diff --git a/src/V2/Support/ParallelChildGroup.php b/src/V2/Support/ParallelChildGroup.php
index 84be4ac9a..d47a48c78 100644
--- a/src/V2/Support/ParallelChildGroup.php
+++ b/src/V2/Support/ParallelChildGroup.php
@@ -283,6 +283,9 @@ public static function shouldWakeParentOnActivityClosure(
             self::normalizedPath($metadata),
             'activity',
             $closedActivityStatus,
+            // Outcome validation may establish a snapshot before the run lock.
+            // The barrier must observe the preceding lock holder's resolution.
+            true,
         );
     }
 
diff --git a/tests/Feature/V2/V2EmbeddedParallelActivityConcurrencyTest.php b/tests/Feature/V2/V2EmbeddedParallelActivityConcurrencyTest.php
new file mode 100644
index 000000000..b70bd8188
--- /dev/null
+++ b/tests/Feature/V2/V2EmbeddedParallelActivityConcurrencyTest.php
@@ -0,0 +1,206 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Feature\V2;
+
+use Illuminate\Database\Eloquent\Builder;
+use Illuminate\Database\Events\QueryExecuted;
+use Illuminate\Support\Facades\DB;
+use Illuminate\Support\Facades\Queue;
+use PHPUnit\Framework\Attributes\DataProvider;
+use Tests\Fixtures\V2\TestNestedParallelActivityWorkflow;
+use Tests\Fixtures\V2\TestParallelActivityBatchWorkflow;
+use Tests\TestCase;
+use Workflow\V2\Enums\HistoryEventType;
+use Workflow\V2\Enums\TaskStatus;
+use Workflow\V2\Enums\TaskType;
+use Workflow\V2\Jobs\RunActivityTask;
+use Workflow\V2\Jobs\RunWorkflowTask;
+use Workflow\V2\Models\ActivityExecution;
+use Workflow\V2\Models\WorkflowHistoryEvent;
+use Workflow\V2\Models\WorkflowTask;
+use Workflow\V2\Support\ActivityOutcomeRecorder;
+use Workflow\V2\WorkflowStub;
+
+final class V2EmbeddedParallelActivityConcurrencyTest extends TestCase
+{
+    public static function completionSnapshots(): iterable
+    {
+        foreach ([false, true] as $named) {
+            foreach ([2, 6, 25, 'nested'] as $shape) {
+                yield ($named ? 'named' : 'default') . '/' . $shape => [$shape, $named];
+            }
+        }
+    }
+
+    #[DataProvider('completionSnapshots')]
+    public function testOlderOutcomeSnapshotObservesTheCommittedSibling(int|string $shape, bool $named): void
+    {
+        self::stopWorkers();
+        Queue::fake();
+        if (DB::connection()->getDriverName() !== 'mysql') {
+            $this->markTestSkipped('MySQL repeatable-read snapshots are required.');
+        }
+
+        $originalStorage = config('workflows.storage.connection');
+        $database = DB::connection()->getConfig();
+        unset($database['name']);
+        $database['url'] = null;
+        config([
+            'database.connections.parallel_join_storage' => $database,
+            'database.connections.parallel_join_sibling' => $database,
+            'workflows.storage.connection' => $named ? 'parallel_join_storage' : null,
+            'workflows.storage.transaction_attempts' => 1,
+        ]);
+        $storage = config('workflows.storage.connection');
+        $primary = DB::connection($storage);
+        $sibling = DB::connection('parallel_join_sibling');
+        $primary->statement('SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ');
+        $sibling->statement('SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ');
+        $this->assertNotSame($primary->getPdo(), $sibling->getPdo());
+
+        $nested = $shape === 'nested';
+        if (! $nested) {
+            $this->assertIsInt($shape);
+        }
+        $count = $nested ? 3 : $shape;
+        $workflow = WorkflowStub::make($nested
+            ? TestNestedParallelActivityWorkflow::class : TestParallelActivityBatchWorkflow::class);
+        if ($nested) {
+            $workflow->start('0', '1', '2');
+        } else {
+            $workflow->start($count);
+        }
+        $runId = $workflow->runId();
+        $this->runWorkflowTask($runId);
+        $tasks = WorkflowTask::query()->where('workflow_run_id', $runId)
+            ->where('task_type', TaskType::Activity->value)->orderBy('id')->get();
+        $this->assertCount($count, $tasks);
+        foreach ($tasks->take($count - 2) as $task) {
+            app()->call([new RunActivityTask($task->id), 'handle']);
+        }
+        $first = $tasks[$count - 2];
+        $last = $tasks[$count - 1];
+        $armed = true;
+        $observed = false;
+        DB::listen(function (QueryExecuted $event) use (
+            &$armed,
+            &$observed,
+            $primary,
+            $storage,
+            $first,
+            $last,
+            $runId,
+        ): void {
+            if (! $armed || $event->connection !== $primary
+                || ! str_starts_with($event->sql, 'select exists(')
+                || ! str_contains($event->sql, 'activity_attempts')
+                || ! in_array($last->id, $event->bindings, true)) {
+                return;
+            }
+            $recording = array_filter(
+                debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS),
+                static fn (array $frame): bool => ($frame['class'] ?? null) === ActivityOutcomeRecorder::class
+                    && ($frame['function'] ?? null) === 'record'
+            );
+            if ($recording === []) {
+                return;
+            }
+            $armed = false;
+            $observed = true;
+            $this->assertGreaterThan(0, $primary->transactionLevel());
+
+            // The last outcome now has its real pre-run-lock read view. Commit
+            // its sibling on an independent PDO before it can take the run lock.
+            config([
+                'workflows.storage.connection' => 'parallel_join_sibling',
+            ]);
+            try {
+                app()->call([new RunActivityTask($first->id), 'handle']);
+                $this->assertSame(0, DB::connection('parallel_join_sibling')->transactionLevel());
+                $this->assertSame(0, $this->openWorkflowTasks($runId)->count());
+            } finally {
+                config([
+                    'workflows.storage.connection' => $storage,
+                ]);
+            }
+        });
+
+        try {
+            app()->call([new RunActivityTask($last->id), 'handle']);
+            $this->assertTrue($observed, 'The production outcome snapshot boundary was not exercised.');
+            $this->assertSame(0, $primary->transactionLevel());
+            $this->assertSame($count, ActivityExecution::query()->where('workflow_run_id', $runId)
+                ->where('status', 'completed')
+                ->where('attempt_count', 1)
+                ->count());
+            $this->assertSame($count, WorkflowHistoryEvent::query()->where('workflow_run_id', $runId)
+                ->where('event_type', HistoryEventType::ActivityCompleted->value)->count());
+            $this->assertSame(1, $this->openWorkflowTasks($runId)->count());
+
+            // Duplicate delivery must neither repeat an outcome nor add a wake-up.
+            app()
+                ->call([new RunActivityTask($last->id), 'handle']);
+            $this->assertSame(1, $this->openWorkflowTasks($runId)->count());
+            $this->assertSame($count, WorkflowHistoryEvent::query()->where('workflow_run_id', $runId)
+                ->where('event_type', HistoryEventType::ActivityCompleted->value)->count());
+            $this->runWorkflowTask($runId);
+            $this->assertTrue($workflow->refresh()->completed());
+            $expected = array_map(static fn (int $index): string => "Hello, {$index}!", range(0, $count - 1));
+            $this->assertSame(
+                $nested ? [$expected[0], [$expected[1], $expected[2]]] : $expected,
+                $nested ? $workflow->output()['results'] : $workflow->output()
+            );
+            $this->assertSame(0, $this->openWorkflowTasks($runId)->count());
+        } finally {
+            $armed = false;
+            config([
+                'workflows.storage.connection' => $originalStorage,
+            ]);
+            DB::purge('parallel_join_storage');
+            DB::purge('parallel_join_sibling');
+        }
+    }
+
+    public static function batchSizes(): iterable
+    {
+        yield 'six' => [6];
+        yield 'twenty-five' => [25];
+    }
+
+    #[DataProvider('batchSizes')]
+    public function testParallelActivitiesCompleteWithQueueWorkers(int $count): void
+    {
+        $workflow = WorkflowStub::make(TestParallelActivityBatchWorkflow::class);
+        $workflow->start($count);
+        $this->waitForWorkflow($workflow, static fn (WorkflowStub $run): bool => ! $run->refresh()->running());
+        $this->assertTrue($workflow->refresh()->completed());
+        $this->assertSame(
+            array_map(static fn (int $index): string => "Hello, {$index}!", range(0, $count - 1)),
+            $workflow->output()
+        );
+        $this->assertSame($count, ActivityExecution::query()->where('workflow_run_id', $workflow->runId())
+            ->where('status', 'completed')
+            ->where('attempt_count', 1)
+            ->count());
+        $this->assertSame(2, WorkflowTask::query()->where('workflow_run_id', $workflow->runId())
+            ->where('task_type', TaskType::Workflow->value)->count());
+        $this->assertSame(0, $this->openWorkflowTasks($workflow->runId())->count());
+    }
+
+    private function openWorkflowTasks(string $runId): Builder
+    {
+        return WorkflowTask::query()->where('workflow_run_id', $runId)
+            ->where('task_type', TaskType::Workflow->value)
+            ->whereIn('status', [TaskStatus::Ready->value, TaskStatus::Leased->value]);
+    }
+
+    private function runWorkflowTask(string $runId): void
+    {
+        $task = $this->openWorkflowTasks($runId)
+            ->sole();
+        app()
+            ->call([new RunWorkflowTask($task->id), 'handle']);
+    }
+}
diff --git a/tests/Fixtures/V2/TestParallelActivityBatchWorkflow.php b/tests/Fixtures/V2/TestParallelActivityBatchWorkflow.php
new file mode 100644
index 000000000..8b7e0e1c4
--- /dev/null
+++ b/tests/Fixtures/V2/TestParallelActivityBatchWorkflow.php
@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Fixtures\V2;
+
+use function Workflow\V2\activity;
+use function Workflow\V2\all;
+use Workflow\V2\Workflow;
+
+final class TestParallelActivityBatchWorkflow extends Workflow
+{
+    public function handle(int $count): array
+    {
+        return all(array_map(
+            static fn (int $index) => static fn () => activity(TestGreetingActivity::class, (string) $index),
+            range(0, $count - 1),
+        ));
+    }
+}