diff --git a/tests/Unit/Services/ApplyPromoCodeTaskConcurrencyTest.php b/tests/Unit/Services/ApplyPromoCodeTaskConcurrencyTest.php
new file mode 100644
index 000000000..fbb7174ec
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+++ b/tests/Unit/Services/ApplyPromoCodeTaskConcurrencyTest.php
@@ -0,0 +1,328 @@
+<?php namespace Tests\Unit\Services;
+/**
+ * Copyright 2026 OpenStack Foundation
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * http://www.apache.org/licenses/LICENSE-2.0
+ **/
+
+use App\Services\Model\ApplyPromoCodeTask;
+use App\Services\Utils\ILockManagerService;
+use libs\utils\ITransactionService;
+use models\exceptions\ValidationException;
+use models\main\Member;
+use models\summit\IDomainAuthorizedPromoCode;
+use models\summit\ISummitRegistrationPromoCodeRepository;
+use models\summit\ISummitRepository;
+use models\summit\Summit;
+use models\summit\SummitRegistrationPromoCode;
+use models\summit\SummitTicketType;
+use Mockery;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Simulates concurrent QuantityPerAccount enforcement in ApplyPromoCodeTask
+ * by controlling what getTicketCountByMemberAndPromoCode returns for each call.
+ *
+ * The race condition: two ReserveOrderTask executions commit tickets before
+ * either ApplyPromoCodeTask runs, so both tasks see the combined count and
+ * both reject — even though individually each was valid. These tests document
+ * that behavior and verify the serialized (correct) path.
+ *
+ * No real DB or pcntl_fork needed — the race is deterministic once we control
+ * the mock repository return values for each task invocation.
+ *
+ * See PR #525 for full context on the TOCTOU risk.
+ */
+class ApplyPromoCodeTaskConcurrencyTest extends TestCase
+{
+    protected function setUp(): void
+    {
+        parent::setUp();
+        \Illuminate\Support\Facades\Facade::clearResolvedInstances();
+        $container = new \Illuminate\Container\Container();
+        $container->instance('app', $container);
+        $container->instance('log', new class {
+            public function __call($name, $args) { /* swallow */ }
+        });
+        \Illuminate\Support\Facades\Facade::setFacadeApplication($container);
+    }
+
+    protected function tearDown(): void
+    {
+        \Illuminate\Support\Facades\Facade::clearResolvedInstances();
+        \Illuminate\Support\Facades\Facade::setFacadeApplication(null);
+        Mockery::close();
+        parent::tearDown();
+    }
+
+    /**
+     * Serialized execution (FOR UPDATE lock works correctly):
+     *
+     * - Limit = 1, each request buys 1 ticket.
+     * - Task A runs first: count = 1 (own ticket only, B hasn't committed yet).
+     *   Guard: 1 > 1 = false → passes, calls addUsage.
+     * - Task B runs second: count = 2 (A's committed ticket + B's own).
+     *   Guard: 2 > 1 = true → rejects.
+     *
+     * This is the correct behavior under serialization.
+     */
+    public function testSerializedExecution_FirstSucceeds_SecondRejects(): void
+    {
+        $promo_code_value = 'DOMAIN-CODE-1';
+        $ticket_type_id = 42;
+        $quantityPerAccountLimit = 1;
+
+        $ticket_type = Mockery::mock(SummitTicketType::class);
+
+        $summit = Mockery::mock(Summit::class);
+        $summit->shouldReceive('getId')->andReturn(1);
+        $summit->shouldReceive('getTicketTypeById')->with($ticket_type_id)->andReturn($ticket_type);
+
+        $promo_code = Mockery::mock(SummitRegistrationPromoCode::class, IDomainAuthorizedPromoCode::class);
+        $promo_code->shouldReceive('getSummitId')->andReturn(1);
+        $promo_code->shouldReceive('getId')->andReturn(101);
+        $promo_code->shouldReceive('getCode')->andReturn($promo_code_value);
+        $promo_code->shouldReceive('validate');
+        $promo_code->shouldReceive('canBeAppliedTo')->with($ticket_type)->andReturn(true);
+        $promo_code->shouldReceive('getQuantityPerAccount')->andReturn($quantityPerAccountLimit);
+        // Only Task A succeeds — exactly one addUsage call.
+        $promo_code->shouldReceive('addUsage')->once();
+
+        $owner = Mockery::mock(Member::class);
+
+        $repo = Mockery::mock(ISummitRegistrationPromoCodeRepository::class);
+        $repo->shouldReceive('getByValueExclusiveLock')
+            ->with($summit, $promo_code_value)
+            ->andReturn($promo_code);
+
+        // Serialized: Task A sees count=1, Task B sees count=2.
+        $repo->shouldReceive('getTicketCountByMemberAndPromoCode')
+            ->with($owner, $promo_code)
+            ->twice()
+            ->andReturnValues([1, 2]);
+
+        $tx_service = Mockery::mock(ITransactionService::class);
+        $tx_service->shouldReceive('transaction')->andReturnUsing(fn($fn) => $fn());
+
+        $lock_service = Mockery::mock(ILockManagerService::class);
+        $lock_service->shouldReceive('lock')->andReturnUsing(fn($_k, $fn) => $fn());
+
+        $this->bindSummitRepository($summit);
+
+        $formerState = [
+            'promo_codes_usage' => [
+                $promo_code_value => [
+                    'qty'   => 1,
+                    'types' => [$ticket_type_id],
+                ],
+            ],
+        ];
+
+        // --- Task A: should succeed ---
+        $taskA = new ApplyPromoCodeTask(
+            $summit, ['owner_email' => 'buyer@example.com'], $owner,
+            $repo, $tx_service, $lock_service,
+        );
+        $resultA = $taskA->run($formerState);
+        $this->assertTrue($resultA['promo_codes_usage'][$promo_code_value]['redeem']);
+
+        // --- Task B: should reject ---
+        $taskB = new ApplyPromoCodeTask(
+            $summit, ['owner_email' => 'buyer@example.com'], $owner,
+            $repo, $tx_service, $lock_service,
+        );
+
+        $this->expectException(ValidationException::class);
+        $this->expectExceptionMessageMatches('/reached the maximum of 1/');
+        $taskB->run($formerState);
+    }
+
+    /**
+     * TOCTOU double-rejection bug — demonstrates the race condition.
+     *
+     * Both ReserveOrderTask executions commit before either ApplyPromoCodeTask runs.
+     * Both tasks see count = 2 (both requests' tickets visible in the DB).
+     *
+     * - Limit = 1, each request buys 1 ticket.
+     * - CORRECT behavior: Task A should succeed (it was the first valid request),
+     *   only Task B should reject.
+     * - ACTUAL behavior (bug): both see count = 2 → 2 > 1 → both reject.
+     *
+     * This test asserts the CORRECT behavior and is expected to FAIL until
+     * the TOCTOU race is fixed (e.g., by moving the count inside the
+     * exclusive lock or deducting the current order's own tickets from the count).
+     */
+    public function testDoubleRejection_BothReservedBeforeEitherValidates(): void
+    {
+        $promo_code_value = 'DOMAIN-CODE-1';
+        $ticket_type_id = 42;
+        $quantityPerAccountLimit = 1;
+
+        $ticket_type = Mockery::mock(SummitTicketType::class);
+
+        $summit = Mockery::mock(Summit::class);
+        $summit->shouldReceive('getId')->andReturn(1);
+        $summit->shouldReceive('getTicketTypeById')->with($ticket_type_id)->andReturn($ticket_type);
+
+        $promo_code = Mockery::mock(SummitRegistrationPromoCode::class, IDomainAuthorizedPromoCode::class);
+        $promo_code->shouldReceive('getSummitId')->andReturn(1);
+        $promo_code->shouldReceive('getId')->andReturn(101);
+        $promo_code->shouldReceive('getCode')->andReturn($promo_code_value);
+        $promo_code->shouldReceive('validate');
+        $promo_code->shouldReceive('canBeAppliedTo')->with($ticket_type)->andReturn(true);
+        $promo_code->shouldReceive('getQuantityPerAccount')->andReturn($quantityPerAccountLimit);
+        // Permissive — correct behavior calls addUsage once, bug calls it zero times.
+        $promo_code->shouldReceive('addUsage')->zeroOrMoreTimes();
+
+        $owner = Mockery::mock(Member::class);
+
+        $repo = Mockery::mock(ISummitRegistrationPromoCodeRepository::class);
+        $repo->shouldReceive('getByValueExclusiveLock')
+            ->with($summit, $promo_code_value)
+            ->andReturn($promo_code);
+
+        // Both tasks see the inflated count (both orders' tickets visible).
+        $repo->shouldReceive('getTicketCountByMemberAndPromoCode')
+            ->with($owner, $promo_code)
+            ->andReturn(2);
+
+        $tx_service = Mockery::mock(ITransactionService::class);
+        $tx_service->shouldReceive('transaction')->andReturnUsing(fn($fn) => $fn());
+
+        // Permissive — correct behavior reaches lock, bug throws before it.
+        $lock_service = Mockery::mock(ILockManagerService::class);
+        $lock_service->shouldReceive('lock')->zeroOrMoreTimes()->andReturnUsing(fn($_k, $fn) => $fn());
+
+        $this->bindSummitRepository($summit);
+
+        $formerState = [
+            'promo_codes_usage' => [
+                $promo_code_value => [
+                    'qty'   => 1,
+                    'types' => [$ticket_type_id],
+                ],
+            ],
+        ];
+
+        // --- Task A: SHOULD succeed (first valid request) ---
+        // BUG: Task A sees count=2 (includes Task B's tickets) and rejects.
+        $taskA = new ApplyPromoCodeTask(
+            $summit, ['owner_email' => 'buyer@example.com'], $owner,
+            $repo, $tx_service, $lock_service,
+        );
+        try {
+            $resultA = $taskA->run($formerState);
+        } catch (ValidationException $ex) {
+            $this->fail(
+                'TOCTOU BUG: Task A was incorrectly rejected. '
+                . 'When two ReserveOrderTask executions commit before either ApplyPromoCodeTask runs, '
+                . 'both tasks see an inflated ticket count (2) and both reject — even though '
+                . 'Task A is a valid first request within the limit of 1. '
+                . 'Exception: ' . $ex->getMessage()
+            );
+        }
+        $this->assertTrue($resultA['promo_codes_usage'][$promo_code_value]['redeem']);
+
+        // --- Task B: should reject (over limit) ---
+        $taskB = new ApplyPromoCodeTask(
+            $summit, ['owner_email' => 'buyer@example.com'], $owner,
+            $repo, $tx_service, $lock_service,
+        );
+        $this->expectException(ValidationException::class);
+        $this->expectExceptionMessageMatches('/reached the maximum of 1/');
+        $taskB->run($formerState);
+    }
+
+    /**
+     * Serialized execution with higher limit — both requests succeed:
+     *
+     * - Limit = 2, each request buys 1 ticket.
+     * - Task A runs first: count = 1 → 1 > 2 = false → passes.
+     * - Task B runs second: count = 2 → 2 > 2 = false → passes.
+     *
+     * Confirms that serialized execution correctly allows both requests
+     * when the combined total stays within the limit.
+     */
+    public function testSerializedExecution_BothAllowedWithinLimit(): void
+    {
+        $promo_code_value = 'DOMAIN-CODE-1';
+        $ticket_type_id = 42;
+        $quantityPerAccountLimit = 2;
+
+        $ticket_type = Mockery::mock(SummitTicketType::class);
+
+        $summit = Mockery::mock(Summit::class);
+        $summit->shouldReceive('getId')->andReturn(1);
+        $summit->shouldReceive('getTicketTypeById')->with($ticket_type_id)->andReturn($ticket_type);
+
+        $promo_code = Mockery::mock(SummitRegistrationPromoCode::class, IDomainAuthorizedPromoCode::class);
+        $promo_code->shouldReceive('getSummitId')->andReturn(1);
+        $promo_code->shouldReceive('getId')->andReturn(101);
+        $promo_code->shouldReceive('getCode')->andReturn($promo_code_value);
+        $promo_code->shouldReceive('validate');
+        $promo_code->shouldReceive('canBeAppliedTo')->with($ticket_type)->andReturn(true);
+        $promo_code->shouldReceive('getQuantityPerAccount')->andReturn($quantityPerAccountLimit);
+        // Both tasks succeed — two addUsage calls.
+        $promo_code->shouldReceive('addUsage')->twice();
+
+        $owner = Mockery::mock(Member::class);
+
+        $repo = Mockery::mock(ISummitRegistrationPromoCodeRepository::class);
+        $repo->shouldReceive('getByValueExclusiveLock')
+            ->with($summit, $promo_code_value)
+            ->andReturn($promo_code);
+
+        // Serialized: Task A sees count=1, Task B sees count=2.
+        $repo->shouldReceive('getTicketCountByMemberAndPromoCode')
+            ->with($owner, $promo_code)
+            ->twice()
+            ->andReturnValues([1, 2]);
+
+        $tx_service = Mockery::mock(ITransactionService::class);
+        $tx_service->shouldReceive('transaction')->andReturnUsing(fn($fn) => $fn());
+
+        $lock_service = Mockery::mock(ILockManagerService::class);
+        $lock_service->shouldReceive('lock')->andReturnUsing(fn($_k, $fn) => $fn());
+
+        $this->bindSummitRepository($summit);
+
+        $formerState = [
+            'promo_codes_usage' => [
+                $promo_code_value => [
+                    'qty'   => 1,
+                    'types' => [$ticket_type_id],
+                ],
+            ],
+        ];
+
+        // --- Task A: should succeed ---
+        $taskA = new ApplyPromoCodeTask(
+            $summit, ['owner_email' => 'buyer@example.com'], $owner,
+            $repo, $tx_service, $lock_service,
+        );
+        $resultA = $taskA->run($formerState);
+        $this->assertTrue($resultA['promo_codes_usage'][$promo_code_value]['redeem']);
+
+        // --- Task B: should also succeed ---
+        $taskB = new ApplyPromoCodeTask(
+            $summit, ['owner_email' => 'buyer@example.com'], $owner,
+            $repo, $tx_service, $lock_service,
+        );
+        $resultB = $taskB->run($formerState);
+        $this->assertTrue($resultB['promo_codes_usage'][$promo_code_value]['redeem']);
+    }
+
+    /**
+     * Bind a mock ISummitRepository so ApplyPromoCodeTask::run() can re-attach the summit.
+     */
+    private function bindSummitRepository(Summit $summit): void
+    {
+        $summit_repo = Mockery::mock(ISummitRepository::class);
+        $summit_repo->shouldReceive('getById')->andReturn($summit);
+
+        $container = \Illuminate\Support\Facades\Facade::getFacadeApplication();
+        $container->instance(ISummitRepository::class, $summit_repo);
+    }
+}