diff --git a/lib/game/engine/game_engine.dart b/lib/game/engine/game_engine.dart
new file mode 100644
index 0000000..a0466ba
--- /dev/null
+++ b/lib/game/engine/game_engine.dart
@@ -0,0 +1,200 @@
+import '../../core/rng.dart';
+import '../models/cell.dart';
+import '../models/grid.dart';
+import '../models/objective.dart';
+import '../models/piece.dart';
+import '../models/stage.dart';
+import 'game_event.dart';
+import 'line_clear.dart';
+import 'piece_generator.dart';
+import 'placement.dart';
+import 'scoring.dart';
+
+enum GamePhase { playing, won, stuck, lost }
+
+enum StuckReason { outOfMoves, boardDead }
+
+class PlacementResult {
+  const PlacementResult({
+    required this.events,
+    required this.pointsGained,
+    required this.linesCleared,
+    required this.gemsCleared,
+  });
+
+  final List<GameEvent> events;
+  final int pointsGained;
+  final int linesCleared;
+  final int gemsCleared;
+}
+
+/// The single stateful pure-Dart session object for one stage attempt.
+/// UI layers wrap this behind a notifier; it never imports Flutter.
+class GameEngine {
+  GameEngine(this._stage, {int attempt = 0, PieceGenerator? generator})
+      : _generator =
+            generator ?? PieceGenerator(SeededRng(_stage.seed ^ attempt)),
+        _grid = _stage.initialGrid(),
+        _objectives = List.of(_stage.objectives),
+        _moveLimit = _stage.moveLimit {
+    _tray = _generator.nextTray(_grid);
+    _checkStuck();
+  }
+
+  static const int extraMovesGrant = 5;
+  static const int continueRowsCleared = 2;
+
+  final StageConfig _stage;
+  final PieceGenerator _generator;
+
+  GridState _grid;
+  late List<Piece> _tray;
+  List<Objective> _objectives;
+  int _moveLimit;
+  int _movesUsed = 0;
+  int _score = 0;
+  ComboState _combo = ComboState.initial;
+  GamePhase _phase = GamePhase.playing;
+  StuckReason? _stuckReason;
+  bool _rescueUsed = false;
+
+  GridState get grid => _grid;
+  List<Piece> get tray => List.unmodifiable(_tray);
+  int get score => _score;
+  ComboState get combo => _combo;
+  int get movesUsed => _movesUsed;
+  int get movesLeft => _moveLimit - _movesUsed;
+  List<Objective> get objectives => List.unmodifiable(_objectives);
+  GamePhase get phase => _phase;
+  StuckReason? get stuckReason => _stuckReason;
+
+  int get starsEarned => _phase == GamePhase.won
+      ? _stage.stars.starsFor(movesLeft: movesLeft)
+      : 0;
+
+  /// Overall objective completion in [0, 1] for the near-miss screen.
+  double get objectiveProgress {
+    if (_objectives.isEmpty) return 0;
+    var total = 0.0;
+    for (final obj in _objectives) {
+      final frac = obj.current / obj.target;
+      total += frac > 1 ? 1 : frac;
+    }
+    return total / _objectives.length;
+  }
+
+  /// Read-only legality probe (drag ghost preview, AI players, tests).
+  bool tryPlaceWouldSucceed(int trayIndex, int x, int y) =>
+      _phase == GamePhase.playing && canPlace(_grid, _tray[trayIndex], x, y);
+
+  PlacementResult? tryPlace(int trayIndex, int x, int y) {
+    if (_phase != GamePhase.playing) return null;
+    final piece = _tray[trayIndex];
+    if (!canPlace(_grid, piece, x, y)) return null;
+
+    _tray.removeAt(trayIndex);
+    _movesUsed++;
+
+    final placed = place(_grid, piece, x, y);
+    final clear = detectAndClear(placed);
+    _grid = clear.grid;
+
+    final delta = scorePlacement(
+      cellsPlaced: piece.size,
+      linesCleared: clear.linesCleared,
+      combo: _combo,
+    );
+    _score += delta.points;
+    _combo = delta.combo;
+
+    final events = <GameEvent>[
+      PiecePlaced(piece: piece, x: x, y: y),
+      if (clear.linesCleared > 0)
+        LinesCleared(lines: clear.linesCleared, gems: clear.gemsCleared),
+      ScoreChanged(total: _score),
+    ];
+    _objectives = [
+      for (final obj in _objectives)
+        events.fold(obj, (o, event) => o.onEvent(event)),
+    ];
+
+    if (_objectives.every((o) => o.isComplete)) {
+      _phase = GamePhase.won;
+    } else {
+      if (_tray.isEmpty) _tray = _generator.nextTray(_grid);
+      _checkStuck();
+    }
+
+    return PlacementResult(
+      events: events,
+      pointsGained: delta.points,
+      linesCleared: clear.linesCleared,
+      gemsCleared: clear.gemsCleared,
+    );
+  }
+
+  void _checkStuck() {
+    if (movesLeft <= 0) {
+      _phase = GamePhase.stuck;
+      _stuckReason = StuckReason.outOfMoves;
+    } else if (!anyPlacementExists(_grid, _tray)) {
+      _phase = GamePhase.stuck;
+      _stuckReason = StuckReason.boardDead;
+    } else {
+      _phase = GamePhase.playing;
+      _stuckReason = null;
+    }
+  }
+
+  /// Rewarded-ad continue for a dead board: clears the two most-filled rows
+  /// and redeals. One rescue (continue or extra moves) per attempt.
+  void useContinue() {
+    if (_phase != GamePhase.stuck ||
+        _stuckReason != StuckReason.boardDead ||
+        _rescueUsed) {
+      throw StateError('continue not available');
+    }
+    _rescueUsed = true;
+
+    final rowCounts = <(int, int)>[];
+    for (var y = 0; y < GridState.size; y++) {
+      var count = 0;
+      for (var x = 0; x < GridState.size; x++) {
+        if (_grid.isOccupied(x, y)) count++;
+      }
+      rowCounts.add((y, count));
+    }
+    rowCounts.sort((a, b) {
+      final c = b.$2.compareTo(a.$2);
+      return c != 0 ? c : a.$1.compareTo(b.$1);
+    });
+    for (final (y, _) in rowCounts.take(continueRowsCleared)) {
+      for (var x = 0; x < GridState.size; x++) {
+        _grid = _grid.withCell(x, y, const Cell(CellType.empty));
+      }
+    }
+
+    _tray = _generator.nextTray(_grid);
+    _checkStuck();
+  }
+
+  /// Rewarded-ad rescue for move exhaustion: +5 moves. One rescue per attempt.
+  void addExtraMoves() {
+    if (_phase != GamePhase.stuck ||
+        _stuckReason != StuckReason.outOfMoves ||
+        _rescueUsed) {
+      throw StateError('extra moves not available');
+    }
+    _rescueUsed = true;
+    _moveLimit += extraMovesGrant;
+    _checkStuck();
+  }
+
+  /// Player declines rescue (or forfeits): the attempt is lost.
+  void declineAndLose() {
+    if (_phase == GamePhase.won || _phase == GamePhase.lost) {
+      throw StateError('attempt already finished');
+    }
+    _phase = GamePhase.lost;
+  }
+}
diff --git a/lib/game/engine/game_event.dart b/lib/game/engine/game_event.dart
new file mode 100644
index 0000000..cf97dc4
--- /dev/null
+++ b/lib/game/engine/game_event.dart
@@ -0,0 +1,28 @@
+import '../models/piece.dart';
+
+/// Events emitted by the engine during a placement, consumed by objectives
+/// and (later) the UI effects layer.
+sealed class GameEvent {
+  const GameEvent();
+}
+
+class PiecePlaced extends GameEvent {
+  const PiecePlaced({required this.piece, required this.x, required this.y});
+
+  final Piece piece;
+  final int x;
+  final int y;
+}
+
+class LinesCleared extends GameEvent {
+  const LinesCleared({required this.lines, required this.gems});
+
+  final int lines;
+  final int gems;
+}
+
+class ScoreChanged extends GameEvent {
+  const ScoreChanged({required this.total});
+
+  final int total;
+}
diff --git a/lib/game/engine/piece_generator.dart b/lib/game/engine/piece_generator.dart
index 390e68d..be1d17f 100644
--- a/lib/game/engine/piece_generator.dart
+++ b/lib/game/engine/piece_generator.dart
@@ -97,7 +97,7 @@ class PieceGenerator {
         break;
       }
     }
-    final tray = <Piece>[if (largestFitting != null) largestFitting];
+    final tray = <Piece>[?largestFitting];
     for (final p in bySizeDesc.reversed) {
       if (tray.length == traySize) break;
       if (tray.any((t) => t.id == p.id)) continue;
diff --git a/lib/game/models/objective.dart b/lib/game/models/objective.dart
new file mode 100644
index 0000000..368e02a
--- /dev/null
+++ b/lib/game/models/objective.dart
@@ -0,0 +1,88 @@
+import '../engine/game_event.dart';
+
+/// Stage win condition with immutable progress tracking. New objective types
+/// plug in as subclasses with a JSON tag.
+sealed class Objective {
+  const Objective();
+
+  factory Objective.clearGems(int count) => ClearGemsObjective(count, 0);
+  factory Objective.reachScore(int target) => ReachScoreObjective(target, 0);
+  factory Objective.clearLines(int count) => ClearLinesObjective(count, 0);
+
+  factory Objective.fromJson(Map<String, dynamic> json) {
+    switch (json['type']) {
+      case 'clearGems':
+        return Objective.clearGems(json['count'] as int);
+      case 'reachScore':
+        return Objective.reachScore(json['target'] as int);
+      case 'clearLines':
+        return Objective.clearLines(json['count'] as int);
+      default:
+        throw FormatException('Unknown objective type: ${json['type']}');
+    }
+  }
+
+  int get target;
+  int get current;
+  bool get isComplete => current >= target;
+
+  Objective onEvent(GameEvent event);
+
+  Map<String, dynamic> toJson();
+}
+
+class ClearGemsObjective extends Objective {
+  const ClearGemsObjective(this.target, this.current);
+
+  @override
+  final int target;
+  @override
+  final int current;
+
+  @override
+  Objective onEvent(GameEvent event) => switch (event) {
+        LinesCleared(:final gems) =>
+          ClearGemsObjective(target, current + gems),
+        _ => this,
+      };
+
+  @override
+  Map<String, dynamic> toJson() => {'type': 'clearGems', 'count': target};
+}
+
+class ReachScoreObjective extends Objective {
+  const ReachScoreObjective(this.target, this.current);
+
+  @override
+  final int target;
+  @override
+  final int current;
+
+  @override
+  Objective onEvent(GameEvent event) => switch (event) {
+        ScoreChanged(:final total) => ReachScoreObjective(target, total),
+        _ => this,
+      };
+
+  @override
+  Map<String, dynamic> toJson() => {'type': 'reachScore', 'target': target};
+}
+
+class ClearLinesObjective extends Objective {
+  const ClearLinesObjective(this.target, this.current);
+
+  @override
+  final int target;
+  @override
+  final int current;
+
+  @override
+  Objective onEvent(GameEvent event) => switch (event) {
+        LinesCleared(:final lines) =>
+          ClearLinesObjective(target, current + lines),
+        _ => this,
+      };
+
+  @override
+  Map<String, dynamic> toJson() => {'type': 'clearLines', 'count': target};
+}
diff --git a/lib/game/models/stage.dart b/lib/game/models/stage.dart
new file mode 100644
index 0000000..c186785
--- /dev/null
+++ b/lib/game/models/stage.dart
@@ -0,0 +1,124 @@
+import 'cell.dart';
+import 'grid.dart';
+import 'objective.dart';
+
+/// A pre-placed cell in a stage layout.
+class PresetCell {
+  const PresetCell({
+    required this.x,
+    required this.y,
+    required this.type,
+    this.colorId = 0,
+  });
+
+  factory PresetCell.fromJson(Map<String, dynamic> json) => PresetCell(
+        x: json['x'] as int,
+        y: json['y'] as int,
+        type: switch (json['t']) {
+          'gem' => CellType.gem,
+          'filled' => CellType.filled,
+          _ => throw FormatException('Unknown preset cell type: ${json['t']}'),
+        },
+        colorId: (json['c'] as int?) ?? 0,
+      );
+
+  final int x;
+  final int y;
+  final CellType type;
+  final int colorId;
+
+  Map<String, dynamic> toJson() => {
+        'x': x,
+        'y': y,
+        't': type.name,
+        if (type == CellType.filled) 'c': colorId,
+      };
+}
+
+/// Star rating from moves remaining at the win.
+class StarThresholds {
+  const StarThresholds({
+    required this.twoMovesLeft,
+    required this.threeMovesLeft,
+  });
+
+  factory StarThresholds.fromJson(Map<String, dynamic> json) =>
+      StarThresholds(
+        twoMovesLeft: (json['two'] as Map<String, dynamic>)['movesLeft'] as int,
+        threeMovesLeft:
+            (json['three'] as Map<String, dynamic>)['movesLeft'] as int,
+      );
+
+  final int twoMovesLeft;
+  final int threeMovesLeft;
+
+  int starsFor({required int movesLeft}) {
+    if (movesLeft >= threeMovesLeft) return 3;
+    if (movesLeft >= twoMovesLeft) return 2;
+    return 1;
+  }
+
+  Map<String, dynamic> toJson() => {
+        'two': {'movesLeft': twoMovesLeft},
+        'three': {'movesLeft': threeMovesLeft},
+      };
+}
+
+/// Immutable stage definition, the unit of season pack content.
+class StageConfig {
+  const StageConfig({
+    required this.id,
+    required this.seed,
+    required this.moveLimit,
+    required this.preset,
+    required this.objectives,
+    required this.stars,
+    required this.generatorProfile,
+  });
+
+  factory StageConfig.fromJson(Map<String, dynamic> json) => StageConfig(
+        id: json['id'] as String,
+        seed: json['seed'] as int,
+        moveLimit: json['moveLimit'] as int,
+        preset: [
+          for (final cell in (json['preset'] as List? ?? const []))
+            PresetCell.fromJson(cell as Map<String, dynamic>),
+        ],
+        objectives: [
+          for (final obj in json['objectives'] as List)
+            Objective.fromJson(obj as Map<String, dynamic>),
+        ],
+        stars: StarThresholds.fromJson(json['stars'] as Map<String, dynamic>),
+        generatorProfile: (json['generatorProfile'] as String?) ?? 'mid',
+      );
+
+  final String id;
+  final int seed;
+  final int moveLimit;
+  final List<PresetCell> preset;
+  final List<Objective> objectives;
+  final StarThresholds stars;
+  final String generatorProfile;
+
+  GridState initialGrid() {
+    var grid = GridState.empty();
+    for (final cell in preset) {
+      grid = grid.withCell(
+        cell.x,
+        cell.y,
+        Cell(cell.type, colorId: cell.colorId),
+      );
+    }
+    return grid;
+  }
+
+  Map<String, dynamic> toJson() => {
+        'id': id,
+        'seed': seed,
+        'moveLimit': moveLimit,
+        'preset': [for (final cell in preset) cell.toJson()],
+        'objectives': [for (final obj in objectives) obj.toJson()],
+        'stars': stars.toJson(),
+        'generatorProfile': generatorProfile,
+      };
+}
diff --git a/test/architecture_test.dart b/test/architecture_test.dart
new file mode 100644
index 0000000..0fee8d4
--- /dev/null
+++ b/test/architecture_test.dart
@@ -0,0 +1,24 @@
+import 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+
+/// The game engine must stay pure Dart: unit-testable headless, reusable by
+/// the stage generator CLI, and free of widget-layer coupling.
+void main() {
+  test('lib/game and lib/core never import Flutter', () {
+    final dirs = [Directory('lib/game'), Directory('lib/core')];
+    final violations = <String>[];
+    for (final dir in dirs) {
+      for (final entity in dir.listSync(recursive: true)) {
+        if (entity is! File || !entity.path.endsWith('.dart')) continue;
+        final source = entity.readAsStringSync();
+        if (source.contains("import 'package:flutter") ||
+            source.contains('import "package:flutter')) {
+          violations.add(entity.path);
+        }
+      }
+    }
+    expect(violations, isEmpty,
+        reason: 'Flutter imports found in pure-Dart layers: $violations');
+  });
+}
diff --git a/test/game/engine/game_engine_test.dart b/test/game/engine/game_engine_test.dart
new file mode 100644
index 0000000..07c0842
--- /dev/null
+++ b/test/game/engine/game_engine_test.dart
@@ -0,0 +1,230 @@
+import 'package:block_seasons/core/rng.dart';
+import 'package:block_seasons/game/engine/game_engine.dart';
+import 'package:block_seasons/game/engine/piece_generator.dart';
+import 'package:block_seasons/game/models/cell.dart';
+import 'package:block_seasons/game/models/grid.dart';
+import 'package:block_seasons/game/models/piece_library.dart';
+import 'package:block_seasons/game/models/stage.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+StageConfig _stage({
+  int seed = 1234,
+  int moveLimit = 20,
+  List<PresetCell> preset = const [],
+  List<Map<String, dynamic>> objectives = const [
+    {'type': 'reachScore', 'target': 999999},
+  ],
+  int twoStars = 5,
+  int threeStars = 8,
+}) {
+  return StageConfig.fromJson({
+    'id': 'test_stage',
+    'seed': seed,
+    'moveLimit': moveLimit,
+    'preset': [for (final c in preset) c.toJson()],
+    'objectives': objectives,
+    'stars': {
+      'two': {'movesLeft': twoStars},
+      'three': {'movesLeft': threeStars},
+    },
+    'generatorProfile': 'mid',
+  });
+}
+
+/// Row [y] filled except column 0; placing a mono at (0, y) clears it.
+List<PresetCell> _almostFullRow(int y) => [
+      for (var x = 1; x < GridState.size; x++)
+        PresetCell(x: x, y: y, type: CellType.filled),
+    ];
+
+PieceGenerator _smallPool(int seed) => PieceGenerator(
+      SeededRng(seed),
+      pool: [
+        PieceLibrary.byId('mono'),
+        PieceLibrary.byId('domino_h'),
+        PieceLibrary.byId('domino_v'),
+      ],
+    );
+
+void main() {
+  group('initial state', () {
+    test('starts with preset grid, full tray, zero progress', () {
+      final engine = GameEngine(_stage(preset: _almostFullRow(3)));
+      expect(engine.phase, GamePhase.playing);
+      expect(engine.tray, hasLength(3));
+      expect(engine.score, 0);
+      expect(engine.movesUsed, 0);
+      expect(engine.grid.occupiedCount, 7);
+    });
+
+    test('same stage and attempt deal identical trays', () {
+      final a = GameEngine(_stage());
+      final b = GameEngine(_stage());
+      expect(a.tray.map((p) => p.id), b.tray.map((p) => p.id));
+    });
+
+    test('different attempts deal different trays', () {
+      final trays = <String>{};
+      for (var attempt = 0; attempt < 5; attempt++) {
+        final engine = GameEngine(_stage(), attempt: attempt);
+        trays.add(engine.tray.map((p) => p.id).join(','));
+      }
+      expect(trays.length, greaterThan(1));
+    });
+  });
+
+  group('tryPlace', () {
+    test('rejects illegal placements without consuming state', () {
+      final engine = GameEngine(_stage(preset: _almostFullRow(3)));
+      final before = engine.tray.length;
+      // (1,3) is occupied by the preset.
+      final result = engine.tryPlace(0, 1, 3);
+      expect(result, isNull);
+      expect(engine.tray.length, before);
+      expect(engine.movesUsed, 0);
+    });
+
+    test('placement consumes the piece, scores cells, advances moves', () {
+      final engine = GameEngine(_stage(), generator: _smallPool(1));
+      final piece = engine.tray[0];
+      final result = engine.tryPlace(0, 0, 0);
+      expect(result, isNotNull);
+      expect(engine.movesUsed, 1);
+      expect(engine.tray, hasLength(2));
+      expect(engine.score, piece.size);
+    });
+
+    test('tray refills to 3 after all pieces are used', () {
+      final engine = GameEngine(_stage(), generator: _smallPool(1));
+      engine.tryPlace(0, 0, 0);
+      engine.tryPlace(0, 0, 3);
+      expect(engine.tray, hasLength(1));
+      engine.tryPlace(0, 0, 6);
+      expect(engine.tray, hasLength(3));
+    });
+
+    test('completing a row clears it and applies combo-multiplied score', () {
+      final engine = GameEngine(
+        _stage(preset: _almostFullRow(3)),
+        generator: _smallPool(1),
+      );
+      final monoIndex = engine.tray.indexWhere((p) => p.id == 'mono');
+      final result = engine.tryPlace(monoIndex, 0, 3)!;
+      expect(result.linesCleared, 1);
+      // 1 cell + round(100 * 1.5) = 151
+      expect(engine.score, 151);
+      expect(engine.grid.occupiedCount, 0);
+    });
+  });
+
+  group('win and stars', () {
+    test('completing all objectives wins with stars from moves left', () {
+      final engine = GameEngine(
+        _stage(
+          preset: _almostFullRow(3),
+          objectives: [
+            {'type': 'clearLines', 'count': 1},
+          ],
+          moveLimit: 10,
+        ),
+        generator: _smallPool(1),
+      );
+      final monoIndex = engine.tray.indexWhere((p) => p.id == 'mono');
+      engine.tryPlace(monoIndex, 0, 3);
+      expect(engine.phase, GamePhase.won);
+      // Won on move 1 of 10 -> 9 left >= 8 -> 3 stars.
+      expect(engine.starsEarned, 3);
+    });
+
+    test('gem clears feed the clearGems objective', () {
+      final engine = GameEngine(
+        _stage(
+          preset: [
+            const PresetCell(x: 7, y: 3, type: CellType.gem),
+            ..._almostFullRow(3).where((c) => c.x != 7),
+          ],
+          objectives: [
+            {'type': 'clearGems', 'count': 1},
+          ],
+        ),
+        generator: _smallPool(1),
+      );
+      final monoIndex = engine.tray.indexWhere((p) => p.id == 'mono');
+      engine.tryPlace(monoIndex, 0, 3);
+      expect(engine.phase, GamePhase.won);
+    });
+  });
+
+  group('out of moves and rescue', () {
+    test('exhausting the move limit gets stuck, extra moves resume', () {
+      final engine = GameEngine(
+        _stage(moveLimit: 1),
+        generator: _smallPool(1),
+      );
+      engine.tryPlace(0, 0, 0);
+      expect(engine.phase, GamePhase.stuck);
+      expect(engine.stuckReason, StuckReason.outOfMoves);
+
+      engine.addExtraMoves();
+      expect(engine.phase, GamePhase.playing);
+      expect(engine.movesLeft, 5);
+
+      engine.declineAndLose();
+      expect(engine.phase, GamePhase.lost);
+    });
+
+    test('rescue is one-shot per attempt', () {
+      final engine = GameEngine(
+        _stage(moveLimit: 1),
+        generator: _smallPool(1),
+      );
+      engine.tryPlace(0, 0, 0);
+      engine.addExtraMoves();
+      // Burn the granted moves without winning.
+      engine.tryPlace(0, 0, 2);
+      engine.tryPlace(0, 0, 4);
+      engine.tryPlace(0, 2, 0);
+      engine.tryPlace(0, 2, 2);
+      engine.tryPlace(0, 2, 4);
+      expect(engine.phase, GamePhase.stuck);
+      expect(() => engine.addExtraMoves(), throwsStateError);
+    });
+  });
+
+  group('dead board and continue', () {
+    StageConfig deadStage() {
+      // Checkerboard: only monos fit, and the injected pool has none small
+      // enough, so the board is dead from the deal.
+      final preset = <PresetCell>[
+        for (var y = 0; y < GridState.size; y++)
+          for (var x = 0; x < GridState.size; x++)
+            if ((x + y).isEven) PresetCell(x: x, y: y, type: CellType.filled),
+      ];
+      return _stage(preset: preset, moveLimit: 50);
+    }
+
+    PieceGenerator bigPool(int seed) => PieceGenerator(
+          SeededRng(seed),
+          pool: [
+            PieceLibrary.byId('square3'),
+            PieceLibrary.byId('line5_h'),
+            PieceLibrary.byId('line5_v'),
+          ],
+        );
+
+    test('a dead deal is detected immediately', () {
+      final engine = GameEngine(deadStage(), generator: bigPool(1));
+      expect(engine.phase, GamePhase.stuck);
+      expect(engine.stuckReason, StuckReason.boardDead);
+    });
+
+    test('continue clears the two most-filled rows and redeals', () {
+      final engine = GameEngine(deadStage(), generator: bigPool(1));
+      engine.useContinue();
+      expect(engine.phase, GamePhase.playing);
+      // 32 checkerboard cells minus two cleared rows (4 each).
+      expect(engine.grid.occupiedCount, 24);
+      expect(() => engine.useContinue(), throwsStateError);
+    });
+  });
+}
diff --git a/test/game/engine/headless_stress_test.dart b/test/game/engine/headless_stress_test.dart
new file mode 100644
index 0000000..aa08148
--- /dev/null
+++ b/test/game/engine/headless_stress_test.dart
@@ -0,0 +1,63 @@
+import 'package:block_seasons/core/rng.dart';
+import 'package:block_seasons/game/engine/game_engine.dart';
+import 'package:block_seasons/game/models/grid.dart';
+import 'package:block_seasons/game/models/stage.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+StageConfig _endlessStage(int seed) => StageConfig.fromJson({
+      'id': 'stress_$seed',
+      'seed': seed,
+      'moveLimit': 40,
+      'preset': const <Map<String, dynamic>>[],
+      'objectives': [
+        {'type': 'reachScore', 'target': 99999999},
+      ],
+      'stars': {
+        'two': {'movesLeft': 5},
+        'three': {'movesLeft': 10},
+      },
+      'generatorProfile': 'mid',
+    });
+
+void main() {
+  test('100 seeded random games run to a terminal state without violations',
+      () {
+    for (var seed = 0; seed < 100; seed++) {
+      final engine = GameEngine(_endlessStage(seed));
+      final rng = SeededRng(seed * 7919 + 1);
+      var guard = 0;
+
+      while (engine.phase == GamePhase.playing) {
+        guard++;
+        expect(guard, lessThan(300), reason: 'seed $seed: game never ended');
+
+        // Enumerate all legal moves; the engine promises at least one exists
+        // while phase == playing.
+        final moves = <(int, int, int)>[];
+        for (var i = 0; i < engine.tray.length; i++) {
+          for (var y = 0; y < GridState.size; y++) {
+            for (var x = 0; x < GridState.size; x++) {
+              if (engine.tryPlaceWouldSucceed(i, x, y)) moves.add((i, x, y));
+            }
+          }
+        }
+        expect(moves, isNotEmpty,
+            reason: 'seed $seed: playing phase but no legal move');
+
+        final (i, x, y) = moves[rng.nextInt(moves.length)];
+        final result = engine.tryPlace(i, x, y);
+        expect(result, isNotNull, reason: 'seed $seed: legal move rejected');
+
+        // Invariants after every move.
+        expect(engine.tray.length, inInclusiveRange(1, 3),
+            reason: 'seed $seed');
+        expect(engine.movesUsed, lessThanOrEqualTo(40), reason: 'seed $seed');
+        expect(engine.score, greaterThanOrEqualTo(0), reason: 'seed $seed');
+      }
+
+      // The score objective is unreachable, so every game ends stuck.
+      expect(engine.phase, GamePhase.stuck, reason: 'seed $seed');
+      expect(engine.stuckReason, isNotNull, reason: 'seed $seed');
+    }
+  });
+}
diff --git a/test/game/models/objective_test.dart b/test/game/models/objective_test.dart
new file mode 100644
index 0000000..2d1e17a
--- /dev/null
+++ b/test/game/models/objective_test.dart
@@ -0,0 +1,66 @@
+import 'package:block_seasons/game/engine/game_event.dart';
+import 'package:block_seasons/game/models/objective.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  group('ClearGems', () {
+    test('accumulates gems from clear events', () {
+      var obj = Objective.clearGems(6);
+      obj = obj.onEvent(const LinesCleared(lines: 1, gems: 2));
+      obj = obj.onEvent(const LinesCleared(lines: 2, gems: 3));
+      expect(obj.current, 5);
+      expect(obj.isComplete, isFalse);
+      obj = obj.onEvent(const LinesCleared(lines: 1, gems: 1));
+      expect(obj.isComplete, isTrue);
+    });
+
+    test('ignores unrelated events', () {
+      var obj = Objective.clearGems(3);
+      obj = obj.onEvent(const ScoreChanged(total: 9999));
+      expect(obj.current, 0);
+    });
+  });
+
+  group('ReachScore', () {
+    test('tracks the running total score', () {
+      var obj = Objective.reachScore(500);
+      obj = obj.onEvent(const ScoreChanged(total: 320));
+      expect(obj.current, 320);
+      expect(obj.isComplete, isFalse);
+      obj = obj.onEvent(const ScoreChanged(total: 510));
+      expect(obj.isComplete, isTrue);
+    });
+  });
+
+  group('ClearLines', () {
+    test('accumulates cleared lines', () {
+      var obj = Objective.clearLines(4);
+      obj = obj.onEvent(const LinesCleared(lines: 3, gems: 0));
+      expect(obj.current, 3);
+      obj = obj.onEvent(const LinesCleared(lines: 1, gems: 0));
+      expect(obj.isComplete, isTrue);
+    });
+  });
+
+  group('JSON', () {
+    test('round-trips all objective types', () {
+      for (final json in [
+        {'type': 'clearGems', 'count': 6},
+        {'type': 'reachScore', 'target': 1200},
+        {'type': 'clearLines', 'count': 10},
+      ]) {
+        final obj = Objective.fromJson(json);
+        expect(obj.toJson(), json);
+        expect(obj.current, 0);
+        expect(obj.target, greaterThan(0));
+      }
+    });
+
+    test('throws on unknown type', () {
+      expect(
+        () => Objective.fromJson({'type': 'collectStars', 'count': 1}),
+        throwsFormatException,
+      );
+    });
+  });
+}
diff --git a/test/game/models/stage_test.dart b/test/game/models/stage_test.dart
new file mode 100644
index 0000000..f069327
--- /dev/null
+++ b/test/game/models/stage_test.dart
@@ -0,0 +1,62 @@
+import 'package:block_seasons/game/models/cell.dart';
+import 'package:block_seasons/game/models/objective.dart';
+import 'package:block_seasons/game/models/stage.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+const sampleJson = {
+  'id': 's001_007',
+  'seed': 8841273,
+  'moveLimit': 22,
+  'preset': [
+    {'x': 3, 'y': 4, 't': 'gem'},
+    {'x': 0, 'y': 7, 't': 'filled', 'c': 2},
+  ],
+  'objectives': [
+    {'type': 'clearGems', 'count': 6},
+  ],
+  'stars': {
+    'two': {'movesLeft': 4},
+    'three': {'movesLeft': 8},
+  },
+  'generatorProfile': 'mid',
+};
+
+void main() {
+  group('StageConfig', () {
+    test('parses the season pack stage schema', () {
+      final stage = StageConfig.fromJson(sampleJson);
+      expect(stage.id, 's001_007');
+      expect(stage.seed, 8841273);
+      expect(stage.moveLimit, 22);
+      expect(stage.preset, hasLength(2));
+      expect(stage.preset[0].type, CellType.gem);
+      expect(stage.preset[1].colorId, 2);
+      expect(stage.objectives.single, isA<Objective>());
+      expect(stage.generatorProfile, 'mid');
+    });
+
+    test('round-trips to JSON', () {
+      final stage = StageConfig.fromJson(sampleJson);
+      expect(stage.toJson(), sampleJson);
+    });
+
+    test('star thresholds map moves left to stars', () {
+      final stage = StageConfig.fromJson(sampleJson);
+      expect(stage.stars.starsFor(movesLeft: 0), 1);
+      expect(stage.stars.starsFor(movesLeft: 3), 1);
+      expect(stage.stars.starsFor(movesLeft: 4), 2);
+      expect(stage.stars.starsFor(movesLeft: 7), 2);
+      expect(stage.stars.starsFor(movesLeft: 8), 3);
+      expect(stage.stars.starsFor(movesLeft: 20), 3);
+    });
+
+    test('builds the initial grid from preset cells', () {
+      final stage = StageConfig.fromJson(sampleJson);
+      final grid = stage.initialGrid();
+      expect(grid.cellAt(3, 4).type, CellType.gem);
+      expect(grid.cellAt(0, 7).type, CellType.filled);
+      expect(grid.cellAt(0, 7).colorId, 2);
+      expect(grid.occupiedCount, 2);
+    });
+  });
+}