diff --git a/.php-cs-fixer.dist.php b/.php-cs-fixer.dist.php
index 30655446..ad4f3336 100644
--- a/.php-cs-fixer.dist.php
+++ b/.php-cs-fixer.dist.php
@@ -9,6 +9,7 @@
 $finder = (new Finder())->in([
     __DIR__ . '/config',
     __DIR__ . '/src',
+    __DIR__ . '/stubs',
     __DIR__ . '/tests',
 ]);
 
diff --git a/psalm.xml b/psalm.xml
index 849e4c85..c3c1b590 100644
--- a/psalm.xml
+++ b/psalm.xml
@@ -12,16 +12,8 @@
     <projectFiles>
         <directory name="src" />
         <directory name="stubs" />
-        <directory name="tests" />
         <ignoreFiles>
             <directory name="vendor" />
         </ignoreFiles>
     </projectFiles>
-    <issueHandlers>
-        <PropertyNotSetInConstructor>
-            <errorLevel type="suppress">
-                <directory name="tests" />
-            </errorLevel>
-        </PropertyNotSetInConstructor>
-    </issueHandlers>
 </psalm>
diff --git a/tests/App/MemoryAdapter.php b/stubs/InMemoryAdapter.php
similarity index 67%
rename from tests/App/MemoryAdapter.php
rename to stubs/InMemoryAdapter.php
index 53c1c5cc..917fa14b 100644
--- a/tests/App/MemoryAdapter.php
+++ b/stubs/InMemoryAdapter.php
@@ -2,7 +2,7 @@
 
 declare(strict_types=1);
 
-namespace Yiisoft\Queue\Tests\App;
+namespace Yiisoft\Queue\Stubs;
 
 use Yiisoft\Queue\Adapter\AdapterInterface;
 use Yiisoft\Queue\Message\IdEnvelope;
@@ -11,9 +11,21 @@
 
 use function count;
 
-final class MemoryAdapter implements AdapterInterface
+/**
+ * In-memory implementation of {@see AdapterInterface} for testing and development purposes.
+ *
+ * This adapter stores messages in a local array and processes them sequentially within the same process without any
+ * external queue backend. Messages are assigned incremental integer identifiers and are handled in FIFO order.
+ *
+ * Note: This implementation is not persistent and should not be used in production, as all data is lost when the
+ * process ends.
+ */
+final class InMemoryAdapter implements AdapterInterface
 {
-    /** @var array<int, MessageInterface> */
+    /**
+     * @var MessageInterface[]
+     * @psalm-var array<int, MessageInterface>
+     */
     private array $messages = [];
     private int $current = 0;
 
diff --git a/stubs/StubAdapter.php b/stubs/StubAdapter.php
deleted file mode 100644
index 6acbbac5..00000000
--- a/stubs/StubAdapter.php
+++ /dev/null
@@ -1,33 +0,0 @@
-<?php
-
-declare(strict_types=1);
-
-namespace Yiisoft\Queue\Stubs;
-
-use Yiisoft\Queue\Adapter\AdapterInterface;
-use Yiisoft\Queue\MessageStatus;
-use Yiisoft\Queue\Message\MessageInterface;
-
-/**
- * Stub adapter that does nothing. Message status is always "done".
- */
-final class StubAdapter implements AdapterInterface
-{
-    public function runExisting(callable $handlerCallback): void
-    {
-    }
-
-    public function status(int|string $id): MessageStatus
-    {
-        return MessageStatus::DONE;
-    }
-
-    public function push(MessageInterface $message): MessageInterface
-    {
-        return $message;
-    }
-
-    public function subscribe(callable $handlerCallback): void
-    {
-    }
-}
diff --git a/stubs/StubLoop.php b/stubs/StubLoop.php
index 20a238b5..414a4af7 100644
--- a/stubs/StubLoop.php
+++ b/stubs/StubLoop.php
@@ -13,8 +13,7 @@ final class StubLoop implements LoopInterface
 {
     public function __construct(
         private readonly bool $canContinue = true,
-    ) {
-    }
+    ) {}
 
     public function canContinue(): bool
     {
diff --git a/tests/Unit/Provider/QueueFactoryProviderTest.php b/tests/Unit/Provider/QueueFactoryProviderTest.php
index cdc0d53c..3e9aadcf 100644
--- a/tests/Unit/Provider/QueueFactoryProviderTest.php
+++ b/tests/Unit/Provider/QueueFactoryProviderTest.php
@@ -11,7 +11,7 @@
 use Yiisoft\Queue\Provider\QueueFactoryProvider;
 use Yiisoft\Queue\Provider\QueueNotFoundException;
 use Yiisoft\Queue\QueueInterface;
-use Yiisoft\Queue\Stubs\StubAdapter;
+use Yiisoft\Queue\Stubs\InMemoryAdapter;
 use Yiisoft\Queue\Stubs\StubLoop;
 use Yiisoft\Queue\Stubs\StubQueue;
 use Yiisoft\Queue\Tests\Unit\Support\StringEnum;
@@ -116,9 +116,8 @@ public function testGetHasByStringEnum(): void
 
     public function testWithContainer(): void
     {
-        $adapter = new StubAdapter();
         $container = new SimpleContainer([
-            AdapterInterface::class => $adapter,
+            AdapterInterface::class => new InMemoryAdapter(),
         ]);
 
         $provider = new QueueFactoryProvider(
diff --git a/tests/Unit/QueueTest.php b/tests/Unit/QueueTest.php
index 2f9236f9..fa571f83 100644
--- a/tests/Unit/QueueTest.php
+++ b/tests/Unit/QueueTest.php
@@ -8,8 +8,8 @@
 use Yiisoft\Queue\Message\IdEnvelope;
 use Yiisoft\Queue\Message\Message;
 use Yiisoft\Queue\MessageStatus;
+use Yiisoft\Queue\Stubs\InMemoryAdapter;
 use Yiisoft\Queue\Tests\App\FakeAdapter;
-use Yiisoft\Queue\Tests\App\MemoryAdapter;
 use Yiisoft\Queue\Tests\TestCase;
 
 use function extension_loaded;
@@ -73,7 +73,7 @@ public function testStatusReturnsNotFoundWithoutAdapter(): void
 
     public function testRunWithAdapter(): void
     {
-        $queue = $this->createQueue(new MemoryAdapter());
+        $queue = $this->createQueue(new InMemoryAdapter());
         $message = new Message('simple', null);
         $queue->push($message);
         $queue->push(clone $message);
@@ -84,7 +84,7 @@ public function testRunWithAdapter(): void
 
     public function testRunPartlyWithAdapter(): void
     {
-        $queue = $this->createQueue(new MemoryAdapter());
+        $queue = $this->createQueue(new InMemoryAdapter());
         $message = new Message('simple', null);
         $queue->push($message);
         $queue->push(clone $message);
@@ -95,7 +95,7 @@ public function testRunPartlyWithAdapter(): void
 
     public function testListenWithAdapter(): void
     {
-        $queue = $this->createQueue(new MemoryAdapter());
+        $queue = $this->createQueue(new InMemoryAdapter());
         $message = new Message('simple', null);
         $queue->push($message);
         $queue->push(clone $message);
@@ -107,7 +107,7 @@ public function testListenWithAdapter(): void
 
     public function testStatusWithAdapter(): void
     {
-        $queue = $this->createQueue(new MemoryAdapter());
+        $queue = $this->createQueue(new InMemoryAdapter());
         $envelope = $queue->push(new Message('simple', null));
 
         self::assertArrayHasKey(IdEnvelope::MESSAGE_ID_KEY, $envelope->getMetadata());
diff --git a/tests/Unit/Stubs/InMemoryAdapterTest.php b/tests/Unit/Stubs/InMemoryAdapterTest.php
new file mode 100644
index 00000000..a286e7db
--- /dev/null
+++ b/tests/Unit/Stubs/InMemoryAdapterTest.php
@@ -0,0 +1,173 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Yiisoft\Queue\Tests\Unit\Stubs;
+
+use PHPUnit\Framework\TestCase;
+use Yiisoft\Queue\Message\IdEnvelope;
+use Yiisoft\Queue\Message\Message;
+use Yiisoft\Queue\Message\MessageInterface;
+use Yiisoft\Queue\MessageStatus;
+use Yiisoft\Queue\Stubs\InMemoryAdapter;
+
+final class InMemoryAdapterTest extends TestCase
+{
+    public function testPush(): void
+    {
+        $adapter = new InMemoryAdapter();
+
+        $envelope1 = $adapter->push(new Message('test', 'a'));
+        $envelope2 = $adapter->push(new Message('test', 'b'));
+        $envelope3 = $adapter->push(new Message('test', 'c'));
+
+        $this->assertInstanceOf(IdEnvelope::class, $envelope1);
+        $this->assertInstanceOf(IdEnvelope::class, $envelope2);
+        $this->assertInstanceOf(IdEnvelope::class, $envelope3);
+
+        $this->assertSame(0, $envelope1->getId());
+        $this->assertSame('a', $envelope1->getMessage()->getData());
+        $this->assertSame(1, $envelope2->getId());
+        $this->assertSame('b', $envelope2->getMessage()->getData());
+        $this->assertSame(2, $envelope3->getId());
+        $this->assertSame('c', $envelope3->getMessage()->getData());
+    }
+
+    public function testStatusWaitingForPushedMessage(): void
+    {
+        $adapter = new InMemoryAdapter();
+        $envelope = $adapter->push(new Message('test', null));
+
+        $this->assertInstanceOf(IdEnvelope::class, $envelope);
+        $this->assertSame(MessageStatus::WAITING, $adapter->status($envelope->getId()));
+    }
+
+    public function testStatusDoneAfterProcessing(): void
+    {
+        $adapter = new InMemoryAdapter();
+        $envelope = $adapter->push(new Message('test', null));
+
+        $adapter->runExisting(static fn() => true);
+
+        $this->assertInstanceOf(IdEnvelope::class, $envelope);
+        $this->assertSame(MessageStatus::DONE, $adapter->status($envelope->getId()));
+    }
+
+    public function testStatusNotFoundForNonExistentId(): void
+    {
+        $adapter = new InMemoryAdapter();
+
+        $this->assertSame(MessageStatus::NOT_FOUND, $adapter->status(99));
+    }
+
+    public function testStatusNotFoundForNegativeId(): void
+    {
+        $adapter = new InMemoryAdapter();
+
+        $this->assertSame(MessageStatus::NOT_FOUND, $adapter->status(-1));
+    }
+
+    public function testStatusAcceptsStringId(): void
+    {
+        $adapter = new InMemoryAdapter();
+        $envelope = $adapter->push(new Message('test', null));
+
+        $this->assertInstanceOf(IdEnvelope::class, $envelope);
+        $this->assertSame(MessageStatus::WAITING, $adapter->status((string) $envelope->getId()));
+    }
+
+    public function testRunExistingProcessesAllMessages(): void
+    {
+        $adapter = new InMemoryAdapter();
+        $adapter->push(new Message('test', 'a'));
+        $adapter->push(new Message('test', 'b'));
+        $adapter->push(new Message('test', 'c'));
+
+        $processed = [];
+        $adapter->runExisting(
+            static function (MessageInterface $message) use (&$processed): bool {
+                $processed[] = $message->getData();
+                return true;
+            },
+        );
+
+        $this->assertSame(['a', 'b', 'c'], $processed);
+    }
+
+    public function testRunExistingStopsWhenHandlerReturnsFalse(): void
+    {
+        $adapter = new InMemoryAdapter();
+        $adapter->push(new Message('test', 'a'));
+        $adapter->push(new Message('test', 'b'));
+        $adapter->push(new Message('test', 'c'));
+
+        $processed = [];
+        $adapter->runExisting(
+            static function (MessageInterface $message) use (&$processed): bool {
+                $processed[] = $message->getData();
+                return false;
+            },
+        );
+
+        $this->assertSame(['a'], $processed);
+    }
+
+    public function testRunExistingOnEmptyQueue(): void
+    {
+        $adapter = new InMemoryAdapter();
+
+        $called = false;
+        $adapter->runExisting(static function () use (&$called): bool {
+            $called = true;
+            return true;
+        });
+
+        $this->assertFalse($called);
+    }
+
+    public function testRunExistingDoesNotReprocessMessages(): void
+    {
+        $adapter = new InMemoryAdapter();
+        $adapter->push(new Message('test', 'x'));
+
+        $count = 0;
+        $handler = static function () use (&$count): bool {
+            $count++;
+            return true;
+        };
+        $adapter->runExisting($handler);
+        $adapter->runExisting($handler);
+
+        $this->assertSame(1, $count);
+    }
+
+    public function testIdContinuesAfterProcessing(): void
+    {
+        $adapter = new InMemoryAdapter();
+        $adapter->push(new Message('test', null));
+        $adapter->runExisting(static fn() => true);
+
+        $envelope = $adapter->push(new Message('test', null));
+
+        $this->assertInstanceOf(IdEnvelope::class, $envelope);
+        $this->assertSame(1, $envelope->getId());
+        $this->assertSame(MessageStatus::WAITING, $adapter->status($envelope->getId()));
+    }
+
+    public function testSubscribeProcessesExistingMessages(): void
+    {
+        $adapter = new InMemoryAdapter();
+        $adapter->push(new Message('test', 'a'));
+        $adapter->push(new Message('test', 'b'));
+
+        $processed = [];
+        $adapter->subscribe(
+            static function (MessageInterface $message) use (&$processed): bool {
+                $processed[] = $message->getData();
+                return true;
+            },
+        );
+
+        $this->assertSame(['a', 'b'], $processed);
+    }
+}
diff --git a/tests/Unit/Stubs/StubAdapterTest.php b/tests/Unit/Stubs/StubAdapterTest.php
deleted file mode 100644
index 8f455c6e..00000000
--- a/tests/Unit/Stubs/StubAdapterTest.php
+++ /dev/null
@@ -1,25 +0,0 @@
-<?php
-
-declare(strict_types=1);
-
-namespace Yiisoft\Queue\Tests\Unit\Stubs;
-
-use PHPUnit\Framework\TestCase;
-use Yiisoft\Queue\MessageStatus;
-use Yiisoft\Queue\Message\Message;
-use Yiisoft\Queue\Message\MessageInterface;
-use Yiisoft\Queue\Stubs\StubAdapter;
-
-final class StubAdapterTest extends TestCase
-{
-    public function testBase(): void
-    {
-        $message = new Message('test', 42);
-        $adapter = new StubAdapter();
-
-        $this->assertSame($message, $adapter->push($message));
-        $this->assertSame(MessageStatus::DONE, $adapter->status('test'));
-        $adapter->runExisting(static fn(MessageInterface $message): bool => true);
-        $adapter->subscribe(static fn(MessageInterface $message): bool => true);
-    }
-}