diff --git a/src/lib/moodle-sync.test.ts b/src/lib/moodle-sync.test.ts
new file mode 100644
index 0000000..1272912
--- /dev/null
+++ b/src/lib/moodle-sync.test.ts
@@ -0,0 +1,42 @@
+import { describe, it, expect } from 'vitest';
+import { modulesToSync } from './moodle-sync';
+import type { ReadMark } from './grist';
+
+let nextId = 100;
+const mark = (module_n: number, chapitre_n: number, moodle_synced = false): ReadMark => ({
+  id: nextId++, course_slug: 'kicad-makers', module_n, chapitre_n, moodle_synced,
+});
+const mod = (id: number, chapterCount: number) => ({
+  id,
+  chapters: Array.from({ length: chapterCount }, (_, i) => ({ title: `c${i + 1}`, fileUrl: '' })),
+});
+
+describe('modulesToSync', () => {
+  it('includes a complete module with unsynced rows', () => {
+    const reads = [mark(1, 1), mark(1, 2)];
+    const out = modulesToSync(reads, [mod(27, 2)]);
+    expect(out).toHaveLength(1);
+    expect(out[0].mi).toBe(0);
+    expect(out[0].cmid).toBe(27);
+    expect(out[0].rowIds.sort()).toEqual(reads.map((r) => r.id).sort());
+  });
+
+  it('excludes a complete module whose rows are all synced', () => {
+    expect(modulesToSync([mark(1, 1, true), mark(1, 2, true)], [mod(27, 2)])).toEqual([]);
+  });
+
+  it('excludes an incomplete module even with unsynced rows', () => {
+    expect(modulesToSync([mark(1, 1)], [mod(27, 2)])).toEqual([]);
+  });
+
+  it('handles multiple modules independently', () => {
+    const reads = [
+      mark(1, 1, true), mark(1, 2, true), // M1 complete, all synced -> out
+      mark(2, 1), mark(2, 2),             // M2 complete, unsynced -> in
+      mark(3, 1),                         // M3 incomplete -> out
+    ];
+    const out = modulesToSync(reads, [mod(10, 2), mod(20, 2), mod(30, 2)]);
+    expect(out.map((t) => t.cmid)).toEqual([20]);
+    expect(out[0].mi).toBe(1);
+  });
+});
diff --git a/src/lib/moodle-sync.ts b/src/lib/moodle-sync.ts
index 4858c54..ea445a4 100644
--- a/src/lib/moodle-sync.ts
+++ b/src/lib/moodle-sync.ts
@@ -1,5 +1,104 @@
-/** Best-effort Moodle completion sync — STUB, implemented in SP2 Task 4. */
+import { randomBytes } from 'node:crypto';
+import { wsCall, getCourseBooks, type BookModule } from './moodle';
+import { getReads, markSynced, isBookComplete, type ReadMark } from './grist';
+import { bySlug } from '../config/courses';
+
+/** Token of the dedicated `formations_sync` WS service (wssync user).
+ *  Allowed functions ONLY: core_user_get_users_by_field, core_user_create_users,
+ *  core_user_update_users, enrol_manual_enrol_users, core_course_get_contents,
+ *  core_completion_override_activity_completion_status. */
+const TOKEN = () => process.env.MOODLE_SYNC_TOKEN || '';
+
+const sync = (fn: string, params: Record<string, string>) => wsCall(TOKEN(), fn, params);
+
+const userIdCache = new Map<string, number>(); // email -> moodle user id
+const enrolCache = new Set<string>(); // `${userId}:${courseId}`
+
+export async function ensureMoodleUser(email: string, name: string): Promise<number> {
+  const cached = userIdCache.get(email);
+  if (cached !== undefined) return cached;
+
+  const found = (await sync('core_user_get_users_by_field', {
+    field: 'email', 'values[0]': email,
+  })) as { id: number }[];
+  if (found.length > 0) {
+    userIdCache.set(email, found[0].id);
+    return found[0].id;
+  }
+
+  const local = (email.split('@')[0] ?? '').toLowerCase().replace(/[^a-z0-9._-]/g, '') || 'apprenant';
+  const username = `${local}-${randomBytes(3).toString('hex')}`;
+  const parts = name.trim().split(/\s+/).filter(Boolean);
+  const firstname = parts[0] || 'Apprenant';
+  const lastname = parts.slice(1).join(' ') || 'X';
+  const password = `${randomBytes(12).toString('base64url')}aA1!`; // random + policy classes
+  const created = (await sync('core_user_create_users', {
+    'users[0][username]': username,
+    'users[0][password]': password,
+    'users[0][firstname]': firstname,
+    'users[0][lastname]': lastname,
+    'users[0][email]': email,
+    'users[0][auth]': 'manual',
+  })) as { id: number }[];
+  userIdCache.set(email, created[0].id);
+  return created[0].id;
+}
+
+export async function ensureEnrolled(userId: number, courseId: number): Promise<void> {
+  const key = `${userId}:${courseId}`;
+  if (enrolCache.has(key)) return;
+  await sync('enrol_manual_enrol_users', {
+    'enrolments[0][roleid]': '5', // student
+    'enrolments[0][userid]': String(userId),
+    'enrolments[0][courseid]': String(courseId),
+  });
+  enrolCache.add(key);
+}
+
+export interface SyncTarget { mi: number; cmid: number; rowIds: number[]; }
+
+/** Pure decision logic: modules fully read whose Grist rows aren't all synced.
+ *  `BookModule.id` IS the cmid for completion override. */
+export function modulesToSync(
+  reads: ReadMark[],
+  modules: Pick<BookModule, 'id' | 'chapters'>[],
+): SyncTarget[] {
+  const out: SyncTarget[] = [];
+  modules.forEach((mod, mi) => {
+    const rows = reads.filter((r) => r.module_n === mi + 1);
+    if (!rows.some((r) => !r.moodle_synced)) return; // nothing new to sync
+    if (!isBookComplete(reads, mi + 1, mod.chapters.length)) return;
+    out.push({ mi, cmid: mod.id, rowIds: rows.map((r) => r.id) });
+  });
+  return out;
+}
+
+/** Best-effort: mirror fully-read books into Moodle completion. Never throws. */
 export async function syncToMoodle(
-  _u: { sub: string; email: string; name: string },
-  _slug: string,
-): Promise<void> {}
+  user: { sub: string; email: string; name: string },
+  courseSlug: string,
+): Promise<void> {
+  try {
+    const course = bySlug(courseSlug);
+    if (!course) return;
+    const [reads, modules] = await Promise.all([
+      getReads(user.sub, courseSlug),
+      getCourseBooks(course.id),
+    ]);
+    const targets = modulesToSync(reads, modules);
+    if (!targets.length) return;
+
+    const userId = await ensureMoodleUser(user.email, user.name);
+    await ensureEnrolled(userId, course.id);
+    for (const t of targets) {
+      await sync('core_completion_override_activity_completion_status', {
+        cmid: String(t.cmid),
+        userid: String(userId),
+        newstate: '1',
+      });
+      await markSynced(t.rowIds);
+    }
+  } catch (err) {
+    console.error('[moodle-sync]', (err as Error).message);
+  }
+}
diff --git a/src/lib/moodle.ts b/src/lib/moodle.ts
index bcddb1d..74e476e 100644
--- a/src/lib/moodle.ts
+++ b/src/lib/moodle.ts
@@ -7,9 +7,10 @@ const BASE = process.env.MOODLE_BASE_URL || 'https://moodle.saillant.cc';
 const TOKEN = process.env.MOODLE_WS_TOKEN || '';
 const TTL = Number(process.env.MOODLE_CACHE_TTL_MS || 5 * 60_000);
 
-async function ws(fn: string, params: Record<string, string>): Promise<unknown> {
+/** Low-level Moodle REST call with an explicit token (shared with moodle-sync). */
+export async function wsCall(token: string, fn: string, params: Record<string, string>): Promise<unknown> {
   const body = new URLSearchParams({
-    wstoken: TOKEN, wsfunction: fn, moodlewsrestformat: 'json', ...params,
+    wstoken: token, wsfunction: fn, moodlewsrestformat: 'json', ...params,
   });
   const res = await fetch(`${BASE}/webservice/rest/server.php`, {
     method: 'POST',
@@ -23,6 +24,8 @@ async function ws(fn: string, params: Record<string, string>): Promise<unknown>
   return data;
 }
 
+const ws = (fn: string, params: Record<string, string>) => wsCall(TOKEN, fn, params);
+
 interface StructureItem { title: string; href: string; level: number; subitems?: StructureItem[]; }
 
 interface ModContent {
diff --git a/src/pages/cours/[slug]/index.astro b/src/pages/cours/[slug]/index.astro
index b4e167f..c45d292 100644
--- a/src/pages/cours/[slug]/index.astro
+++ b/src/pages/cours/[slug]/index.astro
@@ -5,6 +5,8 @@ import Sommaire from '../../../components/Sommaire.astro';
 import { bySlug } from '../../../config/courses';
 import { getCourseBooks } from '../../../lib/moodle';
 import { verifySession, SESSION_COOKIE } from '../../../lib/session';
+import { getReads, type ReadMark } from '../../../lib/grist';
+import { syncToMoodle } from '../../../lib/moodle-sync';
 
 const user = verifySession(Astro.cookies.get(SESSION_COOKIE)?.value);
 const course = bySlug(Astro.params.slug ?? '');
@@ -16,6 +18,15 @@ try {
 } catch {
   return new Response('Formation momentanément indisponible — réessayez dans quelques minutes.', { status: 503, headers: { 'Content-Type': 'text/plain; charset=utf-8' } });
 }
+
+let reads: ReadMark[] = [];
+if (user) {
+  try { reads = await getReads(user.sub, course.slug); } catch { reads = []; }
+  // Best-effort retry of pending Moodle syncs — fire-and-forget, never blocks render.
+  if (reads.some((r) => !r.moodle_synced))
+    syncToMoodle({ sub: user.sub, email: user.email, name: user.name }, course.slug).catch(() => {});
+}
+
 const hubData = {
   slug: course.slug,
   modules: modules.map((m, mi) => ({