diff --git a/Dockerfile b/Dockerfile
index b1cbf1b..3e43bbd 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -113,6 +113,10 @@ RUN <<-EOF
 	chmod +x bin/console; sync
 EOF
 
+RUN chown -R 1000:1000 vendor
+
+RUN chmod -R 775 vendor
+
 # Collect shared libraries needed by FrankenPHP and PHP extensions
 # hadolint ignore=DL3008,SC3054,DL4006
 RUN <<-'EOF'
diff --git a/claude.md b/claude.md
index 70fe5a3..11ddae1 100644
--- a/claude.md
+++ b/claude.md
@@ -256,6 +256,26 @@ Card
 ```
 
 Filters on Card endpoint that target CardGroup fields go through `CardGroupAliasFilter` / `CardGroupOrderFilter`, which create (or reuse) a single `cardGroup` join.
+
+---
+
+## Deck formats
+
+Formats are not a DB table — they're a `DeckFormat` enum (`src/Enum/DeckFormat.php`) plus one `DeckFormatValidator` per format (`src/Validator/Format/`), all extending `AbstractDeckFormatValidator`. Adding a format needs no migration: just an enum case + a validator class (auto-tagged via `DeckFormatValidatorInterface`) + a metadata entry in `FormatController`.
+
+### Frontier format — allowlist via `gameplayFormat` on card data
+
+`FrontierFormatValidator` extends `StandardFormatValidator` (same rules: 39-59 cards, max 3 copies/name, max 3 Unique, max 15 rare, max 3 exalted) and adds one more check: every Unique card in the deck must carry the `"frontier"` key in its `gameplayFormat` array.
+
+`gameplayFormat` (`string[]`) is a field on `CardGroup` in `altered-core-cards-api`, synced from the Altered Reunion formats manifest and flattened onto each `Card`'s JSON by `CardNormalizer` — it arrives for free in the payload `AlteredCoreClient::getCardsByReferences()` already fetches, no extra call needed:
+
+```php
+$gameplayFormats = $cardsData[$ref]['gameplayFormat'] ?? [];
+$isFrontierLegal = in_array('frontier', $gameplayFormats, true);
+```
+
+This replaces the previous design that called a sibling `uniques-search-api` service for a live allowlist lookup (`UniquesSearchApiClient` / `UNIQUES_SEARCH_API_URL` — both removed). No fail-closed handling is needed anymore: if `AlteredCoreClient` can't fetch card data at all, validation already fails upstream for unrelated reasons.
+
 ---
 
 ## Response shape reference
diff --git a/src/Controller/FormatController.php b/src/Controller/FormatController.php
index c7cc356..d4fdaed 100644
--- a/src/Controller/FormatController.php
+++ b/src/Controller/FormatController.php
@@ -76,6 +76,21 @@ public function __invoke(Request $request): JsonResponse
                     'maxCopiesPerRarity' => null,
                 ],
             ],
+            [
+                'code' => 'frontier',
+                'label' => 'Frontier',
+                'minCards' => 39,
+                'maxCards' => 59,
+                'allowBanned' => false,
+                'allowSuspended' => false,
+                'limits' => [
+                    'unique' => 3,
+                    'rare' => 15,
+                    'exalted' => 3,
+                    'maxCopiesPerName' => 3,
+                    'maxCopiesPerRarity' => null,
+                ],
+            ],
             [
                 'code' => 'singleton',
                 'label' => 'Singleton',
diff --git a/src/Enum/DeckFormat.php b/src/Enum/DeckFormat.php
index 527b67d..5758e15 100644
--- a/src/Enum/DeckFormat.php
+++ b/src/Enum/DeckFormat.php
@@ -10,6 +10,7 @@ enum DeckFormat: string
     case SingletonNuc = 'singleton_nuc';
     case Sandbox = 'sandbox';
     case Test = 'test';
+    case Frontier = 'frontier';
 
     public static function fromInput(string $value): self
     {
@@ -20,6 +21,7 @@ public static function fromInput(string $value): self
             'singleton_nuc', 'singleton_no_unique' => self::SingletonNuc,
             'sandbox' => self::Sandbox,
             'test' => self::Test,
+            'frontier' => self::Frontier,
             default => throw new \ValueError(sprintf('"%s" is not a valid DeckFormat.', $value)),
         };
     }
diff --git a/src/Validator/Format/FrontierFormatValidator.php b/src/Validator/Format/FrontierFormatValidator.php
new file mode 100644
index 0000000..a0ef4c6
--- /dev/null
+++ b/src/Validator/Format/FrontierFormatValidator.php
@@ -0,0 +1,57 @@
+<?php
+
+namespace App\Validator\Format;
+
+use App\Entity\Deck;
+
+/**
+ * Frontier format rules:
+ * Same as Standard, plus every Unique card in the deck must carry the
+ * "frontier" gameplay format key. That key is synced onto CardGroup by
+ * altered-core-cards-api from the Altered Reunion formats manifest and
+ * exposed on card data as `gameplayFormat` (string[]).
+ */
+class FrontierFormatValidator extends StandardFormatValidator
+{
+    private const string GAMEPLAY_FORMAT_KEY = 'frontier';
+
+    public function getFormat(): string
+    {
+        return 'frontier';
+    }
+
+    public function validate(Deck $deck, array $cardsData): array
+    {
+        return array_merge(parent::validate($deck, $cardsData), $this->validateFrontierUniques($deck, $cardsData));
+    }
+
+    public function computeLegalityDetail(Deck $deck, array $cardsData): array
+    {
+        $detail = parent::computeLegalityDetail($deck, $cardsData);
+        $detail['frontierUniques'] = [] === $this->validateFrontierUniques($deck, $cardsData);
+        $detail['global'] = $detail['global'] && $detail['frontierUniques'];
+
+        return $detail;
+    }
+
+    /**
+     * @return string[]
+     */
+    private function validateFrontierUniques(Deck $deck, array $cardsData): array
+    {
+        $errors = [];
+        foreach ($deck->getDeckCards() as $deckCard) {
+            $data = $cardsData[$deckCard->getCardReference()] ?? [];
+            if ('U' !== $this->getRarityCode($data)) {
+                continue;
+            }
+
+            $gameplayFormats = $data['gameplayFormat'] ?? [];
+            if (!in_array(self::GAMEPLAY_FORMAT_KEY, $gameplayFormats, true)) {
+                $errors[] = sprintf('Unique card "%s" is not part of the Frontier format allowlist.', $this->getCardName($data));
+            }
+        }
+
+        return $errors;
+    }
+}
diff --git a/tests/Controller/FormatControllerTest.php b/tests/Controller/FormatControllerTest.php
index d048896..9f1ec09 100644
--- a/tests/Controller/FormatControllerTest.php
+++ b/tests/Controller/FormatControllerTest.php
@@ -22,6 +22,7 @@ public function testFormatsEndpointReturnsAllFormats(): void
         self::assertContains('standard', $codes);
         self::assertContains('singleton', $codes);
         self::assertContains('singleton_nuc', $codes);
+        self::assertContains('frontier', $codes);
     }
 
     public function testHiddenFormatsAreNotReturnedByDefault(): void
@@ -83,6 +84,7 @@ public static function strictFormatsProvider(): array
             ['standard'],
             ['singleton'],
             ['singleton_nuc'],
+            ['frontier'],
         ];
     }
 
diff --git a/tests/Validator/Format/FrontierFormatValidatorTest.php b/tests/Validator/Format/FrontierFormatValidatorTest.php
new file mode 100644
index 0000000..763a381
--- /dev/null
+++ b/tests/Validator/Format/FrontierFormatValidatorTest.php
@@ -0,0 +1,154 @@
+<?php
+
+namespace App\Tests\Validator\Format;
+
+use App\Entity\Deck;
+use App\Entity\DeckCard;
+use App\Validator\Format\FrontierFormatValidator;
+use Doctrine\Common\Collections\ArrayCollection;
+use PHPUnit\Framework\TestCase;
+
+class FrontierFormatValidatorTest extends TestCase
+{
+    private FrontierFormatValidator $validator;
+
+    protected function setUp(): void
+    {
+        $this->validator = new FrontierFormatValidator();
+    }
+
+    // ── Helpers ───────────────────────────────────────────────────────────────
+
+    private function card(string $ref, int $qty = 1): DeckCard
+    {
+        $card = new DeckCard();
+        $card->setCardReference($ref);
+        $card->setQuantity($qty);
+
+        return $card;
+    }
+
+    private function deck(DeckCard ...$cards): Deck
+    {
+        $deck = $this->createStub(Deck::class);
+        $deck->method('getDeckCards')->willReturn(new ArrayCollection($cards));
+
+        return $deck;
+    }
+
+    private function data(
+        string $ref,
+        string $typeRef = 'PERMANENT',
+        string $faction = 'AX',
+        string $rarityRef = 'COMMON',
+        string $name = '',
+        array $gameplayFormat = [],
+    ): array {
+        return [
+            'reference' => $ref,
+            'cardType' => ['reference' => $typeRef],
+            'faction' => ['code' => $faction],
+            'rarity' => ['reference' => $rarityRef],
+            'name' => $name ?: $ref,
+            'gameplayFormat' => $gameplayFormat,
+        ];
+    }
+
+    /**
+     * Builds 1 hero + 13 distinct commons × qty 3 = 39 non-hero cards.
+     *
+     * @return array{0: array<string, array>, 1: DeckCard[]}
+     */
+    private function buildMinimalValidDeck(string $faction = 'AX'): array
+    {
+        $cardsData = [];
+        $deckCards = [];
+
+        $heroRef = 'ALT_CORE_B_AX_0_C';
+        $deckCards[] = $this->card($heroRef, 1);
+        $cardsData[$heroRef] = $this->data($heroRef, 'HERO_MAIN', $faction);
+
+        for ($i = 1; $i <= 13; ++$i) {
+            $ref = sprintf('ALT_CORE_B_%s_%d_C', $faction, $i);
+            $deckCards[] = $this->card($ref, 3);
+            $cardsData[$ref] = $this->data($ref, 'PERMANENT', $faction);
+        }
+
+        return [$cardsData, $deckCards];
+    }
+
+    // ── Happy path ────────────────────────────────────────────────────────────
+
+    public function testDeckWithNoUniqueIsValid(): void
+    {
+        [$cardsData, $deckCards] = $this->buildMinimalValidDeck();
+
+        $errors = $this->validator->validate($this->deck(...$deckCards), $cardsData);
+
+        self::assertSame([], $errors);
+    }
+
+    public function testAllUniquesFlaggedFrontierIsValid(): void
+    {
+        [$cardsData, $deckCards] = $this->buildMinimalValidDeck();
+
+        for ($i = 1; $i <= 3; ++$i) {
+            $ref = sprintf('ALT_CORE_B_AX_%d_U', $i);
+            $deckCards[] = $this->card($ref, 1);
+            $cardsData[$ref] = $this->data($ref, 'PERMANENT', 'AX', 'UNIQUE', "Unique $i", ['frontier']);
+        }
+
+        $errors = $this->validator->validate($this->deck(...$deckCards), $cardsData);
+
+        self::assertSame([], $errors);
+    }
+
+    // ── Frontier allowlist ────────────────────────────────────────────────────
+
+    public function testUniqueNotFlaggedFrontierReturnsError(): void
+    {
+        [$cardsData, $deckCards] = $this->buildMinimalValidDeck();
+
+        $allowedRef = 'ALT_CORE_B_AX_1_U';
+        $rejectedRef = 'ALT_CORE_B_AX_2_U';
+        $deckCards[] = $this->card($allowedRef, 1);
+        $deckCards[] = $this->card($rejectedRef, 1);
+        $cardsData[$allowedRef] = $this->data($allowedRef, 'PERMANENT', 'AX', 'UNIQUE', 'Allowed Unique', ['frontier']);
+        $cardsData[$rejectedRef] = $this->data($rejectedRef, 'PERMANENT', 'AX', 'UNIQUE', 'Rejected Unique', []);
+
+        $errors = $this->validator->validate($this->deck(...$deckCards), $cardsData);
+
+        self::assertNotEmpty(array_filter($errors, fn ($e) => str_contains($e, 'Rejected Unique') && str_contains($e, 'Frontier')));
+    }
+
+    public function testUniqueNotFlaggedFrontierMarksLegalityDetailFalse(): void
+    {
+        [$cardsData, $deckCards] = $this->buildMinimalValidDeck();
+
+        $rejectedRef = 'ALT_CORE_B_AX_1_U';
+        $deckCards[] = $this->card($rejectedRef, 1);
+        $cardsData[$rejectedRef] = $this->data($rejectedRef, 'PERMANENT', 'AX', 'UNIQUE', 'Rejected Unique', []);
+
+        $detail = $this->validator->computeLegalityDetail($this->deck(...$deckCards), $cardsData);
+
+        self::assertFalse($detail['frontierUniques']);
+        self::assertFalse($detail['global']);
+    }
+
+    // ── Inherited Standard rules still apply ─────────────────────────────────
+
+    public function testInheritsStandardMaxUniqueLimit(): void
+    {
+        [$cardsData, $deckCards] = $this->buildMinimalValidDeck();
+
+        for ($i = 1; $i <= 4; ++$i) {
+            $ref = sprintf('ALT_CORE_B_AX_%d_U', $i);
+            $deckCards[] = $this->card($ref, 1);
+            $cardsData[$ref] = $this->data($ref, 'PERMANENT', 'AX', 'UNIQUE', "Unique $i", ['frontier']);
+        }
+
+        $errors = $this->validator->validate($this->deck(...$deckCards), $cardsData);
+
+        self::assertNotEmpty(array_filter($errors, fn ($e) => str_contains($e, 'Unique cards')));
+    }
+}