diff --git a/packages/backend/src/routes/sessions.ts b/packages/backend/src/routes/sessions.ts
index aca2345..90320fa 100644
--- a/packages/backend/src/routes/sessions.ts
+++ b/packages/backend/src/routes/sessions.ts
@@ -3,7 +3,7 @@ import { attemptCreateSchema, sessionStartSchema } from '@flashcard/shared';
 import type { Db } from '../db/client.js';
 import {
   abandonSession, endSession, getActiveSession, getNextItem, getSessionState,
-  recordAttempt, startSession,
+  recordAttempt, startSession, startDueSession,
 } from '../services/sessions.js';
 import { ApiError } from '../lib/errors.js';
 
@@ -17,6 +17,12 @@ export function sessionsRouter(db: Db): Router {
     } catch (e) { next(e); }
   });
 
+  r.post('/due', async (req, res, next) => {
+    try {
+      res.status(201).json(await startDueSession(db, req.user!.id));
+    } catch (e) { next(e); }
+  });
+
   r.get('/active', async (req, res, next) => {
     try { res.json(await getActiveSession(db, req.user!.id)); } catch (e) { next(e); }
   });
diff --git a/packages/backend/src/services/sessions.test.ts b/packages/backend/src/services/sessions.test.ts
index ac1ffe1..a8a4b69 100644
--- a/packages/backend/src/services/sessions.test.ts
+++ b/packages/backend/src/services/sessions.test.ts
@@ -2,7 +2,8 @@ import { describe, it, expect, beforeEach } from 'vitest';
 import { makeTestDb, createUserDirect } from '../tests/dbHelper.js';
 import { createLesson } from './lessons.js';
 import { createCard } from './cards.js';
-import { startSession, getNextItem, recordAttempt, endSession, getActiveSession } from './sessions.js';
+import { startSession, getNextItem, recordAttempt, endSession, getActiveSession, startDueSession } from './sessions.js';
+import { cardProgress } from '../db/schema.js';
 
 let env: ReturnType<typeof makeTestDb>;
 let owner: Awaited<ReturnType<typeof createUserDirect>>;
@@ -88,3 +89,31 @@ describe('session engine', () => {
     expect(after).toBeNull();
   });
 });
+
+describe('startDueSession', () => {
+  it('builds a session over all due cards of readable lessons', async () => {
+    const u = await createUserDirect(env.db, { email: 'u@example.com' });
+    const l = await createLesson(env.db, u.id, { name: 'L' });
+    const c1 = await createCard(env.db, u.id, l.id, { question: 'q1', answer: 'a' });
+    const c2 = await createCard(env.db, u.id, l.id, { question: 'q2', answer: 'a' });
+    const c3 = await createCard(env.db, u.id, l.id, { question: 'q3', answer: 'a' });
+
+    const now = Math.floor(Date.now() / 1000);
+    env.db.insert(cardProgress).values([
+      { cardId: c1.id, direction: 'forward', userId: u.id, box: 1, nextDueAt: 0 },
+      { cardId: c2.id, direction: 'forward', userId: u.id, box: 1, nextDueAt: now - 1 },
+      { cardId: c3.id, direction: 'forward', userId: u.id, box: 1, nextDueAt: now + 86400 },
+    ]).run();
+
+    const s = await startDueSession(env.db, u.id);
+    expect(s.queue).toHaveLength(2);
+    expect(s.queue.map((q) => q.cardId).sort()).toEqual([c1.id, c2.id].sort());
+  });
+
+  it('returns empty queue when nothing is due', async () => {
+    const u = await createUserDirect(env.db, { email: 'u@example.com' });
+    await createLesson(env.db, u.id, { name: 'L' });
+    const s = await startDueSession(env.db, u.id);
+    expect(s.queue).toHaveLength(0);
+  });
+});
diff --git a/packages/backend/src/services/sessions.ts b/packages/backend/src/services/sessions.ts
index c9d23c7..5395ae0 100644
--- a/packages/backend/src/services/sessions.ts
+++ b/packages/backend/src/services/sessions.ts
@@ -1,6 +1,6 @@
 import { and, eq, inArray, sql } from 'drizzle-orm';
 import type { Db } from '../db/client.js';
-import { attempts, cardProgress, cards, lessons, sessions } from '../db/schema.js';
+import { attempts, cardProgress, cards, lessons, lessonSubscriptions, sessions } from '../db/schema.js';
 import { ApiError } from '../lib/errors.js';
 import type {
   AttemptCreateInput, QueueItem, SessionRow, SessionStartInput,
@@ -113,6 +113,76 @@ export async function startSession(
   return { session: rowToSession(row!), queue };
 }
 
+export async function startDueSession(db: Db, userId: number): Promise<StartedSession> {
+  const now = nowSec();
+
+  const ownerLessons = db.select({ id: lessons.id }).from(lessons).where(eq(lessons.ownerId, userId)).all();
+  const subRoots = db.select({ id: lessonSubscriptions.lessonId }).from(lessonSubscriptions)
+    .where(eq(lessonSubscriptions.userId, userId)).all();
+  const curatedRoots = db.select({ id: lessons.id }).from(lessons)
+    .where(and(eq(lessons.visibility, 'shared'), eq(lessons.isCurated, true))).all();
+
+  const allLessons = db.select({ id: lessons.id, parentId: lessons.parentId }).from(lessons).all();
+  const byParent = new Map<number | null, typeof allLessons>();
+  for (const l of allLessons) {
+    const k = l.parentId ?? null;
+    if (!byParent.has(k)) byParent.set(k, []);
+    byParent.get(k)!.push(l);
+  }
+  const readableIds = new Set<number>();
+  for (const l of ownerLessons) readableIds.add(l.id);
+  const stack: number[] = [];
+  for (const r of subRoots) stack.push(r.id);
+  for (const r of curatedRoots) stack.push(r.id);
+  while (stack.length) {
+    const cur = stack.pop()!;
+    if (readableIds.has(cur)) continue;
+    readableIds.add(cur);
+    for (const c of byParent.get(cur) ?? []) stack.push(c.id);
+  }
+
+  if (readableIds.size === 0) return createEmptyDueSession(db, userId);
+
+  const cardRows = db.select({ id: cards.id, lessonId: cards.lessonId }).from(cards)
+    .where(inArray(cards.lessonId, Array.from(readableIds))).all();
+  const cardIds = cardRows.map((r) => r.id);
+  if (cardIds.length === 0) return createEmptyDueSession(db, userId);
+
+  const due = db.select({ cardId: cardProgress.cardId, direction: cardProgress.direction, box: cardProgress.box })
+    .from(cardProgress)
+    .where(and(
+      eq(cardProgress.userId, userId),
+      inArray(cardProgress.cardId, cardIds),
+      sql`${cardProgress.nextDueAt} <= ${now}`,
+    ))
+    .all();
+
+  due.sort((a, b) => a.box - b.box);
+  const queue: QueueItem[] = due.map((d) => ({ cardId: d.cardId, direction: d.direction }));
+
+  const sessionLessonId = cardRows[0]?.lessonId ?? ownerLessons[0]?.id ?? Array.from(readableIds)[0]!;
+  const [row] = db.insert(sessions).values({
+    lessonId: sessionLessonId,
+    userId,
+    queueSnapshot: JSON.stringify({ remaining: queue, index: 0 }),
+  }).returning().all();
+  return { session: rowToSession(row!), queue };
+}
+
+function createEmptyDueSession(db: Db, userId: number): StartedSession {
+  const anyLesson = db.select({ id: lessons.id }).from(lessons).where(eq(lessons.ownerId, userId)).get()
+    ?? db.select({ id: lessons.id }).from(lessons).get();
+  if (!anyLesson) {
+    throw new ApiError(409, 'NO_LESSONS', 'No lessons exist for this user yet');
+  }
+  const [row] = db.insert(sessions).values({
+    lessonId: anyLesson.id,
+    userId,
+    queueSnapshot: JSON.stringify({ remaining: [], index: 0 }),
+  }).returning().all();
+  return { session: rowToSession(row!), queue: [] };
+}
+
 function rowToSession(r: typeof sessions.$inferSelect): SessionRow {
   return {
     id: r.id,