diff --git a/packages/backend/src/services/fork.test.ts b/packages/backend/src/services/fork.test.ts
new file mode 100644
index 0000000..4fe2611
--- /dev/null
+++ b/packages/backend/src/services/fork.test.ts
@@ -0,0 +1,59 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import { eq } from 'drizzle-orm';
+import { makeTestDb, createUserDirect, createLessonOwnedBy } from '../tests/dbHelper.js';
+import { createCard, listCardsByLesson } from './cards.js';
+import { lessons } from '../db/schema.js';
+import { forkLesson } from './fork.js';
+
+let env: ReturnType<typeof makeTestDb>;
+beforeEach(() => { env = makeTestDb(); });
+
+describe('fork', () => {
+  it('forks a single shared lesson with cards', async () => {
+    const o = await createUserDirect(env.db, { email: 'o@example.com' });
+    const u = await createUserDirect(env.db, { email: 'u@example.com' });
+    const l = await createLessonOwnedBy(env.db, o.id, { name: 'L', visibility: 'shared' });
+    await createCard(env.db, o.id, l.id, { question: 'q1', answer: 'a1' });
+    await createCard(env.db, o.id, l.id, { question: 'q2', answer: 'a2' });
+
+    const fork = await forkLesson(env.db, u.id, l.id);
+    expect(fork.ownerId).toBe(u.id);
+    expect(fork.visibility).toBe('private');
+    expect(fork.sourceLessonId).toBe(l.id);
+    expect(fork.parentId).toBeNull();
+    const forkCards = await listCardsByLesson(env.db, u.id, fork.id);
+    expect(forkCards).toHaveLength(2);
+  });
+
+  it('forks the whole subtree and rewrites parent_id', async () => {
+    const o = await createUserDirect(env.db, { email: 'o@example.com' });
+    const u = await createUserDirect(env.db, { email: 'u@example.com' });
+    const root = await createLessonOwnedBy(env.db, o.id, { name: 'R', visibility: 'shared' });
+    const child = await createLessonOwnedBy(env.db, o.id, { name: 'C', parentId: root.id });
+    await createCard(env.db, o.id, child.id, { question: 'qc', answer: 'ac' });
+
+    const fork = await forkLesson(env.db, u.id, root.id);
+    const allUser = env.db.select().from(lessons).where(eq(lessons.ownerId, u.id)).all();
+    expect(allUser).toHaveLength(2);
+    const forkChild = allUser.find((x) => x.id !== fork.id)!;
+    expect(forkChild.parentId).toBe(fork.id);
+    expect(forkChild.name).toBe('C');
+    const childCards = await listCardsByLesson(env.db, u.id, forkChild.id);
+    expect(childCards).toHaveLength(1);
+  });
+
+  it('rejects forking a private lesson owned by someone else', async () => {
+    const o = await createUserDirect(env.db, { email: 'o@example.com' });
+    const u = await createUserDirect(env.db, { email: 'u@example.com' });
+    const l = await createLessonOwnedBy(env.db, o.id, { name: 'L', visibility: 'private' });
+    await expect(forkLesson(env.db, u.id, l.id)).rejects.toThrow(/forbid|private|cannot/i);
+  });
+
+  it('allows forking own lesson', async () => {
+    const u = await createUserDirect(env.db, { email: 'u@example.com' });
+    const l = await createLessonOwnedBy(env.db, u.id, { name: 'L', visibility: 'private' });
+    const fork = await forkLesson(env.db, u.id, l.id);
+    expect(fork.ownerId).toBe(u.id);
+    expect(fork.sourceLessonId).toBe(l.id);
+  });
+});
diff --git a/packages/backend/src/services/fork.ts b/packages/backend/src/services/fork.ts
new file mode 100644
index 0000000..8c4c4ea
--- /dev/null
+++ b/packages/backend/src/services/fork.ts
@@ -0,0 +1,84 @@
+import { eq, inArray } from 'drizzle-orm';
+import type { Db } from '../db/client.js';
+import { cards, lessons } from '../db/schema.js';
+import { getDescendantLessonIds } from './lessons.js';
+import { ApiError } from '../lib/errors.js';
+import type { Lesson } from '@flashcard/shared';
+
+function rowToLesson(r: typeof lessons.$inferSelect): Lesson {
+  return {
+    id: r.id,
+    parentId: r.parentId ?? null,
+    name: r.name,
+    description: r.description ?? null,
+    position: r.position,
+    bidirectional: r.bidirectional,
+    ownerId: r.ownerId ?? null,
+    visibility: r.visibility,
+    isCurated: r.isCurated,
+    sourceLessonId: r.sourceLessonId ?? null,
+    createdAt: r.createdAt,
+    updatedAt: r.updatedAt,
+  };
+}
+
+export async function forkLesson(db: Db, userId: number, lessonId: number): Promise<Lesson> {
+  const source = db.select().from(lessons).where(eq(lessons.id, lessonId)).get();
+  if (!source) throw ApiError.notFound('Lesson');
+  const isOwner = source.ownerId === userId;
+  if (!isOwner && source.visibility !== 'shared') {
+    throw new ApiError(403, 'FORBIDDEN_LESSON', 'Cannot fork a private lesson you do not own');
+  }
+
+  const descendantIds = await getDescendantLessonIds(db, lessonId);
+  const sourceLessons = db.select().from(lessons).where(inArray(lessons.id, descendantIds)).all();
+  const sourceCards = db.select().from(cards).where(inArray(cards.lessonId, descendantIds)).all();
+
+  // Sort by depth (parents before children).
+  const idSet = new Set(descendantIds);
+  function depth(l: typeof sourceLessons[number]): number {
+    let d = 0;
+    let cur: number | null = l.parentId ?? null;
+    while (cur !== null && idSet.has(cur)) {
+      d += 1;
+      const next = sourceLessons.find((x) => x.id === cur);
+      cur = next?.parentId ?? null;
+    }
+    return d;
+  }
+  const ordered = [...sourceLessons].sort((a, b) => depth(a) - depth(b));
+
+  const idMap = new Map<number, number>();
+  let newRoot: typeof sourceLessons[number] | null = null;
+  for (const L of ordered) {
+    const newParent = L.parentId !== null && idMap.has(L.parentId) ? idMap.get(L.parentId)! : null;
+    const [inserted] = db.insert(lessons).values({
+      name: L.name,
+      description: L.description ?? null,
+      position: L.position,
+      bidirectional: L.bidirectional,
+      parentId: newParent,
+      ownerId: userId,
+      visibility: 'private',
+      isCurated: false,
+      sourceLessonId: L.id,
+    }).returning().all();
+    idMap.set(L.id, inserted!.id);
+    if (L.id === source.id) newRoot = inserted!;
+  }
+
+  for (const C of sourceCards) {
+    const newLessonId = idMap.get(C.lessonId);
+    if (!newLessonId) continue;
+    db.insert(cards).values({
+      lessonId: newLessonId,
+      question: C.question,
+      answer: C.answer,
+      hint: C.hint ?? null,
+      position: C.position,
+    }).run();
+  }
+
+  if (!newRoot) throw new ApiError(500, 'INTERNAL', 'Fork failed: root not produced');
+  return rowToLesson(newRoot);
+}