diff --git a/composer.json b/composer.json
index 60ef28f..afad724 100644
--- a/composer.json
+++ b/composer.json
@@ -28,6 +28,7 @@
         "symfony/runtime": "^5.3 || ^6.0"
     },
     "require-dev": {
+        "guzzlehttp/test-server": "^0.1",
         "illuminate/http": "^8.33",
         "swoole/ide-helper": "^4.6"
     },
diff --git a/src/bref/composer.json b/src/bref/composer.json
index 96516cf..f19b4c6 100644
--- a/src/bref/composer.json
+++ b/src/bref/composer.json
@@ -20,6 +20,7 @@
     },
     "require-dev": {
         "bref/bref": "^1.3",
+        "guzzlehttp/test-server": "^0.1",
         "symfony/http-foundation": "^5.3 || ^6.0",
         "symfony/http-kernel": "^5.4 || ^6.0",
         "symfony/phpunit-bridge": "^5.3"
diff --git a/src/bref/phpunit.xml.dist b/src/bref/phpunit.xml.dist
index ca58bf8..f1701c2 100644
--- a/src/bref/phpunit.xml.dist
+++ b/src/bref/phpunit.xml.dist
@@ -13,7 +13,6 @@
   <testsuites>
     <testsuite name="Test Suite">
       <directory>./tests</directory>
-      <directory suffix=".phpt">./tests/phpt</directory>
     </testsuite>
   </testsuites>
 </phpunit>
diff --git a/src/bref/tests/Lambda/LambdaClientTest.php b/src/bref/tests/Lambda/LambdaClientTest.php
new file mode 100644
index 0000000..32211a3
--- /dev/null
+++ b/src/bref/tests/Lambda/LambdaClientTest.php
@@ -0,0 +1,339 @@
+<?php
+
+namespace Runtime\Bref\Tests;
+
+use Bref\Context\Context;
+use Bref\Event\Handler;
+use GuzzleHttp\Psr7\Response;
+use GuzzleHttp\Server\Server;
+use PHPUnit\Framework\TestCase;
+use Runtime\Bref\Lambda\LambdaClient;
+
+/**
+ * Tests the communication between `LambdaClient` and the Lambda Runtime HTTP API.
+ *
+ * The API is mocked using a fake HTTP server.
+ */
+class LambdaClientTest extends TestCase
+{
+    /** @var LambdaClient */
+    private $lambda;
+
+    protected function setUp(): void
+    {
+        ob_start();
+        Server::start();
+        $this->lambda = new LambdaClient('localhost:8126', 'phpunit');
+    }
+
+    protected function tearDown(): void
+    {
+        Server::stop();
+        ob_end_clean();
+    }
+
+    public function test basic behavior()
+    {
+        $this->givenAnEvent(['Hello' => 'world!']);
+
+        $output = $this->lambda->processNextEvent(new class() implements Handler {
+            public function handle($event, Context $context)
+            {
+                return ['hello' => 'world'];
+            }
+        });
+
+        $this->assertTrue($output);
+        $this->assertInvocationResult(['hello' => 'world']);
+    }
+
+    public function test handler receives context()
+    {
+        $this->givenAnEvent(['Hello' => 'world!']);
+
+        $this->lambda->processNextEvent(new class() implements Handler {
+            public function handle($event, Context $context)
+            {
+                return ['hello' => 'world', 'received-function-arn' => $context->getInvokedFunctionArn()];
+            }
+        });
+
+        $this->assertInvocationResult([
+            'hello' => 'world',
+            'received-function-arn' => 'test-function-name',
+        ]);
+    }
+
+    public function test exceptions in the handler result in an invocation error()
+    {
+        $this->givenAnEvent(['Hello' => 'world!']);
+
+        $output = $this->lambda->processNextEvent(new class() implements Handler {
+            public function handle($event, Context $context)
+            {
+                throw new \RuntimeException('This is an exception');
+            }
+        });
+
+        $this->assertFalse($output);
+        $this->assertInvocationErrorResult('RuntimeException', 'This is an exception');
+        $this->assertErrorInLogs('RuntimeException', 'This is an exception');
+    }
+
+    public function test nested exceptions in the handler result in an invocation error()
+    {
+        $this->givenAnEvent(['Hello' => 'world!']);
+
+        $this->lambda->processNextEvent(new class() implements Handler {
+            public function handle($event, Context $context)
+            {
+                throw new \RuntimeException('This is an exception', 0, new \RuntimeException('The previous exception.', 0, new \Exception('The original exception.')));
+            }
+        });
+
+        $this->assertInvocationErrorResult('RuntimeException', 'This is an exception');
+        $this->assertErrorInLogs('RuntimeException', 'This is an exception');
+        $this->assertPreviousErrorsInLogs([
+            ['errorClass' => 'RuntimeException', 'errorMessage' => 'The previous exception.'],
+            ['errorClass' => 'Exception', 'errorMessage' => 'The original exception.'],
+        ]);
+    }
+
+    public function test an error is thrown if the runtime API returns a wrong response()
+    {
+        $this->expectExceptionMessage('Failed to fetch next Lambda invocation: The requested URL returned error: 404');
+        Server::enqueue([
+            new Response( // lambda event
+                404, // 404 instead of 200
+                [
+                    'lambda-runtime-aws-request-id' => 1,
+                ],
+                '{ "Hello": "world!"}'
+            ),
+        ]);
+
+        $this->lambda->processNextEvent(new class() implements Handler {
+            public function handle($event, Context $context)
+            {
+            }
+        });
+    }
+
+    public function test an error is thrown if the invocation id is missing()
+    {
+        $this->expectExceptionMessage('Failed to determine the Lambda invocation ID');
+        Server::enqueue([
+            new Response( // lambda event
+                200,
+                [], // Missing `lambda-runtime-aws-request-id`
+                '{ "Hello": "world!"}'
+            ),
+        ]);
+
+        $this->lambda->processNextEvent(new class() implements Handler {
+            public function handle($event, Context $context)
+            {
+            }
+        });
+    }
+
+    public function test an error is thrown if the invocation body is empty()
+    {
+        $this->expectExceptionMessage('Empty Lambda runtime API response');
+        Server::enqueue([
+            new Response( // lambda event
+                200,
+                [
+                    'lambda-runtime-aws-request-id' => 1,
+                ]
+            ),
+        ]);
+
+        $this->lambda->processNextEvent(new class() implements Handler {
+            public function handle($event, Context $context)
+            {
+            }
+        });
+    }
+
+    public function test a wrong response from the runtime API turns the invocation into an error()
+    {
+        Server::enqueue([
+            new Response( // lambda event
+                200,
+                [
+                    'lambda-runtime-aws-request-id' => 1,
+                ],
+                '{ "Hello": "world!"}'
+            ),
+            new Response(400), // The Lambda API returns a 400 instead of a 200
+            new Response(200),
+        ]);
+
+        $this->lambda->processNextEvent(new class() implements Handler {
+            public function handle($event, Context $context)
+            {
+                return $event;
+            }
+        });
+        $requests = Server::received();
+        $this->assertCount(3, $requests);
+
+        [$eventRequest, $eventFailureResponse, $eventFailureLog] = $requests;
+        $this->assertSame('GET', $eventRequest->getMethod());
+        $this->assertSame('http://localhost:8126/2018-06-01/runtime/invocation/next', $eventRequest->getUri()->__toString());
+        $this->assertSame('POST', $eventFailureResponse->getMethod());
+        $this->assertSame('http://localhost:8126/2018-06-01/runtime/invocation/1/response', $eventFailureResponse->getUri()->__toString());
+        $this->assertSame('POST', $eventFailureLog->getMethod());
+        $this->assertSame('http://localhost:8126/2018-06-01/runtime/invocation/1/error', $eventFailureLog->getUri()->__toString());
+
+        // Check the lambda result contains the error message
+        $error = json_decode((string) $eventFailureLog->getBody(), true);
+        $this->assertSame('Error while calling the Lambda runtime API: The requested URL returned error: 400 Bad Request', $error['errorMessage']);
+
+        $this->assertErrorInLogs('Exception', 'Error while calling the Lambda runtime API: The requested URL returned error: 400 Bad Request');
+    }
+
+    public function test function results that cannot be encoded are reported as invocation errors()
+    {
+        $this->givenAnEvent(['hello' => 'world!']);
+
+        $this->lambda->processNextEvent(new class() implements Handler {
+            public function handle($event, Context $context)
+            {
+                return "\xB1\x31";
+            }
+        });
+
+        $message = <<<ERROR
+The Lambda response cannot be encoded to JSON.
+This error usually happens when you try to return binary content. If you are writing an HTTP application and you want to return a binary HTTP response (like an image, a PDF, etc.), please read this guide: https://bref.sh/docs/runtimes/http.html#binary-responses
+Here is the original JSON error: 'Malformed UTF-8 characters, possibly incorrectly encoded'
+ERROR;
+        $this->assertInvocationErrorResult('Exception', $message);
+        $this->assertErrorInLogs('Exception', $message);
+    }
+
+    public function test generic event handler()
+    {
+        $handler = new class() implements Handler {
+            /**
+             * @param mixed $event
+             *
+             * @return mixed
+             */
+            public function handle($event, Context $context)
+            {
+                return $event;
+            }
+        };
+
+        $this->givenAnEvent(['foo' => 'bar']);
+
+        $this->lambda->processNextEvent($handler);
+
+        $this->assertInvocationResult(['foo' => 'bar']);
+    }
+
+    /**
+     * @param mixed $event
+     */
+    private function givenAnEvent($event): void
+    {
+        Server::enqueue([
+            new Response( // lambda event
+                200,
+                [
+                    'lambda-runtime-aws-request-id' => '1',
+                    'lambda-runtime-invoked-function-arn' => 'test-function-name',
+                ],
+                json_encode($event)
+            ),
+            new Response(200), // lambda response accepted
+        ]);
+    }
+
+    /**
+     * @param mixed $result
+     */
+    private function assertInvocationResult($result)
+    {
+        $requests = Server::received();
+        $this->assertCount(2, $requests);
+
+        [$eventRequest, $eventResponse] = $requests;
+        $this->assertSame('GET', $eventRequest->getMethod());
+        $this->assertSame('http://localhost:8126/2018-06-01/runtime/invocation/next', $eventRequest->getUri()->__toString());
+        $this->assertSame('POST', $eventResponse->getMethod());
+        $this->assertSame('http://localhost:8126/2018-06-01/runtime/invocation/1/response', $eventResponse->getUri()->__toString());
+        $this->assertEquals($result, json_decode($eventResponse->getBody()->__toString(), true));
+    }
+
+    private function assertInvocationErrorResult(string $errorClass, string $errorMessage)
+    {
+        $requests = Server::received();
+        $this->assertCount(2, $requests);
+
+        [$eventRequest, $eventResponse] = $requests;
+        $this->assertSame('GET', $eventRequest->getMethod());
+        $this->assertSame('http://localhost:8126/2018-06-01/runtime/invocation/next', $eventRequest->getUri()->__toString());
+        $this->assertSame('POST', $eventResponse->getMethod());
+        $this->assertSame('http://localhost:8126/2018-06-01/runtime/invocation/1/error', $eventResponse->getUri()->__toString());
+
+        // Check the content of the result of the lambda
+        $invocationResult = json_decode($eventResponse->getBody()->__toString(), true);
+        $this->assertSame([
+            'errorType',
+            'errorMessage',
+            'stackTrace',
+        ], array_keys($invocationResult));
+        $this->assertEquals($errorClass, $invocationResult['errorType']);
+        $this->assertEquals($errorMessage, $invocationResult['errorMessage']);
+        $this->assertIsArray($invocationResult['stackTrace']);
+    }
+
+    private function assertErrorInLogs(string $errorClass, string $errorMessage): void
+    {
+        // Decode the logs from stdout
+        $stdout = $this->getActualOutput();
+
+        [$requestId, $message, $json] = explode("\t", $stdout);
+
+        $this->assertSame('Invoke Error', $message);
+
+        // Check the request ID matches a UUID
+        $this->assertNotEmpty($requestId);
+
+        $invocationResult = json_decode($json, true);
+        unset($invocationResult['previous']);
+        $this->assertSame([
+            'errorType',
+            'errorMessage',
+            'stack',
+        ], array_keys($invocationResult));
+        $this->assertEquals($errorClass, $invocationResult['errorType']);
+        $this->assertEquals($errorMessage, $invocationResult['errorMessage']);
+        $this->assertIsArray($invocationResult['stack']);
+    }
+
+    private function assertPreviousErrorsInLogs(array $previousErrors)
+    {
+        // Decode the logs from stdout
+        $stdout = $this->getActualOutput();
+
+        [, , $json] = explode("\t", $stdout);
+
+        ['previous' => $previous] = json_decode($json, true);
+        $this->assertCount(count($previousErrors), $previous);
+        foreach ($previous as $index => $error) {
+            $this->assertSame([
+                'errorType',
+                'errorMessage',
+                'stack',
+            ], array_keys($error));
+            $this->assertEquals($previousErrors[$index]['errorClass'], $error['errorType']);
+            $this->assertEquals($previousErrors[$index]['errorMessage'], $error['errorMessage']);
+            $this->assertIsArray($error['stack']);
+        }
+    }
+}