diff --git a/src/Http/Response.php b/src/Http/Response.php
index 06c878bd..4d4485e3 100644
--- a/src/Http/Response.php
+++ b/src/Http/Response.php
@@ -494,14 +494,12 @@ public function header(string $header): string|array|null
      */
     public function isJson(): bool
     {
-        $contentType = $this->header('Content-Type');
+        $contentType = $this->psrResponse->getHeaderLine('Content-Type');
 
-        if (is_null($contentType)) {
+        if ($contentType === '') {
             return false;
         }
 
-        $contentType = is_array($contentType) ? $contentType[0] : $contentType;
-
         return str_contains($contentType, 'json');
     }
 
@@ -510,14 +508,12 @@ public function isJson(): bool
      */
     public function isXml(): bool
     {
-        $contentType = $this->header('Content-Type');
+        $contentType = $this->psrResponse->getHeaderLine('Content-Type');
 
-        if (is_null($contentType)) {
+        if ($contentType === '') {
             return false;
         }
 
-        $contentType = is_array($contentType) ? $contentType[0] : $contentType;
-
         return str_contains($contentType, 'xml');
     }
 
diff --git a/tests/Unit/ResponseTest.php b/tests/Unit/ResponseTest.php
index 14d8706e..02d0e363 100644
--- a/tests/Unit/ResponseTest.php
+++ b/tests/Unit/ResponseTest.php
@@ -415,6 +415,8 @@
         MockResponse::make(['foo' => 'bar'], 200, ['Content-Type' => 'application/json']),
         // JSON with charset
         MockResponse::make(['foo' => 'bar'], 200, ['Content-Type' => 'application/json; charset=utf-8']),
+        // JSON with lowercase header (e.g. FastAPI, HTTP/2)
+        MockResponse::make(['foo' => 'bar'], 200, ['content-type' => 'application/json']),
         // Non-JSON content type
         MockResponse::make('plain text', 200, ['Content-Type' => 'text/plain']),
         // No content type
@@ -429,6 +431,9 @@
     $response = $connector->send(new UserRequest, $mockClient);
     expect($response->isJson())->toBeTrue();
 
+    $response = $connector->send(new UserRequest, $mockClient);
+    expect($response->isJson())->toBeTrue();
+
     $response = $connector->send(new UserRequest, $mockClient);
     expect($response->isJson())->toBeFalse();
 
@@ -442,6 +447,8 @@
         MockResponse::make('<?xml version="1.0"?><root></root>', 200, ['Content-Type' => 'application/xml']),
         // XML with charset
         MockResponse::make('<?xml version="1.0"?><root></root>', 200, ['Content-Type' => 'text/xml; charset=utf-8']),
+        // XML with lowercase header (e.g. FastAPI, HTTP/2)
+        MockResponse::make('<?xml version="1.0"?><root></root>', 200, ['content-type' => 'application/xml']),
         // Non-XML content type
         MockResponse::make('plain text', 200, ['Content-Type' => 'text/plain']),
         // No content type
@@ -456,6 +463,9 @@
     $response = $connector->send(new UserRequest, $mockClient);
     expect($response->isXml())->toBeTrue();
 
+    $response = $connector->send(new UserRequest, $mockClient);
+    expect($response->isXml())->toBeTrue();
+
     $response = $connector->send(new UserRequest, $mockClient);
     expect($response->isXml())->toBeFalse();