diff --git a/tests/ClientBuilderTest.php b/tests/ClientBuilderTest.php
index 3c8eb5ce6..f9be538b9 100644
--- a/tests/ClientBuilderTest.php
+++ b/tests/ClientBuilderTest.php
@@ -4,12 +4,14 @@
 
 namespace Sentry\Tests;
 
-use Http\Client\Common\Plugin;
-use Http\Client\Common\PluginClient;
-use Http\Client\HttpAsyncClient;
-use Http\Message\MessageFactory;
-use Http\Message\UriFactory;
-use Http\Promise\Promise;
+use Http\Client\Common\Plugin as PluginInterface;
+use Http\Client\HttpAsyncClient as HttpAsyncClientInterface;
+use Http\Discovery\MessageFactoryDiscovery;
+use Http\Discovery\UriFactoryDiscovery;
+use Http\Message\MessageFactory as MessageFactoryInterface;
+use Http\Message\UriFactory as UriFactoryInterface;
+use Http\Promise\FulfilledPromise;
+use Http\Promise\Promise as PromiseInterface;
 use Jean85\PrettyVersions;
 use PHPUnit\Framework\MockObject\MockObject;
 use PHPUnit\Framework\TestCase;
@@ -17,6 +19,7 @@
 use Sentry\Client;
 use Sentry\ClientBuilder;
 use Sentry\Event;
+use Sentry\FlushableClientInterface;
 use Sentry\Integration\ErrorListenerIntegration;
 use Sentry\Integration\ExceptionListenerIntegration;
 use Sentry\Integration\FatalErrorListenerIntegration;
@@ -60,13 +63,17 @@ public function testNullTransportIsUsedWhenNoServerIsConfigured(): void
      */
     public function testSetUriFactory(): void
     {
-        /** @var UriFactory|MockObject $uriFactory */
-        $uriFactory = $this->createMock(UriFactory::class);
+        /** @var UriFactoryInterface&MockObject $uriFactory */
+        $uriFactory = $this->createMock(UriFactoryInterface::class);
+        $uriFactory->expects($this->once())
+            ->method('createUri')
+            ->willReturn(UriFactoryDiscovery::find()->createUri('http://www.example.com'));
 
-        $clientBuilder = ClientBuilder::create(['dsn' => 'http://public:secret@example.com/sentry/1']);
-        $clientBuilder->setUriFactory($uriFactory);
+        $client = ClientBuilder::create(['dsn' => 'http://public@example.com/sentry/1'])
+            ->setUriFactory($uriFactory)
+            ->getClient();
 
-        $this->assertAttributeSame($uriFactory, 'uriFactory', $clientBuilder);
+        $client->captureMessage('foo');
     }
 
     /**
@@ -76,17 +83,17 @@ public function testSetUriFactory(): void
      */
     public function testSetMessageFactory(): void
     {
-        /** @var MessageFactory|MockObject $messageFactory */
-        $messageFactory = $this->createMock(MessageFactory::class);
-
-        $clientBuilder = ClientBuilder::create(['dsn' => 'http://public:secret@example.com/sentry/1']);
-        $clientBuilder->setMessageFactory($messageFactory);
+        /** @var MessageFactoryInterface&MockObject $messageFactory */
+        $messageFactory = $this->createMock(MessageFactoryInterface::class);
+        $messageFactory->expects($this->once())
+            ->method('createRequest')
+            ->willReturn(MessageFactoryDiscovery::find()->createRequest('POST', 'http://www.example.com'));
 
-        $this->assertAttributeSame($messageFactory, 'messageFactory', $clientBuilder);
-
-        $transport = $this->getObjectAttribute($clientBuilder->getClient(), 'transport');
+        $client = ClientBuilder::create(['dsn' => 'http://public@example.com/sentry/1'])
+            ->setMessageFactory($messageFactory)
+            ->getClient();
 
-        $this->assertAttributeSame($messageFactory, 'requestFactory', $transport);
+        $client->captureMessage('foo');
     }
 
     /**
@@ -96,14 +103,17 @@ public function testSetMessageFactory(): void
      */
     public function testSetTransport(): void
     {
-        /** @var TransportInterface|MockObject $transport */
+        /** @var TransportInterface&MockObject $transport */
         $transport = $this->createMock(TransportInterface::class);
+        $transport->expects($this->once())
+            ->method('send')
+            ->willReturn('ddb4a0b9ab1941bf92bd2520063663e3');
 
-        $clientBuilder = ClientBuilder::create(['dsn' => 'http://public:secret@example.com/sentry/1']);
-        $clientBuilder->setTransport($transport);
+        $client = ClientBuilder::create(['dsn' => 'http://public@example.com/sentry/1'])
+            ->setTransport($transport)
+            ->getClient();
 
-        $this->assertAttributeSame($transport, 'transport', $clientBuilder);
-        $this->assertAttributeSame($transport, 'transport', $clientBuilder->getClient());
+        $this->assertSame('ddb4a0b9ab1941bf92bd2520063663e3', $client->captureMessage('foo'));
     }
 
     /**
@@ -113,17 +123,19 @@ public function testSetTransport(): void
      */
     public function testSetHttpClient(): void
     {
-        /** @var HttpAsyncClient|MockObject $httpClient */
-        $httpClient = $this->createMock(HttpAsyncClient::class);
-
-        $clientBuilder = ClientBuilder::create(['dsn' => 'http://public:secret@example.com/sentry/1']);
-        $clientBuilder->setHttpClient($httpClient);
-
-        $this->assertAttributeSame($httpClient, 'httpClient', $clientBuilder);
-
-        $transport = $this->getObjectAttribute($clientBuilder->getClient(), 'transport');
-
-        $this->assertAttributeSame($httpClient, 'client', $this->getObjectAttribute($transport, 'httpClient'));
+        /** @var HttpAsyncClientInterface&MockObject $httpClient */
+        $httpClient = $this->createMock(HttpAsyncClientInterface::class);
+        $httpClient->expects($this->once())
+            ->method('sendAsyncRequest')
+            ->willReturn(new FulfilledPromise(true));
+
+        /** @var FlushableClientInterface $client */
+        $client = ClientBuilder::create(['dsn' => 'http://public@example.com/sentry/1'])
+            ->setHttpClient($httpClient)
+            ->getClient();
+
+        $client->captureMessage('foo');
+        $client->flush();
     }
 
     /**
@@ -133,16 +145,19 @@ public function testSetHttpClient(): void
      */
     public function testAddHttpClientPlugin(): void
     {
-        /** @var Plugin|MockObject $plugin */
-        $plugin = $this->createMock(Plugin::class);
-
-        $clientBuilder = new ClientBuilder();
-        $clientBuilder->addHttpClientPlugin($plugin);
-
-        $plugins = $this->getObjectAttribute($clientBuilder, 'httpClientPlugins');
-
-        $this->assertCount(1, $plugins);
-        $this->assertSame($plugin, $plugins[0]);
+        /** @var PluginInterface&MockObject $plugin */
+        $plugin = $this->createMock(PluginInterface::class);
+        $plugin->expects($this->once())
+            ->method('handleRequest')
+            ->willReturn(new FulfilledPromise(true));
+
+        /** @var FlushableClientInterface $client */
+        $client = ClientBuilder::create(['dsn' => 'http://public@example.com/sentry/1'])
+            ->addHttpClientPlugin($plugin)
+            ->getClient();
+
+        $client->captureMessage('foo');
+        $client->flush();
     }
 
     /**
@@ -153,36 +168,30 @@ public function testAddHttpClientPlugin(): void
      */
     public function testRemoveHttpClientPlugin(): void
     {
-        $plugin = new PluginStub1();
-        $plugin2 = new PluginStub2();
-
-        $clientBuilder = new ClientBuilder();
-        $clientBuilder->addHttpClientPlugin($plugin);
-        $clientBuilder->addHttpClientPlugin($plugin);
-        $clientBuilder->addHttpClientPlugin($plugin2);
-
-        $this->assertAttributeCount(3, 'httpClientPlugins', $clientBuilder);
-
-        $clientBuilder->removeHttpClientPlugin(PluginStub1::class);
-
-        $plugins = $this->getObjectAttribute($clientBuilder, 'httpClientPlugins');
-
-        $this->assertCount(1, $plugins);
-        $this->assertSame($plugin2, reset($plugins));
-    }
-
-    public function testGetClient(): void
-    {
-        $clientBuilder = ClientBuilder::create(['dsn' => 'http://public:secret@example.com/sentry/1']);
-        $client = $clientBuilder->getClient();
-
-        $this->assertInstanceOf(Client::class, $client);
-        $this->assertAttributeInstanceOf(HttpTransport::class, 'transport', $client);
-
-        $transport = $this->getObjectAttribute($client, 'transport');
-
-        $this->assertAttributeSame($this->getObjectAttribute($clientBuilder, 'messageFactory'), 'requestFactory', $transport);
-        $this->assertAttributeInstanceOf(PluginClient::class, 'httpClient', $transport);
+        $plugin = new class() implements PluginInterface {
+            public function handleRequest(RequestInterface $request, callable $next, callable $first): PromiseInterface
+            {
+                return new FulfilledPromise(true);
+            }
+        };
+
+        $plugin2 = new class() implements PluginInterface {
+            public function handleRequest(RequestInterface $request, callable $next, callable $first): PromiseInterface
+            {
+                return new FulfilledPromise(true);
+            }
+        };
+
+        /** @var FlushableClientInterface $client */
+        $client = ClientBuilder::create()
+            ->addHttpClientPlugin($plugin)
+            ->addHttpClientPlugin($plugin)
+            ->addHttpClientPlugin($plugin2)
+            ->removeHttpClientPlugin(\get_class($plugin2))
+            ->getClient();
+
+        $client->captureMessage('foo');
+        $client->flush();
     }
 
     /**
@@ -194,8 +203,7 @@ public function testIntegrationsAreAddedToClientCorrectly(bool $defaultIntegrati
         $options->setDefaultIntegrations($defaultIntegrations);
         $options->setIntegrations($integrations);
 
-        $clientBuilder = new ClientBuilder($options);
-        $client = $clientBuilder->getClient();
+        $client = (new ClientBuilder($options))->getClient();
 
         $actualIntegrationsClassNames = array_map('\get_class', $client->getOptions()->getIntegrations());
 
@@ -299,17 +307,3 @@ public function setupOnce(): void
     {
     }
 }
-
-final class PluginStub1 implements Plugin
-{
-    public function handleRequest(RequestInterface $request, callable $next, callable $first): Promise
-    {
-    }
-}
-
-final class PluginStub2 implements Plugin
-{
-    public function handleRequest(RequestInterface $request, callable $next, callable $first): Promise
-    {
-    }
-}
diff --git a/tests/Context/AbstractContextTest.php b/tests/Context/AbstractContextTest.php
index 2237f9012..8e1b82fe7 100644
--- a/tests/Context/AbstractContextTest.php
+++ b/tests/Context/AbstractContextTest.php
@@ -100,7 +100,8 @@ public function testOffsetSet(string $key, $value, ?string $expectedExceptionCla
         $context = $this->createContext();
         $context[$key] = $value;
 
-        $this->assertArraySubset([$key => $value], $context->toArray());
+        $this->assertArrayHasKey($key, $context);
+        $this->assertSame($value, $context[$key]);
     }
 
     /**
diff --git a/tests/Context/ContextTest.php b/tests/Context/ContextTest.php
index 79085e214..76a418776 100644
--- a/tests/Context/ContextTest.php
+++ b/tests/Context/ContextTest.php
@@ -9,14 +9,14 @@
 
 class ContextTest extends TestCase
 {
-    public function testConstructor()
+    public function testConstructor(): void
     {
         $context = new Context(['foo' => 'bar']);
 
-        $this->assertAttributeEquals(['foo' => 'bar'], 'data', $context);
+        $this->assertSame(['foo' => 'bar'], $context->toArray());
     }
 
-    public function testMerge()
+    public function testMerge(): void
     {
         $context = new Context([
             'foo' => 'bar',
@@ -27,45 +27,45 @@ public function testMerge()
 
         $context->merge(['bar' => ['barfoo' => 'foobar']], true);
 
-        $this->assertAttributeEquals(['foo' => 'bar', 'bar' => ['foobar' => 'barfoo', 'barfoo' => 'foobar']], 'data', $context);
+        $this->assertSame(['foo' => 'bar', 'bar' => ['foobar' => 'barfoo', 'barfoo' => 'foobar']], $context->toArray());
 
         $context->merge(['bar' => 'foo']);
 
-        $this->assertAttributeEquals(['foo' => 'bar', 'bar' => 'foo'], 'data', $context);
+        $this->assertSame(['foo' => 'bar', 'bar' => 'foo'], $context->toArray());
     }
 
-    public function testSetData()
+    public function testSetData(): void
     {
         $context = new Context(['foo' => 'bar']);
         $context->setData(['bar' => 'foo']);
 
-        $this->assertAttributeEquals(['foo' => 'bar', 'bar' => 'foo'], 'data', $context);
+        $this->assertSame(['foo' => 'bar', 'bar' => 'foo'], $context->toArray());
 
         $context->setData(['foo' => ['bar' => 'baz']]);
 
-        $this->assertAttributeEquals(['foo' => ['bar' => 'baz'], 'bar' => 'foo'], 'data', $context);
+        $this->assertSame(['foo' => ['bar' => 'baz'], 'bar' => 'foo'], $context->toArray());
     }
 
-    public function testReplaceData()
+    public function testReplaceData(): void
     {
         $context = new Context(['foo' => 'bar']);
         $context->replaceData(['bar' => 'foo']);
 
-        $this->assertAttributeEquals(['bar' => 'foo'], 'data', $context);
+        $this->assertSame(['bar' => 'foo'], $context->toArray());
     }
 
-    public function testClear()
+    public function testClear(): void
     {
         $context = new Context(['foo' => 'bar']);
 
-        $this->assertAttributeEquals(['foo' => 'bar'], 'data', $context);
+        $this->assertSame(['foo' => 'bar'], $context->toArray());
 
         $context->clear();
 
-        $this->assertAttributeEmpty('data', $context);
+        $this->assertSame([], $context->toArray());
     }
 
-    public function testIsEmpty()
+    public function testIsEmpty(): void
     {
         $context = new Context();
 
@@ -76,51 +76,40 @@ public function testIsEmpty()
         $this->assertFalse($context->isEmpty());
     }
 
-    public function testToArray()
+    public function testJsonSerialize(): void
     {
         $context = new Context(['foo' => 'bar']);
 
-        $this->assertEquals(['foo' => 'bar'], $context->toArray());
+        $this->assertSame('{"foo":"bar"}', json_encode($context));
     }
 
-    public function testJsonSerialize()
-    {
-        $context = new Context(['foo' => 'bar']);
-
-        $this->assertEquals('{"foo":"bar"}', json_encode($context));
-    }
-
-    public function testArrayLikeBehaviour()
+    public function testArrayLikeBehaviour(): void
     {
         $context = new Context();
 
-        $this->assertAttributeEquals([], 'data', $context);
-        $this->assertArrayNotHasKey('foo', $context);
-
         // Accessing a key that does not exists in the data object should behave
         // like accessing a non-existent key of an array
         @$context['foo'];
 
         $error = error_get_last();
 
-        $this->assertInternalType('array', $error);
-        $this->assertEquals('Undefined index: foo', $error['message']);
+        $this->assertIsArray($error);
+        $this->assertSame('Undefined index: foo', $error['message']);
 
         $context['foo'] = 'bar';
 
-        $this->assertAttributeEquals(['foo' => 'bar'], 'data', $context);
         $this->assertTrue(isset($context['foo']));
-        $this->assertEquals('bar', $context['foo']);
+        $this->assertSame('bar', $context['foo']);
 
         unset($context['foo']);
 
         $this->assertArrayNotHasKey('foo', $context);
     }
 
-    public function testGetIterator()
+    public function testGetIterator(): void
     {
         $context = new Context(['foo' => 'bar', 'bar' => 'foo']);
 
-        $this->assertEquals($context->toArray(), iterator_to_array($context));
+        $this->assertSame($context->toArray(), iterator_to_array($context));
     }
 }
diff --git a/tests/Transport/HttpTransportTest.php b/tests/Transport/HttpTransportTest.php
index 9f8d3df99..f5a8505ed 100644
--- a/tests/Transport/HttpTransportTest.php
+++ b/tests/Transport/HttpTransportTest.php
@@ -6,7 +6,6 @@
 
 use Http\Client\HttpAsyncClient;
 use Http\Discovery\MessageFactoryDiscovery;
-use Http\Promise\FulfilledPromise;
 use Http\Promise\RejectedPromise;
 use PHPUnit\Framework\MockObject\MockObject;
 use PHPUnit\Framework\TestCase;
@@ -18,39 +17,6 @@
 
 final class HttpTransportTest extends TestCase
 {
-    /**
-     * @group legacy
-     *
-     * @expectedDeprecationMessage Delaying the sending of the events using the "Sentry\Transport\HttpTransport" class is deprecated since version 2.2 and will not work in 3.0.
-     */
-    public function testSendDelaysExecutionUntilShutdown(): void
-    {
-        $promise = new FulfilledPromise('foo');
-
-        /** @var HttpAsyncClient&MockObject $httpClient */
-        $httpClient = $this->createMock(HttpAsyncClient::class);
-        $httpClient->expects($this->once())
-            ->method('sendAsyncRequest')
-            ->willReturn($promise);
-
-        $transport = new HttpTransport(
-            new Options(['dsn' => 'http://public@example.com/sentry/1']),
-            $httpClient,
-            MessageFactoryDiscovery::find(),
-            true
-        );
-
-        $this->assertAttributeEmpty('pendingRequests', $transport);
-
-        $transport->send(new Event());
-
-        $this->assertAttributeNotEmpty('pendingRequests', $transport);
-
-        $transport->close();
-
-        $this->assertAttributeEmpty('pendingRequests', $transport);
-    }
-
     public function testSendDoesNotDelayExecutionUntilShutdownWhenConfiguredToNotDoIt(): void
     {
         $promise = new RejectedPromise(new \Exception());
@@ -69,8 +35,6 @@ public function testSendDoesNotDelayExecutionUntilShutdownWhenConfiguredToNotDoI
         );
 
         $transport->send(new Event());
-
-        $this->assertAttributeEmpty('pendingRequests', $transport);
     }
 
     public function testSendThrowsOnMissingProjectIdCredential(): void
diff --git a/tests/Transport/SpoolTransportTest.php b/tests/Transport/SpoolTransportTest.php
index 11b5e808d..501eefa61 100644
--- a/tests/Transport/SpoolTransportTest.php
+++ b/tests/Transport/SpoolTransportTest.php
@@ -33,14 +33,30 @@ public function testGetSpool(): void
         $this->assertSame($this->spool, $this->transport->getSpool());
     }
 
-    public function testSend(): void
+    /**
+     * @dataProvider sendDataProvider
+     */
+    public function testSend(bool $isSendingSuccessful): void
     {
         $event = new Event();
 
         $this->spool->expects($this->once())
             ->method('queueEvent')
-            ->with($event);
+            ->with($event)
+            ->willReturn($isSendingSuccessful);
+
+        $eventId = $this->transport->send($event);
 
-        $this->transport->send($event);
+        if ($isSendingSuccessful) {
+            $this->assertSame($event->getId(), $eventId);
+        } else {
+            $this->assertNull($eventId);
+        }
+    }
+
+    public function sendDataProvider(): \Generator
+    {
+        yield [true];
+        yield [false];
     }
 }
diff --git a/tests/phpt/http_transport_send_event_delayed.phpt b/tests/phpt/http_transport_send_event_delayed.phpt
new file mode 100644
index 000000000..b64a44028
--- /dev/null
+++ b/tests/phpt/http_transport_send_event_delayed.phpt
@@ -0,0 +1,59 @@
+--TEST--
+Test that the HttpTransport transport delays the event sending until the shutdown
+--FILE--
+<?php
+
+declare(strict_types=1);
+
+namespace Sentry\Tests;
+
+use Http\Client\HttpAsyncClient as HttpAsyncClientInterface;
+use Http\Discovery\MessageFactoryDiscovery;
+use Http\Promise\RejectedPromise;
+use Psr\Http\Message\RequestInterface;
+use Sentry\Event;
+use Sentry\Options;
+use Sentry\Transport\HttpTransport;
+
+$vendor = __DIR__;
+
+while (!file_exists($vendor . '/vendor')) {
+    $vendor = \dirname($vendor);
+}
+
+require $vendor . '/vendor/autoload.php';
+
+$httpClientInvokationCount = 0;
+$httpClient = new class ($httpClientInvokationCount) implements HttpAsyncClientInterface {
+    private $httpClientInvokationCount;
+
+    public function __construct(int &$httpClientInvokationCount)
+    {
+        $this->httpClientInvokationCount = &$httpClientInvokationCount;
+    }
+
+    public function sendAsyncRequest(RequestInterface $request)
+    {
+        ++$this->httpClientInvokationCount;
+
+        return new RejectedPromise(new \Exception());
+    }
+};
+
+$transport = new HttpTransport(
+    new Options(['dsn' => 'http://public@example.com/sentry/1']),
+    $httpClient,
+    MessageFactoryDiscovery::find()
+);
+
+$transport->send(new Event());
+
+var_dump($httpClientInvokationCount);
+
+register_shutdown_function('register_shutdown_function', static function () use (&$httpClientInvokationCount): void {
+    var_dump($httpClientInvokationCount);
+});
+?>
+--EXPECT--
+int(0)
+int(1)
diff --git a/tests/phpt/spool_drain.phpt b/tests/phpt/spool_drain.phpt
deleted file mode 100644
index 882361f9f..000000000
--- a/tests/phpt/spool_drain.phpt
+++ /dev/null
@@ -1,68 +0,0 @@
---TEST--
-Test emptying spool transport
---FILE--
-<?php
-
-declare(strict_types=1);
-
-namespace Sentry\Tests;
-
-use PHPUnit\Framework\Assert;
-use Sentry\ClientBuilder;
-use Sentry\Options;
-use Sentry\SentrySdk;
-use Sentry\Spool\MemorySpool;
-use Sentry\Spool\SpoolInterface;
-use Sentry\Transport\NullTransport;
-use Sentry\Transport\SpoolTransport;
-use Sentry\Transport\TransportFactoryInterface;
-use Sentry\Transport\TransportInterface;
-
-$vendor = __DIR__;
-
-while (!file_exists($vendor . '/vendor')) {
-    $vendor = dirname($vendor);
-}
-
-require $vendor . '/vendor/autoload.php';
-
-$spool = new MemorySpool();
-$nullTransport = new NullTransport();
-
-$transportFactory = new class($spool) implements TransportFactoryInterface {
-    private $spool;
-
-    public function __construct(SpoolInterface $spool)
-    {
-        $this->spool = $spool;
-    }
-
-    public function create(Options $options): TransportInterface
-    {
-        return new SpoolTransport($this->spool);
-    }
-};
-
-$client = ClientBuilder::create()
-    ->setTransportFactory($transportFactory)
-    ->getClient();
-
-SentrySdk::getCurrentHub()->bindClient($client);
-
-register_shutdown_function('register_shutdown_function', static function () use ($spool, $nullTransport): void {
-    Assert::assertAttributeCount(1, 'events', $spool);
-
-    $spool->flushQueue($nullTransport);
-
-    Assert::assertAttributeCount(0, 'events', $spool);
-
-    echo 'Shutdown function called';
-});
-
-\Foo\Bar::baz();
-?>
---EXPECTF--
-Fatal error: Uncaught Error: Class '%s' not found in %s:%d
-Stack trace:
-%a
-Shutdown function called