diff --git a/lib/presentation/workout_template_screen.dart b/lib/presentation/workout_template_screen.dart
index 9666990..a95d7ed 100644
--- a/lib/presentation/workout_template_screen.dart
+++ b/lib/presentation/workout_template_screen.dart
@@ -111,6 +111,19 @@ final class _WorkoutTemplateListScreenState
   }
 
   Future<void> _start(WorkoutTemplate template) async {
+    final openSession = await widget.activeUseCases.findOpen();
+    if (!mounted) return;
+    if (openSession != null) {
+      ScaffoldMessenger.of(context).showSnackBar(
+        const SnackBar(
+          content: Text(
+            'Une séance est déjà en cours. Termine-la ou reprends-la avant '
+            'd’en lancer une nouvelle.',
+          ),
+        ),
+      );
+      return;
+    }
     final session = await widget.activeUseCases.startFromTemplate(
       template.metadata.id,
     );
diff --git a/test/presentation/workout_template_screen_test.dart b/test/presentation/workout_template_screen_test.dart
index 6e3dfd1..bb323a7 100644
--- a/test/presentation/workout_template_screen_test.dart
+++ b/test/presentation/workout_template_screen_test.dart
@@ -1,5 +1,9 @@
+import 'dart:convert';
+
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:gametime/application/application.dart';
+import 'package:gametime/domain/domain.dart';
 import 'package:gametime/presentation/presentation.dart';
 
 void main() {
@@ -39,4 +43,303 @@ void main() {
       expect(find.text('5 séries · Temps + Répétitions'), findsOneWidget);
     },
   );
+
+  testWidgets(
+    'lancer une séance alors qu’une autre est en cours affiche un message',
+    (tester) async {
+      final clock = _FakeClock(DateTime.utc(2026, 7, 17, 12));
+      final templateRepository = _FakeWorkoutTemplateRepository([
+        _workoutTemplate(),
+      ]);
+      final activeRepository = _FakeActiveSessionRepository()
+        ..session = _activeSession();
+      final programRepository = _FakeProgramRepository();
+      final historyRepository = _FakeWorkoutHistoryRepository();
+
+      await tester.pumpWidget(
+        MaterialApp(
+          home: WorkoutTemplateListScreen(
+            workoutTemplateUseCases: WorkoutTemplateUseCases(
+              templateRepository: templateRepository,
+              programRepository: programRepository,
+              clock: clock,
+              ids: _FakeIds(),
+              originDeviceId: 'device-1',
+            ),
+            programUseCases: ProgramUseCases(
+              programRepository: programRepository,
+              exerciseRepository: _FakeExerciseRepository(),
+              clock: clock,
+              ids: _FakeIds(),
+              originDeviceId: 'device-1',
+            ),
+            activeUseCases: ActiveWorkoutSessionUseCases(
+              sessionRepository: activeRepository,
+              templateRepository: templateRepository,
+              clock: clock,
+              ids: _FakeIds(),
+              originDeviceId: 'device-1',
+            ),
+            closeUseCase: CloseWorkoutSessionUseCase(
+              sessionRepository: activeRepository,
+              historyRepository: historyRepository,
+              clock: clock,
+              ids: _FakeIds(),
+              originDeviceId: 'device-1',
+            ),
+            historyUseCases: WorkoutHistoryUseCases(
+              repository: historyRepository,
+              clock: clock,
+            ),
+          ),
+        ),
+      );
+      await tester.pump();
+      await tester.pump();
+
+      await tester.tap(find.text('Lancer'));
+      await tester.pump();
+      await tester.pump();
+
+      expect(
+        find.textContaining('Une séance est déjà en cours.'),
+        findsOneWidget,
+      );
+      expect(activeRepository.saveCount, 0);
+    },
+  );
+}
+
+WorkoutTemplate _workoutTemplate() {
+  return WorkoutTemplate(
+    metadata: _metadata('template-1'),
+    name: 'Séance jambes',
+    programs: [
+      WorkoutTemplateProgram(
+        metadata: _metadata('template-program-1'),
+        workoutTemplateId: 'template-1',
+        position: 0,
+        programNameSnapshot: 'Programme jambes',
+        defaultRestSecondsSnapshot: 60,
+        programSnapshotJson: _programSnapshotJson(),
+      ),
+    ],
+  );
+}
+
+ActiveWorkoutSession _activeSession() {
+  return ActiveWorkoutSession(
+    metadata: _metadata('session-1'),
+    sourceWorkoutTemplateId: 'template-active',
+    status: ActiveWorkoutStatus.running,
+    startedAt: DateTime.utc(2026, 7, 17, 12),
+    lastPersistedAt: DateTime.utc(2026, 7, 17, 12),
+    elapsedActiveMs: 0,
+    currentProgramIndex: 0,
+    currentExerciseIndex: 0,
+    currentSetIndex: 0,
+    resolvedTemplateSnapshotJson: jsonEncode({
+      'name': 'Séance active',
+      'programs': [
+        {
+          'id': 'template-program-1',
+          'programNameSnapshot': 'Programme jambes',
+          'programSnapshotJson': _programSnapshotJson(),
+        },
+      ],
+      'overrides': const [],
+    }),
+  );
+}
+
+String _programSnapshotJson() {
+  return jsonEncode({
+    'exercises': [
+      {
+        'id': 'exercise-1',
+        'exerciseNameSnapshot': 'Squat',
+        'setsCount': 3,
+        'timeEnabled': true,
+        'repsEnabled': false,
+        'scoreEnabled': false,
+        'restSecondsOverride': 60,
+      },
+    ],
+  });
+}
+
+EntityMetadata _metadata(String id) {
+  return EntityMetadata(
+    id: id,
+    createdAt: DateTime.utc(2026, 7, 17),
+    updatedAt: DateTime.utc(2026, 7, 17),
+    originDeviceId: 'device-1',
+  );
+}
+
+final class _FakeClock implements Clock {
+  const _FakeClock(this.value);
+
+  final DateTime value;
+
+  @override
+  DateTime now() => value;
+}
+
+final class _FakeIds implements IdGenerator {
+  var _next = 0;
+
+  @override
+  String newId() {
+    _next += 1;
+    return 'id-$_next';
+  }
+}
+
+final class _FakeWorkoutTemplateRepository
+    implements WorkoutTemplateRepository {
+  _FakeWorkoutTemplateRepository(this.templates);
+
+  final List<WorkoutTemplate> templates;
+
+  @override
+  Future<WorkoutTemplate?> findById(String id) async {
+    return templates.cast<WorkoutTemplate?>().firstWhere(
+      (template) => template?.metadata.id == id,
+      orElse: () => null,
+    );
+  }
+
+  @override
+  Future<List<WorkoutTemplate>> listActive() async => templates;
+
+  @override
+  Future<void> replaceComposition(
+    WorkoutTemplate template,
+    DateTime deletedAt,
+  ) async {}
+
+  @override
+  Future<void> save(WorkoutTemplate template) async {}
+
+  @override
+  Future<void> saveOverride(WorkoutTemplateExerciseOverride override) async {}
+
+  @override
+  Future<void> saveProgram(WorkoutTemplateProgram program) async {}
+}
+
+final class _FakeActiveSessionRepository implements ActiveSessionRepository {
+  ActiveWorkoutSession? session;
+  var saveCount = 0;
+
+  @override
+  Future<ActiveWorkoutSession?> findById(String id) async {
+    return session?.metadata.id == id ? session : null;
+  }
+
+  @override
+  Future<ActiveWorkoutSession?> findOpen() async => session;
+
+  @override
+  Future<ActiveRestState?> findRestStateById(String id) async => null;
+
+  @override
+  Future<ActiveScoreStopwatchState?> findScoreStopwatchState({
+    required String sessionId,
+    required int programIndex,
+    required int exerciseIndex,
+    required int setIndex,
+  }) async {
+    return null;
+  }
+
+  @override
+  Future<List<ActiveRestState>> listRestStates(String sessionId) async {
+    return const [];
+  }
+
+  @override
+  Future<List<ActiveScoreStopwatchState>> listScoreStopwatchStates(
+    String sessionId,
+  ) async {
+    return const [];
+  }
+
+  @override
+  Future<List<ActiveSetResult>> listSetResults(String sessionId) async {
+    return const [];
+  }
+
+  @override
+  Future<void> save(ActiveWorkoutSession session) async {
+    saveCount += 1;
+    this.session = session;
+  }
+
+  @override
+  Future<void> saveRestState(ActiveRestState restState) async {}
+
+  @override
+  Future<void> saveScoreStopwatchState(ActiveScoreStopwatchState state) async {}
+
+  @override
+  Future<void> deleteScoreStopwatchState({
+    required String sessionId,
+    required int programIndex,
+    required int exerciseIndex,
+    required int setIndex,
+    required DateTime deletedAt,
+  }) async {}
+
+  @override
+  Future<void> saveSetResult(ActiveSetResult result) async {}
+}
+
+final class _FakeProgramRepository implements ProgramRepository {
+  @override
+  Future<Program?> findById(String id) async => null;
+
+  @override
+  Future<List<Program>> listActive() async => const [];
+
+  @override
+  Future<void> replaceExercises(Program program, DateTime deletedAt) async {}
+
+  @override
+  Future<void> save(Program program) async {}
+
+  @override
+  Future<void> saveExercise(ProgramExercise exercise) async {}
+}
+
+final class _FakeExerciseRepository implements ExerciseRepository {
+  @override
+  Future<Exercise?> findById(String id) async => null;
+
+  @override
+  Future<bool> isReferencedByProgram(String id) async => false;
+
+  @override
+  Future<List<Exercise>> listActive() async => const [];
+
+  @override
+  Future<void> save(Exercise exercise) async {}
+}
+
+final class _FakeWorkoutHistoryRepository implements WorkoutHistoryRepository {
+  @override
+  Future<void> delete(String id, DateTime deletedAt) async {}
+
+  @override
+  Future<WorkoutHistory?> findById(String id) async => null;
+
+  @override
+  Future<List<WorkoutHistory>> listActive() async => const [];
+
+  @override
+  Future<void> save(WorkoutHistory history) async {}
+
+  @override
+  Future<void> saveSetResult(WorkoutHistorySetResult result) async {}
 }