diff --git a/src/Client/CurseForgeAPIClient.php b/src/Client/CurseForgeAPIClient.php
index 0301e0e..aa7bbe8 100644
--- a/src/Client/CurseForgeAPIClient.php
+++ b/src/Client/CurseForgeAPIClient.php
@@ -72,7 +72,7 @@ class CurseForgeAPIClient
     public function __construct(string $apiToken, ?Configuration $configuration = null, ?ClientInterface $httpClient = null)
     {
         $this->httpClient = $httpClient;
-        $this->configuration = $configuration ?? (Configuration::getDefaultConfiguration())
+        $this->configuration = $configuration ?? (new Configuration())
             ->setUserAgent(static::getDefaultUserAgent());
         $this->setApiToken($apiToken);
     }
diff --git a/tests/Unit/Client/ClientTestCase.php b/tests/Unit/Client/ClientTestCase.php
index 62b45d6..9782448 100644
--- a/tests/Unit/Client/ClientTestCase.php
+++ b/tests/Unit/Client/ClientTestCase.php
@@ -3,7 +3,6 @@
 namespace Aternos\CurseForgeApi\Tests\Unit\Client;
 
 use Aternos\CurseForgeApi\Client\CurseForgeAPIClient;
-use Aternos\CurseForgeApi\Configuration;
 use GuzzleHttp\Client;
 use GuzzleHttp\Handler\MockHandler;
 use GuzzleHttp\HandlerStack;
@@ -37,7 +36,6 @@ public function setUp(): void
      * Create a client that answers every request with one of the given responses, in order,
      * and records all requests in {@see $requests}.
      *
-     * The client gets its own configuration, because the default configuration is shared by all clients.
      * @param Response[] $responses
      * @return CurseForgeAPIClient
      */
@@ -45,7 +43,7 @@ protected function createClient(array $responses = []): CurseForgeAPIClient
     {
         $stack = HandlerStack::create(new MockHandler($responses));
         $stack->push(Middleware::history($this->requests));
-        return new CurseForgeAPIClient(static::API_TOKEN, new Configuration(), new Client(['handler' => $stack]));
+        return new CurseForgeAPIClient(static::API_TOKEN, null, new Client(['handler' => $stack]));
     }
 
     /**
diff --git a/tests/Unit/Client/CurseForgeAPIClientTest.php b/tests/Unit/Client/CurseForgeAPIClientTest.php
index 6fe663e..2ca5ffb 100644
--- a/tests/Unit/Client/CurseForgeAPIClientTest.php
+++ b/tests/Unit/Client/CurseForgeAPIClientTest.php
@@ -129,6 +129,52 @@ public function testSetHttpClientReplacesHttpClient(): void
         $this->assertRequestCount(0);
     }
 
+    /**
+     * Clients created without an explicit configuration must not share one,
+     * otherwise setting the API token or user agent of one client would change it for all others.
+     *
+     * @throws ApiException
+     */
+    public function testClientsWithoutExplicitConfigurationDoNotShareOne(): void
+    {
+        $defaultConfiguration = Configuration::getDefaultConfiguration();
+        $defaultApiKey = $defaultConfiguration->getApiKey("x-api-key");
+        $defaultUserAgent = $defaultConfiguration->getUserAgent();
+
+        $firstHistory = [];
+        $secondHistory = [];
+        $first = new CurseForgeAPIClient("first-api-token", null, $this->createHttpClient($firstHistory));
+        $second = new CurseForgeAPIClient("second-api-token", null, $this->createHttpClient($secondHistory));
+        $first->setUserAgent("first-agent/1.0");
+
+        $first->getGame(432);
+        $second->getGame(432);
+
+        $this->assertSame("first-api-token", $firstHistory[0]['request']->getHeaderLine("x-api-key"));
+        $this->assertSame("first-agent/1.0", $firstHistory[0]['request']->getHeaderLine("User-Agent"));
+        $this->assertSame("second-api-token", $secondHistory[0]['request']->getHeaderLine("x-api-key"));
+        $this->assertSame(CurseForgeAPIClient::getDefaultUserAgent(), $secondHistory[0]['request']->getHeaderLine("User-Agent"));
+        $this->assertSame($defaultApiKey, $defaultConfiguration->getApiKey("x-api-key"));
+        $this->assertSame($defaultUserAgent, $defaultConfiguration->getUserAgent());
+    }
+
+    /**
+     * @throws ApiException
+     */
+    public function testExplicitConfigurationIsUsed(): void
+    {
+        $history = [];
+        $configuration = new Configuration();
+        $client = new CurseForgeAPIClient(static::API_TOKEN, $configuration, $this->createHttpClient($history));
+
+        $client->setUserAgent("custom-agent/1.0");
+        $client->getGame(432);
+
+        $this->assertSame(static::API_TOKEN, $configuration->getApiKey("x-api-key"));
+        $this->assertSame("custom-agent/1.0", $configuration->getUserAgent());
+        $this->assertSame("custom-agent/1.0", $history[0]['request']->getHeaderLine("User-Agent"));
+    }
+
     /**
      * ObjectSerializer reads the boolean format from the default configuration,
      * so boolean query parameters must also be sent as strings if the client uses its own configuration.
@@ -608,4 +654,17 @@ public function testGetModFileDownloadURL(): void
         $this->assertGetRequest("/v1/mods/420561/files/8946029/download-url");
         $this->assertSame("https://edge.forgecdn.net/files/8946/29/mclogs-neoforge-26.1-3.3.4.jar", $url);
     }
+
+    /**
+     * Create an HTTP client that answers with a single game and records all requests in the given history
+     *
+     * @param array $history
+     * @return Client
+     */
+    protected function createHttpClient(array &$history): Client
+    {
+        $stack = HandlerStack::create(new MockHandler([new Response(200, [], '{"data": {"id": 432}}')]));
+        $stack->push(Middleware::history($history));
+        return new Client(['handler' => $stack]);
+    }
 }