diff --git a/app/Exceptions/SimilarGamesCapExceededException.php b/app/Exceptions/SimilarGamesCapExceededException.php
new file mode 100644
index 0000000000..9ee38dfcfd
--- /dev/null
+++ b/app/Exceptions/SimilarGamesCapExceededException.php
@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions;
+
+use RuntimeException;
+
+class SimilarGamesCapExceededException extends RuntimeException
+{
+    public function __construct(
+        public readonly int $offendingGameId,
+        public readonly int $cap,
+    ) {
+        parent::__construct("Game #{$offendingGameId} would exceed the {$cap}-similar-game cap.");
+    }
+}
diff --git a/app/Filament/Resources/GameResource/Pages/SimilarGames.php b/app/Filament/Resources/GameResource/Pages/SimilarGames.php
index 048a216644..9bd2df922e 100644
--- a/app/Filament/Resources/GameResource/Pages/SimilarGames.php
+++ b/app/Filament/Resources/GameResource/Pages/SimilarGames.php
@@ -4,6 +4,7 @@
 
 namespace App\Filament\Resources\GameResource\Pages;
 
+use App\Exceptions\SimilarGamesCapExceededException;
 use App\Filament\Actions\ParseIdsFromCsvAction;
 use App\Filament\Resources\GameResource;
 use App\Filament\Resources\SystemResource;
@@ -61,7 +62,8 @@ public static function getNavigationItems(array $urlParameters = []): array
     {
         $item = parent::getNavigationItems($urlParameters)[0];
         if (($record = $urlParameters['record'] ?? null) instanceof Game) {
-            $item->badge((string) $record->similarGamesList->count());
+            $count = $record->similarGamesList->count();
+            $item->badge("{$count} / " . LinkSimilarGamesAction::MAX_SIMILAR_GAMES);
         }
 
         return [$item];
@@ -127,6 +129,10 @@ public function table(Table $table): Table
             ->headerActions([
                 Actions\Action::make('add')
                     ->label('Add similar games')
+                    ->disabled(fn (): bool => $this->isSimilarGamesCapReached())
+                    ->tooltip(fn (): ?string => $this->isSimilarGamesCapReached()
+                        ? 'This game is at the ' . LinkSimilarGamesAction::MAX_SIMILAR_GAMES . '-similar-game cap. Remove a game before adding more.'
+                        : null)
                     ->schema([
                         Forms\Components\TextInput::make('game_ids_csv')
                             ->label('Game IDs (CSV)')
@@ -181,18 +187,34 @@ public function table(Table $table): Table
                         /** @var Game $game */
                         $game = $this->getOwnerRecord();
 
-                        $gameIds = [];
+                        $selectIds = !empty($data['game_ids']) ? array_map('intval', $data['game_ids']) : [];
+                        $csvIds = !empty($data['game_ids_csv'])
+                            ? (new ParseIdsFromCsvAction())->execute($data['game_ids_csv'])
+                            : [];
+                        $gameIds = array_values(array_unique(array_merge($selectIds, $csvIds)));
 
-                        // Handle select field input.
-                        if (!empty($data['game_ids'])) {
-                            $gameIds = $data['game_ids'];
-                            (new LinkSimilarGamesAction())->execute($game, $gameIds);
+                        if (empty($gameIds)) {
+                            return;
                         }
 
-                        // Handle CSV input.
-                        if (!empty($data['game_ids_csv'])) {
-                            $gameIds = (new ParseIdsFromCsvAction())->execute($data['game_ids_csv']);
+                        try {
                             (new LinkSimilarGamesAction())->execute($game, $gameIds);
+                        } catch (SimilarGamesCapExceededException $e) {
+                            if ($e->offendingGameId === $game->id) {
+                                $body = "Adding these games would push this game's similar list over the {$e->cap}-similar-game cap. Remove an entry or add fewer games.";
+                            } else {
+                                $offending = Game::find($e->offendingGameId);
+                                $title = $offending?->title ?? "Game #{$e->offendingGameId}";
+                                $body = "{$title} is already at the {$e->cap}-similar-game cap and cannot be linked.";
+                            }
+
+                            Notification::make()
+                                ->danger()
+                                ->title('Cap exceeded')
+                                ->body($body)
+                                ->send();
+
+                            return;
                         }
 
                         Notification::make()
@@ -214,6 +236,7 @@ public function table(Table $table): Table
                         $game = $this->getOwnerRecord();
 
                         (new UnlinkSimilarGamesAction())->execute($game, [$similarGame->id]);
+                        $this->isSimilarGamesCapReached = null;
 
                         Notification::make()
                             ->success()
@@ -237,6 +260,7 @@ public function table(Table $table): Table
                             $game,
                             $similarGames->pluck('id')->toArray()
                         );
+                        $this->isSimilarGamesCapReached = null;
 
                         $this->deselectAllTableRecords();
 
@@ -249,6 +273,20 @@ public function table(Table $table): Table
             ]);
     }
 
+    private ?bool $isSimilarGamesCapReached = null;
+
+    private function isSimilarGamesCapReached(): bool
+    {
+        if ($this->isSimilarGamesCapReached === null) {
+            /** @var Game $game */
+            $game = $this->getOwnerRecord();
+            $similarGamesCount = $game->similarGamesList()->count();
+            $this->isSimilarGamesCapReached = $similarGamesCount >= LinkSimilarGamesAction::MAX_SIMILAR_GAMES;
+        }
+
+        return $this->isSimilarGamesCapReached;
+    }
+
     /**
      * @param Builder<Game> $query
      * @return Builder<Game>
diff --git a/app/Platform/Actions/LinkSimilarGamesAction.php b/app/Platform/Actions/LinkSimilarGamesAction.php
index f279c6c6b9..a98ed05f8f 100644
--- a/app/Platform/Actions/LinkSimilarGamesAction.php
+++ b/app/Platform/Actions/LinkSimilarGamesAction.php
@@ -4,46 +4,76 @@
 
 namespace App\Platform\Actions;
 
+use App\Exceptions\SimilarGamesCapExceededException;
 use App\Models\Game;
 use App\Models\GameSet;
-use App\Models\GameSetGame;
 use App\Platform\Enums\GameSetType;
+use Illuminate\Support\Facades\DB;
 
 class LinkSimilarGamesAction
 {
+    public const int MAX_SIMILAR_GAMES = 6;
+
     public function execute(Game $parentGame, array $gameIdsToLink): void
     {
-        $parentSimilarGamesSet = GameSet::firstOrCreate([
+        // normalize to a list of ints
+        $gameIdsToLink = array_values(array_unique(array_map('intval', $gameIdsToLink)));
+
+        $parentSimilarGamesSet = GameSet::firstOrNew([
             'type' => GameSetType::SimilarGames,
             'game_id' => $parentGame->id,
         ]);
 
-        $existingSimilarGames = GameSetGame::query()
-            ->where('game_set_id', $parentSimilarGamesSet->id)
-            ->whereIn('game_id', $gameIdsToLink)
-            ->pluck('game_id')
-            ->toArray();
+        $existingParentLinkedIds = $parentSimilarGamesSet->exists
+            ? $parentSimilarGamesSet->games()->pluck('games.id')->all()
+            : [];
+
+        $newSetGameIds = array_values(array_diff($gameIdsToLink, $existingParentLinkedIds));
 
-        $newSetGameIds = array_diff($gameIdsToLink, $existingSimilarGames);
-        if (!empty($newSetGameIds)) {
-            $parentSimilarGamesSet->games()->attach($newSetGameIds);
+        $existingParentCount = count($existingParentLinkedIds);
+        if ($existingParentCount + count($newSetGameIds) > self::MAX_SIMILAR_GAMES) {
+            throw new SimilarGamesCapExceededException($parentGame->id, self::MAX_SIMILAR_GAMES);
         }
 
-        // Link each game's similar games set to include the parent game.
+        /** @var GameSet[] $targetSets */
+        $targetSets = [];
         foreach ($gameIdsToLink as $gameId) {
-            $similarGamesSet = GameSet::firstOrCreate([
+            $candidateSet = GameSet::firstOrNew([
                 'type' => GameSetType::SimilarGames,
                 'game_id' => $gameId,
             ]);
 
-            $isAlreadyAttached = GameSetGame::query()
-                ->where('game_set_id', $similarGamesSet->id)
-                ->where('game_id', $parentGame->id)
-                ->exists();
+            if ($candidateSet->exists && $candidateSet->games()->where('games.id', $parentGame->id)->exists()) {
+                continue;
+            }
 
-            if (!$isAlreadyAttached) {
-                $similarGamesSet->games()->attach($parentGame->id);
+            $candidateCount = $candidateSet->exists ? $candidateSet->games()->count() : 0;
+            if ($candidateCount + 1 > self::MAX_SIMILAR_GAMES) {
+                throw new SimilarGamesCapExceededException($gameId, self::MAX_SIMILAR_GAMES);
             }
+
+            $targetSets[] = $candidateSet;
         }
+
+        if (empty($newSetGameIds) && empty($targetSets)) {
+            return;
+        }
+
+        // writes only happen if validation passed
+        DB::transaction(function () use ($parentGame, $parentSimilarGamesSet, $newSetGameIds, $targetSets): void {
+            if (!empty($newSetGameIds)) {
+                if (!$parentSimilarGamesSet->exists) {
+                    $parentSimilarGamesSet->save();
+                }
+                $parentSimilarGamesSet->games()->attach($newSetGameIds);
+            }
+
+            foreach ($targetSets as $candidateSet) {
+                if (!$candidateSet->exists) {
+                    $candidateSet->save();
+                }
+                $candidateSet->games()->attach($parentGame->id);
+            }
+        });
     }
 }