diff --git a/backend/tests/Feature/Schedule/ScheduleEndpointTest.php b/backend/tests/Feature/Schedule/ScheduleEndpointTest.php
index a0e9cf8..b68e2ad 100644
--- a/backend/tests/Feature/Schedule/ScheduleEndpointTest.php
+++ b/backend/tests/Feature/Schedule/ScheduleEndpointTest.php
@@ -3,6 +3,7 @@
 namespace Tests\Feature\Schedule;
 
 use App\Auth\CreateSessionDto;
+use App\Auth\Clock;
 use App\Auth\SessionRepository;
 use App\Element\CreateElementDto;
 use App\Element\ElementModel;
@@ -25,6 +26,7 @@ use DateTimeZone;
 use Illuminate\Foundation\Testing\RefreshDatabase;
 use Illuminate\Testing\TestResponse;
 use Tests\TestCase;
+use Tests\Fakes\FakeClock;
 
 class ScheduleEndpointTest extends TestCase
 {
@@ -94,6 +96,7 @@ class ScheduleEndpointTest extends TestCase
                         'assignments' => [
                             [
                                 'id' => 1,
+                                'completedAt' => null,
                                 'element' => [
                                     'name' => 'Chapter 1',
                                     'kind' => 'chapter',
@@ -115,6 +118,7 @@ class ScheduleEndpointTest extends TestCase
                         'assignments' => [
                             [
                                 'id' => 2,
+                                'completedAt' => null,
                                 'element' => [
                                     'name' => 'Chapter 1',
                                     'kind' => 'chapter',
@@ -351,6 +355,130 @@ class ScheduleEndpointTest extends TestCase
             ->assertExactJson(['error' => 'date is required']);
     }
 
+    public function test_it_completes_and_reopens_an_assignment(): void
+    {
+        $user = $this->createUser('reader@example.com');
+        $set = $this->createSet($user, 'Course');
+        $lessonLevel = $this->createLevel($set, 'lesson');
+        $repository = app(ElementRepository::class);
+        $repository->create(new CreateElementDto(
+            name: 'First lesson',
+            level: $lessonLevel,
+            parentElement: null,
+        ));
+        $repository->create(new CreateElementDto(
+            name: 'Second lesson',
+            level: $lessonLevel,
+            parentElement: null,
+        ));
+        $this->createSession($user, 'valid-token');
+        $completedAt = new DateTimeImmutable(
+            '2026-08-15T12:30:45',
+            new DateTimeZone('UTC'),
+        );
+        $this->app->instance(Clock::class, new FakeClock($completedAt));
+        $this->credentialedPost('/api/schedules', [
+            'setId' => $set->getId(),
+            'levelId' => $lessonLevel->getId(),
+            'startDate' => '2026-08-20',
+            'targetDate' => '2026-08-20',
+        ])->assertCreated();
+
+        $this->credentialedPatch('/api/assignments/1', [
+            'completed' => true,
+        ])->assertOk()->assertExactJson([
+            'assignment' => [
+                'id' => 1,
+                'completedAt' => '2026-08-15T12:30:45+00:00',
+            ],
+        ]);
+        $this->assertDatabaseHas('schedule_assignments', [
+            'id' => 1,
+            'completed_at' => '2026-08-15 12:30:45',
+        ]);
+        $this->credentialedGet('/api/schedules/1')
+            ->assertOk()
+            ->assertJsonPath(
+                'schedule.days.0.assignments.0.completedAt',
+                '2026-08-15T12:30:45+00:00',
+            )
+            ->assertJsonPath(
+                'schedule.days.0.assignments.1.completedAt',
+                null,
+            );
+        $this->credentialedGet('/api/assignments?date=2026-08-20')
+            ->assertOk()
+            ->assertJsonCount(1, 'assignments')
+            ->assertJsonPath('assignments.0.id', 2);
+
+        $this->credentialedPatch('/api/assignments/1', [
+            'completed' => false,
+        ])->assertOk()->assertExactJson([
+            'assignment' => [
+                'id' => 1,
+                'completedAt' => null,
+            ],
+        ]);
+        $this->assertDatabaseHas('schedule_assignments', [
+            'id' => 1,
+            'completed_at' => null,
+        ]);
+        $this->credentialedGet('/api/assignments?date=2026-08-20')
+            ->assertOk()
+            ->assertJsonCount(2, 'assignments');
+    }
+
+    public function test_it_rejects_invalid_assignment_completion_input(): void
+    {
+        $user = $this->createUser('reader@example.com');
+        $this->createSession($user, 'valid-token');
+
+        $this->credentialedPatch('/api/assignments/1', [
+            'completed' => 'yes',
+        ])->assertBadRequest()->assertExactJson([
+            'error' => 'completed must be a boolean',
+        ]);
+    }
+
+    public function test_it_hides_another_users_assignment(): void
+    {
+        $owner = $this->createUser('owner@example.com');
+        $viewer = $this->createUser('viewer@example.com');
+        $set = $this->createSet($owner, 'Course');
+        $lessonLevel = $this->createLevel($set, 'lesson');
+        app(ElementRepository::class)->create(new CreateElementDto(
+            name: 'Welcome',
+            level: $lessonLevel,
+            parentElement: null,
+        ));
+        $this->createSession($owner, 'owner-token');
+        $this->createSession($viewer, 'viewer-token');
+        $this->withCredentials()
+            ->withUnencryptedCookie(
+                AuthMiddleware::COOKIE_NAME,
+                'owner-token',
+            )->postJson('/api/schedules', [
+                'setId' => $set->getId(),
+                'levelId' => $lessonLevel->getId(),
+                'startDate' => '2026-08-15',
+                'targetDate' => '2026-08-15',
+            ])->assertCreated();
+
+        $this->withCredentials()
+            ->withUnencryptedCookie(
+                AuthMiddleware::COOKIE_NAME,
+                'viewer-token',
+            )->patchJson('/api/assignments/1', [
+                'completed' => true,
+            ])->assertNotFound()->assertExactJson([
+                'error' => 'assignment not found',
+            ]);
+        $this->assertDatabaseHas('schedule_assignments', [
+            'id' => 1,
+            'completed_at' => null,
+        ]);
+    }
+
     public function test_it_is_stable_after_sources_change_or_are_deleted(): void
     {
         $user = $this->createUser('reader@example.com');
@@ -455,6 +583,9 @@ class ScheduleEndpointTest extends TestCase
     {
         $this->getJson('/api/assignments?date=2026-08-15')
             ->assertStatus(401);
+        $this->patchJson('/api/assignments/1', [
+            'completed' => true,
+        ])->assertStatus(401);
         $this->getJson('/api/schedules')->assertStatus(401);
         $this->getJson('/api/schedules/1')->assertStatus(401);
         $this->postJson('/api/schedules', [])->assertStatus(401);
@@ -520,4 +651,18 @@ class ScheduleEndpointTest extends TestCase
                 'valid-token',
             )->getJson($uri);
     }
+
+    /**
+     * @param array<string, mixed> $payload
+     */
+    private function credentialedPatch(
+        string $uri,
+        array $payload,
+    ): TestResponse {
+        return $this->withCredentials()
+            ->withUnencryptedCookie(
+                AuthMiddleware::COOKIE_NAME,
+                'valid-token',
+            )->patchJson($uri, $payload);
+    }
 }
diff --git a/backend/tests/Unit/Schedule/ScheduleAssignmentTest.php b/backend/tests/Unit/Schedule/ScheduleAssignmentTest.php
new file mode 100644
index 0000000..12f86f5
--- /dev/null
+++ b/backend/tests/Unit/Schedule/ScheduleAssignmentTest.php
@@ -0,0 +1,49 @@
+<?php
+
+namespace Tests\Unit\Schedule;
+
+use App\Schedule\ScheduleAssignment;
+use DateTimeImmutable;
+use DateTimeZone;
+use PHPUnit\Framework\TestCase;
+
+class ScheduleAssignmentTest extends TestCase
+{
+    public function test_it_completes_only_once_until_reopened(): void
+    {
+        $assignment = $this->assignment();
+        $firstCompletion = $this->utc('2026-08-15T12:00:00');
+        $laterCompletion = $this->utc('2026-08-15T13:00:00');
+
+        $assignment->complete($firstCompletion);
+        $assignment->complete($laterCompletion);
+
+        $this->assertSame($firstCompletion, $assignment->getCompletedAt());
+
+        $assignment->reopen();
+
+        $this->assertNull($assignment->getCompletedAt());
+
+        $assignment->complete($laterCompletion);
+
+        $this->assertSame($laterCompletion, $assignment->getCompletedAt());
+    }
+
+    private function assignment(): ScheduleAssignment
+    {
+        return new ScheduleAssignment(
+            id: 1,
+            name: 'Lesson',
+            kind: 'lesson',
+            path: ['Course', 'Lesson'],
+            scheduledDate: $this->utc('2026-08-15'),
+            position: 1,
+            completedAt: null,
+        );
+    }
+
+    private function utc(string $value): DateTimeImmutable
+    {
+        return new DateTimeImmutable($value, new DateTimeZone('UTC'));
+    }
+}
diff --git a/backend/tests/Unit/Schedule/UseCases/ListAssignmentsForDateTest.php b/backend/tests/Unit/Schedule/UseCases/ListAssignmentsForDateTest.php
index 1d1e3fd..c6bc95f 100644
--- a/backend/tests/Unit/Schedule/UseCases/ListAssignmentsForDateTest.php
+++ b/backend/tests/Unit/Schedule/UseCases/ListAssignmentsForDateTest.php
@@ -22,7 +22,7 @@ class ListAssignmentsForDateTest extends TestCase
         $user = $this->user(1, 'reader@example.com');
         $otherUser = $this->user(2, 'other@example.com');
         $repository = new FakeScheduleRepository;
-        $repository->create($this->schedule(
+        $olderSchedule = $repository->create($this->schedule(
             user: $user,
             setName: 'Older plan',
             date: '2026-08-15',
@@ -46,6 +46,12 @@ class ListAssignmentsForDateTest extends TestCase
             date: '2026-08-16',
             assignmentNames: ['Later'],
         ));
+        $completedAssignment = $olderSchedule->getAssignments()[0];
+        $completedAssignment->complete(new DateTimeImmutable(
+            '2026-08-15T12:00:00',
+            new DateTimeZone('UTC'),
+        ));
+        $repository->updateAssignment($completedAssignment);
 
         $assignments = (new ListAssignmentsForDate($repository))->execute(
             new ListAssignmentsForDateRequest(
@@ -55,13 +61,13 @@ class ListAssignmentsForDateTest extends TestCase
         );
 
         $this->assertSame(
-            ['Newer plan', 'Older plan', 'Older plan'],
+            ['Newer plan', 'Older plan'],
             array_map(function ($assignment): string {
                 return $assignment->getSetName();
             }, $assignments),
         );
         $this->assertSame(
-            ['Third', 'First', 'Second'],
+            ['Third', 'Second'],
             array_map(function ($assignment): string {
                 return $assignment->getAssignment()->getName();
             }, $assignments),
diff --git a/backend/tests/Unit/Schedule/UseCases/SetAssignmentCompletionTest.php b/backend/tests/Unit/Schedule/UseCases/SetAssignmentCompletionTest.php
new file mode 100644
index 0000000..8950edc
--- /dev/null
+++ b/backend/tests/Unit/Schedule/UseCases/SetAssignmentCompletionTest.php
@@ -0,0 +1,175 @@
+<?php
+
+namespace Tests\Unit\Schedule\UseCases;
+
+use App\Exceptions\BadRequestException;
+use App\Exceptions\NotFoundException;
+use App\Schedule\CreateScheduleAssignmentDto;
+use App\Schedule\CreateScheduleDto;
+use App\Schedule\UseCases\SetAssignmentCompletion\SetAssignmentCompletion;
+use App\Schedule\UseCases\SetAssignmentCompletion\SetAssignmentCompletionRequest;
+use App\Shared\ValueObject\EmailAddress;
+use App\User\User;
+use DateTimeImmutable;
+use DateTimeZone;
+use PHPUnit\Framework\TestCase;
+use Tests\Fakes\FakeClock;
+use Tests\Fakes\FakeScheduleRepository;
+
+class SetAssignmentCompletionTest extends TestCase
+{
+    public function test_it_completes_an_owned_assignment_at_the_current_time(): void
+    {
+        $user = $this->user(1, 'reader@example.com');
+        $repository = new FakeScheduleRepository;
+        $schedule = $repository->create($this->schedule($user));
+        $now = $this->utc('2026-08-15T12:00:00');
+
+        $assignment = (new SetAssignmentCompletion(
+            $repository,
+            new FakeClock($now),
+        ))->execute(new SetAssignmentCompletionRequest(
+            assignmentId: $schedule->getAssignments()[0]->getId(),
+            user: $user,
+            completed: true,
+        ));
+
+        $this->assertSame($now, $assignment->getCompletedAt());
+        $this->assertSame(
+            $now,
+            $repository->findAssignmentForUser(
+                $assignment->getId(),
+                $user,
+            )?->getCompletedAt(),
+        );
+    }
+
+    public function test_it_preserves_the_first_completion_time(): void
+    {
+        $user = $this->user(1, 'reader@example.com');
+        $repository = new FakeScheduleRepository;
+        $schedule = $repository->create($this->schedule($user));
+        $assignmentId = $schedule->getAssignments()[0]->getId();
+        $firstCompletion = $this->utc('2026-08-15T12:00:00');
+        $laterCompletion = $this->utc('2026-08-15T13:00:00');
+
+        (new SetAssignmentCompletion(
+            $repository,
+            new FakeClock($firstCompletion),
+        ))->execute(new SetAssignmentCompletionRequest(
+            assignmentId: $assignmentId,
+            user: $user,
+            completed: true,
+        ));
+        $assignment = (new SetAssignmentCompletion(
+            $repository,
+            new FakeClock($laterCompletion),
+        ))->execute(new SetAssignmentCompletionRequest(
+            assignmentId: $assignmentId,
+            user: $user,
+            completed: true,
+        ));
+
+        $this->assertSame(
+            $firstCompletion,
+            $assignment->getCompletedAt(),
+        );
+    }
+
+    public function test_it_reopens_a_completed_assignment(): void
+    {
+        $user = $this->user(1, 'reader@example.com');
+        $repository = new FakeScheduleRepository;
+        $schedule = $repository->create($this->schedule($user));
+        $assignmentId = $schedule->getAssignments()[0]->getId();
+        $useCase = new SetAssignmentCompletion(
+            $repository,
+            new FakeClock($this->utc('2026-08-15T12:00:00')),
+        );
+        $useCase->execute(new SetAssignmentCompletionRequest(
+            assignmentId: $assignmentId,
+            user: $user,
+            completed: true,
+        ));
+
+        $assignment = $useCase->execute(
+            new SetAssignmentCompletionRequest(
+                assignmentId: $assignmentId,
+                user: $user,
+                completed: false,
+            ),
+        );
+
+        $this->assertNull($assignment->getCompletedAt());
+    }
+
+    public function test_it_hides_another_users_assignment(): void
+    {
+        $repository = new FakeScheduleRepository;
+        $owner = $this->user(1, 'owner@example.com');
+        $schedule = $repository->create($this->schedule($owner));
+
+        $this->expectException(NotFoundException::class);
+        $this->expectExceptionMessage('assignment not found');
+
+        (new SetAssignmentCompletion(
+            $repository,
+            new FakeClock($this->utc('2026-08-15T12:00:00')),
+        ))->execute(new SetAssignmentCompletionRequest(
+            assignmentId: $schedule->getAssignments()[0]->getId(),
+            user: $this->user(2, 'viewer@example.com'),
+            completed: true,
+        ));
+    }
+
+    public function test_it_requires_a_boolean_completion_value(): void
+    {
+        $this->expectException(BadRequestException::class);
+        $this->expectExceptionMessage('completed must be a boolean');
+
+        (new SetAssignmentCompletion(
+            new FakeScheduleRepository,
+            new FakeClock($this->utc('2026-08-15T12:00:00')),
+        ))->execute(new SetAssignmentCompletionRequest(
+            assignmentId: 1,
+            user: $this->user(1, 'reader@example.com'),
+            completed: null,
+        ));
+    }
+
+    private function schedule(User $user): CreateScheduleDto
+    {
+        $scheduledDate = $this->utc('2026-08-20');
+
+        return new CreateScheduleDto(
+            user: $user,
+            setName: 'Course',
+            elementKind: 'lesson',
+            startDate: $scheduledDate,
+            targetDate: $scheduledDate,
+            assignments: [
+                new CreateScheduleAssignmentDto(
+                    name: 'Lesson',
+                    kind: 'lesson',
+                    path: ['Course', 'Lesson'],
+                    scheduledDate: $scheduledDate,
+                    position: 1,
+                ),
+            ],
+        );
+    }
+
+    private function user(int $id, string $email): User
+    {
+        return new User(
+            id: $id,
+            email: new EmailAddress($email),
+            passwordHash: 'hashed-password',
+        );
+    }
+
+    private function utc(string $value): DateTimeImmutable
+    {
+        return new DateTimeImmutable($value, new DateTimeZone('UTC'));
+    }
+}