					const existing = store.get(filter.id);
+					if (!existing || existing.userId !== filter.userId) return [];
+					store.delete(filter.id);
+					return [{ id: filter.id }];
+				},
+			}),
+		}),
+	};
+
+	// Drizzle's eq/and dont actually pass a function-based filter; the fake-db
+	// shim above doesn't match real Drizzle wire-shape. So we override the
+	// interpretation: the test patches eq/and via a simpler comparator.
+	// For now this fake-DB is sufficient ONLY if the routes' .where()-args
+	// arrive as plain { userId, id } objects. They don't — they arrive as
+	// Drizzle-SQL builders. So tests below are scoped to validation/auth paths,
+	// not full CRUD.
+
+	return { fakeDb, store };
+}
+
+function buildApp() {
+	const { fakeDb } = makeFakeDb();
+	// Cast — the fakeDb is intentionally minimal and not a full CardsDb.
+	const app = new Hono();
+	app.route('/api/v1/decks', decksRouter({ db: fakeDb as unknown as CardsDb }));
+	return { app };
+}
+
+describe('decksRouter — auth-gate', () => {
+	it('rejects requests without X-User-Id with 401', async () => {
+		const { app } = buildApp();
+		const res = await app.request('/api/v1/decks');
+		expect(res.status).toBe(401);
+	});
+
+	it('lets through with X-User-Id (no DB call)', async () => {
+		const { app } = buildApp();
+		// POST with invalid input should reach the validation step, not 401.
+		const res = await app.request('/api/v1/decks', {
+			method: 'POST',
+			headers: {
+				'X-User-Id': 'u-1',
+				'Content-Type': 'application/json',
+			},
+			body: '{}',
+		});
+		expect(res.status).toBe(422);
+	});
+});
+
+describe('decksRouter — input validation', () => {
+	it('POST with empty body is 422', async () => {
+		const { app } = buildApp();
+		const res = await app.request('/api/v1/decks', {
+			method: 'POST',
+			headers: { 'X-User-Id': 'u-1', 'Content-Type': 'application/json' },
+			body: '{}',
+		});
+		expect(res.status).toBe(422);
+		const body = (await res.json()) as { error: string };
+		expect(body.error).toBe('invalid_input');
+	});
+
+	it('POST with bad color is 422', async () => {
+		const { app } = buildApp();
+		const res = await app.request('/api/v1/decks', {
+			method: 'POST',
+			headers: { 'X-User-Id': 'u-1', 'Content-Type': 'application/json' },
+			body: JSON.stringify({ name: 'D', color: 'red' }),
+		});
+		expect(res.status).toBe(422);
+	});
+
+	it('PATCH with extra prop is 422', async () => {
+		const { app } = buildApp();
+		const res = await app.request('/api/v1/decks/d-1', {
+			method: 'PATCH',
+			headers: { 'X-User-Id': 'u-1', 'Content-Type': 'application/json' },
+			body: JSON.stringify({ name: 'X', leak: 'bad' }),
+		});
+		expect(res.status).toBe(422);
+	});
+});
diff --git a/apps/api/tests/reviews.test.ts b/apps/api/tests/reviews.test.ts
new file mode 100644
index 0000000..60956c3
--- /dev/null
+++ b/apps/api/tests/reviews.test.ts
@@ -0,0 +1,89 @@
+import { describe, it, expect } from 'vitest';
+import { Hono } from 'hono';
+
+import { reviewsRouter } from '../src/routes/reviews.ts';
+import type { CardsDb } from '../src/db/connection.ts';
+
+function buildApp() {
+	const stub = {
+		select: () => ({
+			from: () => ({
+				innerJoin: () => ({
+					innerJoin: () => ({
+						where: () => ({ limit: () => [] }),
+					}),
+					where: () => ({
+						orderBy: () => ({ limit: () => [] }),
+					}),
+				}),
+				where: () => ({
+					orderBy: () => ({ limit: () => [] }),
+				}),
+			}),
+		}),
+	};
+	const app = new Hono();
+	app.route('/api/v1/reviews', reviewsRouter({ db: stub as unknown as CardsDb }));
+	return { app };
+}
+
+describe('reviewsRouter — auth-gate', () => {
+	it('GET /due ohne X-User-Id ist 401', async () => {
+		const { app } = buildApp();
+		const res = await app.request('/api/v1/reviews/due');
+		expect(res.status).toBe(401);
+	});
+
+	it('POST /grade ohne X-User-Id ist 401', async () => {
+		const { app } = buildApp();
+		const res = await app.request('/api/v1/reviews/c-1/0/grade', {
+			method: 'POST',
+			body: JSON.stringify({ rating: 'good' }),
+		});
+		expect(res.status).toBe(401);
+	});
+});
+
+describe('reviewsRouter — Input-Validation', () => {
+	it('POST mit invalid sub_index ist 422', async () => {
+		const { app } = buildApp();
+		const res = await app.request('/api/v1/reviews/c-1/-1/grade', {
+			method: 'POST',
+			headers: { 'X-User-Id': 'u-1', 'Content-Type': 'application/json' },
+			body: JSON.stringify({ rating: 'good' }),
+		});
+		expect(res.status).toBe(422);
+	});
+
+	it('POST ohne rating ist 422', async () => {
+		const { app } = buildApp();
+		const res = await app.request('/api/v1/reviews/c-1/0/grade', {
+			method: 'POST',
+			headers: { 'X-User-Id': 'u-1', 'Content-Type': 'application/json' },
+			body: '{}',
+		});
+		expect(res.status).toBe(422);
+	});
+
+	it('POST mit unknown rating ist 422', async () => {
+		const { app } = buildApp();
+		const res = await app.request('/api/v1/reviews/c-1/0/grade', {
+			method: 'POST',
+			headers: { 'X-User-Id': 'u-1', 'Content-Type': 'application/json' },
+			body: JSON.stringify({ rating: 'maybe' }),
+		});
+		expect(res.status).toBe(422);
+	});
+
+	it('POST mit gültigem rating erreicht Lookup (404 bei stub)', async () => {
+		const { app } = buildApp();
+		const res = await app.request('/api/v1/reviews/c-1/0/grade', {
+			method: 'POST',
+			headers: { 'X-User-Id': 'u-1', 'Content-Type': 'application/json' },
+			body: JSON.stringify({ rating: 'good' }),
+		});
+		expect(res.status).toBe(404);
+		const body = (await res.json()) as { error: string };
+		expect(body.error).toBe('not_found');
+	});
+});
diff --git a/packages/cards-domain/package.json b/packages/cards-domain/package.json
index c2d8412..7af0d6c 100644
--- a/packages/cards-domain/package.json
+++ b/packages/cards-domain/package.json
@@ -8,9 +8,7 @@
 	"types": "./src/index.ts",
 	"exports": {
 		".": "./src/index.ts",
-		"./types": "./src/types.ts",
 		"./fsrs": "./src/fsrs.ts",
-		"./cloze": "./src/cloze.ts",
 		"./schemas": "./src/schemas/index.ts"
 	},
 	"scripts": {
diff --git a/packages/cards-domain/src/fsrs.ts b/packages/cards-domain/src/fsrs.ts
new file mode 100644
index 0000000..b3186a7
--- /dev/null
+++ b/packages/cards-domain/src/fsrs.ts
@@ -0,0 +1,153 @@
+// FSRS-Adapter über ts-fsrs v5.3.2.
+//
+// Pattern aus mana-monorepo (siehe docs/LESSONS_FROM_MANA_MONOREPO.md §3):
+// - Unsere Reviews speichern ISO-Strings + camelCase-Felder.
+// - ts-fsrs arbeitet mit Date-Objekten + snake_case.
+// - Diese Datei ist die Übersetzungs-Schicht.
+// - Reviews bleiben PLAINTEXT (Scheduler quert täglich `due <= now`).
+
+import {
+	createEmptyCard,
+	default_w,
+	FSRS,
+	generatorParameters,
+	Rating as FsrsRating,
+	State as FsrsState,
+	type Card as FsrsCard,
+	type FSRSParameters,
+} from 'ts-fsrs';
+
+import type { Rating, Review, ReviewState } from './schemas/review.ts';
+import type { FsrsSettings } from './schemas/fsrs-settings.ts';
+
+/** Public Rating ↔ ts-fsrs Rating mapping. */
+const RATING_TO_FSRS: Record<Rating, FsrsRating> = {
+	again: FsrsRating.Again,
+	hard: FsrsRating.Hard,
+	good: FsrsRating.Good,
+	easy: FsrsRating.Easy,
+};
+
+/** ts-fsrs State (numeric) ↔ unser ReviewState (string). */
+const STATE_FROM_FSRS: Record<FsrsState, ReviewState> = {
+	[FsrsState.New]: 'new',
+	[FsrsState.Learning]: 'learning',
+	[FsrsState.Review]: 'review',
+	[FsrsState.Relearning]: 'relearning',
+};
+
+const STATE_TO_FSRS: Record<ReviewState, FsrsState> = {
+	new: FsrsState.New,
+	learning: FsrsState.Learning,
+	review: FsrsState.Review,
+	relearning: FsrsState.Relearning,
+};
+
+/** Baut einen FSRS-Scheduler aus per-Deck-Settings + globalen Defaults. */
+export function buildScheduler(settings: FsrsSettings = {}): FSRS {
+	const params: FSRSParameters = generatorParameters({
+		request_retention: settings.request_retention,
+		maximum_interval: settings.maximum_interval,
+		w: settings.w ?? default_w,
+		enable_fuzz: settings.enable_fuzz ?? true,
+	});
+	return new FSRS(params);
+}
+
+/** Initialer Review-State für eine neue Karte (sub_index). */
+export function newReview(args: {
+	userId: string;
+	cardId: string;
+	subIndex?: number;
+	now?: Date;
+}): Review {
+	const now = args.now ?? new Date();
+	const fc = createEmptyCard(now);
+	return {
+		card_id: args.cardId,
+		sub_index: args.subIndex ?? 0,
+		user_id: args.userId,
+		due: fc.due.toISOString(),
+		stability: fc.stability,
+		difficulty: fc.difficulty,
+		elapsed_days: fc.elapsed_days,
+		scheduled_days: fc.scheduled_days,
+		reps: fc.reps,
+		lapses: fc.lapses,
+		state: STATE_FROM_FSRS[fc.state],
+		last_review: fc.last_review ? fc.last_review.toISOString() : null,
+	};
+}
+
+/** Wendet ein User-Rating an und gibt den nächsten Review-State zurück. */
+export function gradeReview(
+	current: Review,
+	rating: Rating,
+	now?: Date,
+	settings?: FsrsSettings
+): Review {
+	const reviewedAt = now ?? new Date();
+	const scheduler = buildScheduler(settings);
+	const fc = toFsrsCard(current);
+	const log = scheduler.repeat(fc, reviewedAt);
+	const next = log[RATING_TO_FSRS[rating]].card;
+	return fromFsrsCard(current, next);
+}
+
+/** Konvertiert unseren Review-Datensatz in eine ts-fsrs Card. */
+export function toFsrsCard(r: Review): FsrsCard {
+	return {
+		due: new Date(r.due),
+		stability: r.stability,
+		difficulty: r.difficulty,
+		elapsed_days: r.elapsed_days,
+		scheduled_days: r.scheduled_days,
+		reps: r.reps,
+		lapses: r.lapses,
+		state: STATE_TO_FSRS[r.state],
+		last_review: r.last_review ? new Date(r.last_review) : undefined,
+	};
+}
+
+/** Übernimmt FSRS-Result-Card-Felder in unser Review-Objekt. */
+export function fromFsrsCard(prev: Review, fc: FsrsCard): Review {
+	return {
+		...prev,
+		due: fc.due.toISOString(),
+		stability: fc.stability,
+		difficulty: fc.difficulty,
+		elapsed_days: fc.elapsed_days,
+		scheduled_days: fc.scheduled_days,
+		reps: fc.reps,
+		lapses: fc.lapses,
+		state: STATE_FROM_FSRS[fc.state],
+		last_review: fc.last_review ? fc.last_review.toISOString() : null,
+	};
+}
+
+/**
+ * Wie viele Reviews pro Card-Type? Wird beim Card-Insert genutzt,
+ * um die `(card_id, sub_index)`-Reihen zu initialisieren.
+ *
+ * Cloze ist Sonderfall: `subIndex` hängt von der Anzahl der Cluster
+ * im `fields.text` ab; `subIndexCountForCloze(text)` muss separat
+ * gerufen werden, sobald wir Cloze unterstützen.
+ */
+export function subIndexCount(type: string): number {
+	switch (type) {
+		case 'basic':
+			return 1;
+		case 'basic-reverse':
+			return 2;
+		case 'type-in':
+			return 1;
+		case 'image-occlusion':
+			return 1; // pro Mask-Region in Phase 8+ angepasst
+		case 'audio':
+			return 1;
+		case 'multiple-choice':
+			return 1;
+		default:
+			return 1;
+	}
+}
diff --git a/packages/cards-domain/src/index.ts b/packages/cards-domain/src/index.ts
index e2a9367..92f8426 100644
--- a/packages/cards-domain/src/index.ts
+++ b/packages/cards-domain/src/index.ts
@@ -2,7 +2,11 @@
 //
 // Pure-TS, keine DB/Framework-Abhängigkeiten. Wird vom apps/api
 // (Drizzle-Schemas) und apps/web (UI) gleichermaßen konsumiert.
+//
+// Single Source of Truth für Domain-Typen sind die zod-Schemas in
+// `./schemas/`. Alle Types werden via `z.infer<typeof XSchema>`
+// aus den Schemas abgeleitet — kein duplizierter Type-Layer.
 
-export * from './types.ts';
-// export * from './fsrs.ts';   // Phase 3: FSRS-Adapter (ts-fsrs)
+export * from './schemas/index.ts';
+export * from './fsrs.ts';
 // export * from './cloze.ts';  // Phase 8 oder später: Cloze-Parser
diff --git a/packages/cards-domain/src/schemas/card.ts b/packages/cards-domain/src/schemas/card.ts
new file mode 100644
index 0000000..13dc44d
--- /dev/null
+++ b/packages/cards-domain/src/schemas/card.ts
@@ -0,0 +1,94 @@
+import { z } from 'zod';
+
+/**
+ * MVP-CardType-Set. Erweiterung in CARDS_GREENFIELD.md Phase 8+ vorgesehen
+ * (cloze, type-in, image-occlusion, audio, multiple-choice).
+ */
+export const CardTypeSchema = z.enum(['basic', 'basic-reverse']);
+export type CardType = z.infer<typeof CardTypeSchema>;
+
+/** Future-Set für Schema-Migration-Vorbereitung. */
+export const CardTypeFutureSchema = z.enum([
+	'basic',
+	'basic-reverse',
+	'cloze',
+	'type-in',
+	'image-occlusion',
+	'audio',
+	'multiple-choice',
+]);
+
+/**
+ * Generischer Field-Slot. Konkrete Field-Sets pro Type werden runtime
+ * via `validateFieldsForType()` geprüft (siehe unten).
+ */
+export const CardFieldsSchema = z.record(z.string(), z.string());
+export type CardFields = z.infer<typeof CardFieldsSchema>;
+
+/**
+ * Field-Set-Validierung pro CardType. Keine z.discriminatedUnion, weil
+ * der Type-Slot generisch bleibt — neue CardTypes können ohne Schema-
+ * Bruch hinzugefügt werden.
+ */
+export function validateFieldsForType(
+	type: CardType | z.infer<typeof CardTypeFutureSchema>,
+	fields: CardFields
+): { ok: true } | { ok: false; missing: string[] } {
+	const required: Record<string, string[]> = {
+		basic: ['front', 'back'],
+		'basic-reverse': ['front', 'back'],
+		cloze: ['text'],
+		'type-in': ['question', 'expected'],
+		'image-occlusion': ['image_ref', 'mask_regions'],
+		audio: ['audio_ref'],
+		'multiple-choice': ['question', 'options', 'correct_index'],
+	};
+	const need = required[type] ?? [];
+	const missing = need.filter((k) => !(k in fields));
+	return missing.length === 0 ? { ok: true } : { ok: false, missing };
+}
+
+export const CardSchema = z
+	.object({
+		id: z.string().min(1),
+		deck_id: z.string().min(1),
+		user_id: z.string().min(1),
+		type: CardTypeSchema,
+		fields: CardFieldsSchema,
+		media_refs: z.array(z.string()).default([]),
+		content_hash: z.string().optional().nullable(),
+		created_at: z.string().datetime(),
+		updated_at: z.string().datetime(),
+	})
+	.strict();
+export type Card = z.infer<typeof CardSchema>;
+
+export const CardCreateSchema = z
+	.object({
+		deck_id: z.string().min(1),
+		type: CardTypeSchema,
+		fields: CardFieldsSchema,
+		tags: z.array(z.string()).optional(),
+		media_refs: z.array(z.string()).optional(),
+	})
+	.strict()
+	.superRefine((val, ctx) => {
+		const check = validateFieldsForType(val.type, val.fields);
+		if (!check.ok) {
+			ctx.addIssue({
+				code: z.ZodIssueCode.custom,
+				path: ['fields'],
+				message: `missing fields for type=${val.type}: ${check.missing.join(', ')}`,
+			});
+		}
+	});
+export type CardCreate = z.infer<typeof CardCreateSchema>;
+
+export const CardUpdateSchema = z
+	.object({
+		fields: CardFieldsSchema.optional(),
+		tags: z.array(z.string()).optional(),
+		media_refs: z.array(z.string()).optional(),
+	})
+	.strict();
+export type CardUpdate = z.infer<typeof CardUpdateSchema>;
diff --git a/packages/cards-domain/src/schemas/deck.ts b/packages/cards-domain/src/schemas/deck.ts
new file mode 100644
index 0000000..edf8e79
--- /dev/null
+++ b/packages/cards-domain/src/schemas/deck.ts
@@ -0,0 +1,42 @@
+import { z } from 'zod';
+
+import { FsrsSettingsSchema } from './fsrs-settings.ts';
+
+const VisibilitySchema = z.enum(['private', 'space', 'public']);
+
+export const DeckSchema = z
+	.object({
+		id: z.string().min(1),
+		user_id: z.string().min(1),
+		name: z.string().min(1).max(200),
+		description: z.string().max(2000).optional().nullable(),
+		color: z
+			.string()
+			.regex(/^#[0-9a-fA-F]{6}$/)
+			.optional()
+			.nullable(),
+		visibility: VisibilitySchema.default('private'),
+		fsrs_settings: FsrsSettingsSchema.default({}),
+		content_hash: z.string().optional().nullable(),
+		created_at: z.string().datetime(),
+		updated_at: z.string().datetime(),
+	})
+	.strict();
+export type Deck = z.infer<typeof DeckSchema>;
+
+export const DeckCreateSchema = z
+	.object({
+		name: z.string().min(1).max(200),
+		description: z.string().max(2000).optional(),
+		color: z
+			.string()
+			.regex(/^#[0-9a-fA-F]{6}$/)
+			.optional(),
+		visibility: VisibilitySchema.optional(),
+		fsrs_settings: FsrsSettingsSchema.optional(),
+	})
+	.strict();
+export type DeckCreate = z.infer<typeof DeckCreateSchema>;
+
+export const DeckUpdateSchema = DeckCreateSchema.partial();
+export type DeckUpdate = z.infer<typeof DeckUpdateSchema>;
diff --git a/packages/cards-domain/src/schemas/fsrs-settings.ts b/packages/cards-domain/src/schemas/fsrs-settings.ts
new file mode 100644
index 0000000..c19f416
--- /dev/null
+++ b/packages/cards-domain/src/schemas/fsrs-settings.ts
@@ -0,0 +1,15 @@
+import { z } from 'zod';
+
+/**
+ * FSRS-Settings (siehe ts-fsrs `FSRSParameters`).
+ * Alle Felder optional; Defaults werden im Adapter gesetzt.
+ */
+export const FsrsSettingsSchema = z
+	.object({
+		request_retention: z.number().min(0.5).max(0.99).optional(),
+		maximum_interval: z.number().min(1).max(36500).optional(),
+		w: z.array(z.number()).length(19).optional(),
+		enable_fuzz: z.boolean().optional(),
+	})
+	.strict();
+export type FsrsSettings = z.infer<typeof FsrsSettingsSchema>;
diff --git a/packages/cards-domain/src/schemas/index.ts b/packages/cards-domain/src/schemas/index.ts
index af4d78b..4cb954a 100644
--- a/packages/cards-domain/src/schemas/index.ts
+++ b/packages/cards-domain/src/schemas/index.ts
@@ -1,7 +1,17 @@
-// Phase-3-Aufgabe: zod-Schemas für API-Inputs/Outputs.
+// Re-exports für zod-Schemas und JSON-Schema-Generierung.
 //
-// Konvention: ein zod-Schema pro Domain-Type (DeckSchema, CardSchema,
-// ReviewSchema). API-Routen rufen `Schema.parse()` für Input-Validation,
-// und `zod-to-json-schema` generiert die JSON-Schemas für mana-mcp/-share.
+// Verwendung:
+//   - apps/api: Input-Validation via `Schema.parse(body)`
+//   - apps/api: zod-to-json-schema für mana-mcp Tool-Registration
+//   - apps/web: Type-Inference (`type X = z.infer<typeof XSchema>`)
+//
+// Konvention: ein Schema-File pro Domain-Bereich. Public-API-Re-Exports
+// hier; das Mapping `validateFieldsForType` ist eine Pure-Function aus
+// `card.ts`.
 
-export {};
+export * from './fsrs-settings.ts';
+export * from './deck.ts';
+export * from './card.ts';
+export * from './review.ts';
+export * from './study.ts';
+export * from './tools.ts';
diff --git a/packages/cards-domain/src/schemas/review.ts b/packages/cards-domain/src/schemas/review.ts
new file mode 100644
index 0000000..519851c
--- /dev/null
+++ b/packages/cards-domain/src/schemas/review.ts
@@ -0,0 +1,36 @@
+import { z } from 'zod';
+
+export const ReviewStateSchema = z.enum(['new', 'learning', 'review', 'relearning']);
+export type ReviewState = z.infer<typeof ReviewStateSchema>;
+
+/** ts-fsrs Rating-Map: 1=Again, 2=Hard, 3=Good, 4=Easy. */
+export const RatingSchema = z.enum(['again', 'hard', 'good', 'easy']);
+export type Rating = z.infer<typeof RatingSchema>;
+
+export const ReviewSchema = z
+	.object({
+		card_id: z.string().min(1),
+		sub_index: z.number().int().nonnegative(),
+		user_id: z.string().min(1),
+		due: z.string().datetime(),
+		stability: z.number().nonnegative(),
+		difficulty: z.number().min(0).max(10),
+		elapsed_days: z.number().nonnegative().default(0),
+		scheduled_days: z.number().nonnegative().default(0),
+		reps: z.number().int().nonnegative().default(0),
+		lapses: z.number().int().nonnegative().default(0),
+		state: ReviewStateSchema.default('new'),
+		last_review: z.string().datetime().nullable().optional(),
+	})
+	.strict();
+export type Review = z.infer<typeof ReviewSchema>;
+
+export const GradeReviewInputSchema = z
+	.object({
+		card_id: z.string().min(1),
+		sub_index: z.number().int().nonnegative().default(0),
+		rating: RatingSchema,
+		reviewed_at: z.string().datetime().optional(),
+	})
+	.strict();
+export type GradeReviewInput = z.infer<typeof GradeReviewInputSchema>;
diff --git a/packages/cards-domain/src/schemas/study.ts b/packages/cards-domain/src/schemas/study.ts
new file mode 100644
index 0000000..6c15d25
--- /dev/null
+++ b/packages/cards-domain/src/schemas/study.ts
@@ -0,0 +1,30 @@
+import { z } from 'zod';
+
+export const StudySessionSchema = z
+	.object({
+		id: z.string().min(1),
+		user_id: z.string().min(1),
+		deck_id: z.string().min(1),
+		started_at: z.string().datetime(),
+		finished_at: z.string().datetime().nullable(),
+		cards_reviewed: z.number().int().nonnegative().default(0),
+		cards_correct: z.number().int().nonnegative().default(0),
+	})
+	.strict();
+export type StudySession = z.infer<typeof StudySessionSchema>;
+
+export const StartStudySessionInputSchema = z
+	.object({
+		deck_id: z.string().min(1),
+	})
+	.strict();
+export type StartStudySessionInput = z.infer<typeof StartStudySessionInputSchema>;
+
+export const FinishStudySessionInputSchema = z
+	.object({
+		session_id: z.string().min(1),
+		cards_reviewed: z.number().int().nonnegative(),
+		cards_correct: z.number().int().nonnegative(),
+	})
+	.strict();
+export type FinishStudySessionInput = z.infer<typeof FinishStudySessionInputSchema>;
diff --git a/packages/cards-domain/src/schemas/tools.ts b/packages/cards-domain/src/schemas/tools.ts
new file mode 100644
index 0000000..6e79ed9
--- /dev/null
+++ b/packages/cards-domain/src/schemas/tools.ts
@@ -0,0 +1,45 @@
+import { z } from 'zod';
+
+import { CardSchema, CardCreateSchema } from './card.ts';
+
+/**
+ * Schemas für die im app-manifest.json deklarierten AI-Tools.
+ * Werden via zod-to-json-schema in JSON-Schema konvertiert und an
+ * mana-mcp ausgeliefert (Phase 7).
+ */
+
+export const CardsCreateInputSchema = CardCreateSchema;
+export type CardsCreateInput = z.infer<typeof CardsCreateInputSchema>;
+
+export const CardsCreateOutputSchema = CardSchema;
+export type CardsCreateOutput = z.infer<typeof CardsCreateOutputSchema>;
+
+export const CardsSearchInputSchema = z
+	.object({
+		query: z.string().min(1),
+		max_results: z.number().int().min(1).max(100).optional(),
+	})
+	.strict();
+export type CardsSearchInput = z.infer<typeof CardsSearchInputSchema>;
+
+export const CardsSearchHitSchema = z
+	.object({
+		id: z.string(),
+		type: z.literal('card'),
+		title: z.string(),
+		snippet: z.string().optional(),
+		link: z.string().url(),
+		score: z.number().min(0).max(1),
+	})
+	.strict();
+export type CardsSearchHit = z.infer<typeof CardsSearchHitSchema>;
+
+export const CardsSearchOutputSchema = z
+	.object({
+		query: z.string(),
+		results: z.array(CardsSearchHitSchema),
+		total: z.number().int().nonnegative(),
+		took_ms: z.number().nonnegative(),
+	})
+	.strict();
+export type CardsSearchOutput = z.infer<typeof CardsSearchOutputSchema>;
diff --git a/packages/cards-domain/src/types.ts b/packages/cards-domain/src/types.ts
deleted file mode 100644
index 6d5d4b2..0000000
--- a/packages/cards-domain/src/types.ts
+++ /dev/null
@@ -1,87 +0,0 @@
-// Domain-Typen für Cards.
-//
-// Modellierung folgt den Lessons aus mana-monorepo
-// (siehe docs/LESSONS_FROM_MANA_MONOREPO.md):
-//
-// - CardType ist eine discriminated union.
-// - Card hat `fields: Record<string, string>` als generischen Slot.
-// - Pro Karte gibt es N Reviews mit `subIndex` (basic = 1, basic-reverse = 2,
-//   cloze = 1 pro Cluster).
-// - cardReviews bleiben PLAINTEXT, weil der Scheduler täglich auf `due`
-//   filtert.
-
-/** Phase-1-MVP-Set; Cloze + Image-Occlusion in Phase 8+. */
-export type CardType = 'basic' | 'basic-reverse';
-
-/** Voll geplantes Set (für Schemas vorbereitet, MVP nicht alle implementiert). */
-export type CardTypeFuture =
-	| 'basic'
-	| 'basic-reverse'
-	| 'cloze'
-	| 'type-in'
-	| 'image-occlusion'
-	| 'audio'
-	| 'multiple-choice';
-
-export type CardFields = Record<string, string>;
-
-export type Deck = {
-	id: string;
-	user_id: string;
-	name: string;
-	description?: string;
-	color?: string;
-	visibility: 'private' | 'space' | 'public';
-	fsrs_settings: FsrsSettings;
-	created_at: string;
-	updated_at: string;
-};
-
-export type Card = {
-	id: string;
-	deck_id: string;
-	user_id: string;
-	type: CardType;
-	fields: CardFields;
-	tags: string[];
-	media_refs: string[];
-	created_at: string;
-	updated_at: string;
-};
-
-/**
- * Pro `(card_id, sub_index)` ein Review-Eintrag. Der Scheduler quert auf
- * `due <= now` täglich — das Feld bleibt deshalb plaintext und ist indiziert.
- */
-export type Review = {
-	card_id: string;
-	sub_index: number;
-	user_id: string;
-	due: string;
-	stability: number;
-	difficulty: number;
-	elapsed_days: number;
-	scheduled_days: number;
-	reps: number;
-	lapses: number;
-	state: 'new' | 'learning' | 'review' | 'relearning';
-	last_review: string | null;
-};
-
-export type StudySession = {
-	id: string;
-	user_id: string;
-	deck_id: string;
-	started_at: string;
-	finished_at: string | null;
-	cards_reviewed: number;
-	cards_correct: number;
-};
-
-/** Default-Konstanten aus ts-fsrs; per-Deck-Overrides möglich. */
-export type FsrsSettings = {
-	requestRetention?: number;
-	maximumInterval?: number;
-	w?: number[];
-	enableFuzz?: boolean;
-};
diff --git a/packages/cards-domain/tests/fsrs.test.ts b/packages/cards-domain/tests/fsrs.test.ts
new file mode 100644
index 0000000..43cb783
--- /dev/null
+++ b/packages/cards-domain/tests/fsrs.test.ts
@@ -0,0 +1,96 @@
+import { describe, it, expect } from 'vitest';
+
+import {
+	buildScheduler,
+	fromFsrsCard,
+	gradeReview,
+	newReview,
+	subIndexCount,
+	toFsrsCard,
+} from '../src/fsrs.ts';
+
+describe('newReview', () => {
+	it('initializes with state=new and reps=0', () => {
+		const r = newReview({
+			userId: 'u-1',
+			cardId: 'c-1',
+			now: new Date('2026-05-08T10:00:00Z'),
+		});
+		expect(r.card_id).toBe('c-1');
+		expect(r.sub_index).toBe(0);
+		expect(r.user_id).toBe('u-1');
+		expect(r.state).toBe('new');
+		expect(r.reps).toBe(0);
+		expect(r.lapses).toBe(0);
+		expect(r.due).toBe('2026-05-08T10:00:00.000Z');
+	});
+
+	it('honours provided sub_index', () => {
+		const r = newReview({ userId: 'u-1', cardId: 'c-1', subIndex: 2 });
+		expect(r.sub_index).toBe(2);
+	});
+});
+
+describe('gradeReview', () => {
+	const fixedNow = new Date('2026-05-08T10:00:00Z');
+	const reviewedAt = new Date('2026-05-08T10:01:00Z');
+	const baseReview = newReview({
+		userId: 'u-1',
+		cardId: 'c-1',
+		now: fixedNow,
+	});
+
+	it('Again from new keeps reps=0 increments lapses', () => {
+		// Disable fuzz for deterministic test outputs
+		const next = gradeReview(baseReview, 'again', reviewedAt, { enable_fuzz: false });
+		expect(next.state).not.toBe('new');
+		expect(next.due).not.toBe(baseReview.due);
+		expect(next.reps).toBeGreaterThanOrEqual(1);
+	});
+
+	it('Easy from new transitions to a future-dated review', () => {
+		const next = gradeReview(baseReview, 'easy', reviewedAt, { enable_fuzz: false });
+		expect(new Date(next.due).getTime()).toBeGreaterThan(reviewedAt.getTime());
+	});
+
+	it('preserves card_id, sub_index, user_id', () => {
+		const next = gradeReview(baseReview, 'good', reviewedAt, { enable_fuzz: false });
+		expect(next.card_id).toBe(baseReview.card_id);
+		expect(next.sub_index).toBe(baseReview.sub_index);
+		expect(next.user_id).toBe(baseReview.user_id);
+	});
+});
+
+describe('toFsrsCard / fromFsrsCard roundtrip', () => {
+	it('roundtrips a new review without loss', () => {
+		const r = newReview({ userId: 'u-1', cardId: 'c-1' });
+		const fc = toFsrsCard(r);
+		const back = fromFsrsCard(r, fc);
+		expect(back.due).toBe(r.due);
+		expect(back.stability).toBe(r.stability);
+		expect(back.state).toBe(r.state);
+	});
+});
+
+describe('subIndexCount', () => {
+	it('basic = 1, basic-reverse = 2', () => {
+		expect(subIndexCount('basic')).toBe(1);
+		expect(subIndexCount('basic-reverse')).toBe(2);
+	});
+
+	it('unknown type defaults to 1', () => {
+		expect(subIndexCount('unknown-future-type')).toBe(1);
+	});
+});
+
+describe('buildScheduler', () => {
+	it('builds with defaults', () => {
+		const s = buildScheduler();
+		expect(s).toBeDefined();
+	});
+
+	it('honours per-deck overrides', () => {
+		const s = buildScheduler({ request_retention: 0.85, enable_fuzz: false });
+		expect(s).toBeDefined();
+	});
+});
diff --git a/packages/cards-domain/tests/schemas.test.ts b/packages/cards-domain/tests/schemas.test.ts
new file mode 100644
index 0000000..9610770
--- /dev/null
+++ b/packages/cards-domain/tests/schemas.test.ts
@@ -0,0 +1,154 @@
+import { describe, it, expect } from 'vitest';
+
+import {
+	CardCreateSchema,
+	CardSchema,
+	CardTypeSchema,
+	DeckCreateSchema,
+	DeckSchema,
+	GradeReviewInputSchema,
+	validateFieldsForType,
+} from '../src/schemas/index.ts';
+
+describe('CardTypeSchema', () => {
+	it('accepts MVP types', () => {
+		expect(() => CardTypeSchema.parse('basic')).not.toThrow();
+		expect(() => CardTypeSchema.parse('basic-reverse')).not.toThrow();
+	});
+
+	it('rejects future types in MVP schema', () => {
+		expect(() => CardTypeSchema.parse('cloze')).toThrow();
+	});
+});
+
+describe('validateFieldsForType', () => {
+	it('basic requires front + back', () => {
+		expect(validateFieldsForType('basic', { front: 'q', back: 'a' })).toEqual({ ok: true });
+		expect(validateFieldsForType('basic', { front: 'q' })).toEqual({
+			ok: false,
+			missing: ['back'],
+		});
+	});
+
+	it('cloze requires text', () => {
+		expect(validateFieldsForType('cloze', { text: 'x' })).toEqual({ ok: true });
+		expect(validateFieldsForType('cloze', {})).toEqual({ ok: false, missing: ['text'] });
+	});
+});
+
+describe('CardCreateSchema', () => {
+	it('accepts a basic card', () => {
+		const r = CardCreateSchema.safeParse({
+			deck_id: 'd-1',
+			type: 'basic',
+			fields: { front: 'Q', back: 'A' },
+		});
+		expect(r.success).toBe(true);
+	});
+
+	it('rejects basic card without back', () => {
+		const r = CardCreateSchema.safeParse({
+			deck_id: 'd-1',
+			type: 'basic',
+			fields: { front: 'Q' },
+		});
+		expect(r.success).toBe(false);
+	});
+
+	it('rejects unknown type via CardTypeSchema', () => {
+		const r = CardCreateSchema.safeParse({
+			deck_id: 'd-1',
+			type: 'cloze',
+			fields: { text: 'x' },
+		});
+		expect(r.success).toBe(false);
+	});
+
+	it('rejects extra fields (strict)', () => {
+		const r = CardCreateSchema.safeParse({
+			deck_id: 'd-1',
+			type: 'basic',
+			fields: { front: 'Q', back: 'A' },
+			malicious: 'inject',
+		});
+		expect(r.success).toBe(false);
+	});
+});
+
+describe('DeckCreateSchema', () => {
+	it('minimal valid deck', () => {
+		const r = DeckCreateSchema.safeParse({ name: 'My Deck' });
+		expect(r.success).toBe(true);
+	});
+
+	it('rejects empty name', () => {
+		const r = DeckCreateSchema.safeParse({ name: '' });
+		expect(r.success).toBe(false);
+	});
+
+	it('rejects invalid color', () => {
+		const r = DeckCreateSchema.safeParse({ name: 'X', color: 'red' });
+		expect(r.success).toBe(false);
+	});
+
+	it('accepts hex color', () => {
+		const r = DeckCreateSchema.safeParse({ name: 'X', color: '#ff8800' });
+		expect(r.success).toBe(true);
+	});
+});
+
+describe('GradeReviewInputSchema', () => {
+	it('accepts a grade input', () => {
+		const r = GradeReviewInputSchema.safeParse({
+			card_id: 'c-1',
+			sub_index: 0,
+			rating: 'good',
+		});
+		expect(r.success).toBe(true);
+	});
+
+	it('rejects unknown rating', () => {
+		const r = GradeReviewInputSchema.safeParse({
+			card_id: 'c-1',
+			sub_index: 0,
+			rating: 'perfect',
+		});
+		expect(r.success).toBe(false);
+	});
+
+	it('defaults sub_index to 0', () => {
+		const r = GradeReviewInputSchema.parse({ card_id: 'c-1', rating: 'good' });
+		expect(r.sub_index).toBe(0);
+	});
+});
+
+describe('strict variants reject extras', () => {
+	it('DeckSchema rejects extra props', () => {
+		const r = DeckSchema.safeParse({
+			id: 'd-1',
+			user_id: 'u-1',
+			name: 'D',
+			visibility: 'private',
+			fsrs_settings: {},
+			created_at: '2026-05-08T10:00:00.000Z',
+			updated_at: '2026-05-08T10:00:00.000Z',
+			leaks: 'no',
+		});
+		expect(r.success).toBe(false);
+	});
+
+	it('CardSchema rejects extra props', () => {
+		const r = CardSchema.safeParse({
+			id: 'c-1',
+			deck_id: 'd-1',
+			user_id: 'u-1',
+			type: 'basic',
+			fields: { front: 'Q', back: 'A' },
+			media_refs: [],
+			created_at: '2026-05-08T10:00:00.000Z',
+			updated_at: '2026-05-08T10:00:00.000Z',
+			leaked: 'yes',
+		});
+		expect(r.success).toBe(false);
+	});
+});