diff --git a/src/Appwrite/Functions/Validator/Headers.php b/src/Appwrite/Functions/Validator/Headers.php
index 99f2d74249..fcda90e6a3 100644
--- a/src/Appwrite/Functions/Validator/Headers.php
+++ b/src/Appwrite/Functions/Validator/Headers.php
@@ -47,21 +47,33 @@ class Headers extends Validator
             $value = \json_decode($value, true);
         }
 
-        if (\json_last_error() == JSON_ERROR_NONE) {
+        if (json_last_error() !== JSON_ERROR_NONE) {
+            return false;
+        } else {
             if (\is_array($value)) {
                 foreach ($value as $key => $val) {
-                    // Check for invalid characters in key and value
-                    if (!preg_match('/^[a-zA-Z0-9_-]+$/', $key)) {
+                    // Reject non-string keys
+                    if (!\is_string($key) || \strlen($key) === 0) {
+                        return false;
+                    }
+
+                    // Check if the key is a single character and ensure it is an alphabetic character
+                    if (\strlen($key) === 1 && !preg_match('/^[a-zA-Z]$/', $key)) {
+                        return false;
+                    }
+
+                    // Check for invalid characters in keys longer than one character
+                    if (\strlen($key) > 1 && !preg_match('/^[a-zA-Z0-9][a-zA-Z0-9-]*[a-zA-Z0-9]$/', $key)) {
                         return false;
                     }
                     // Check for x-appwrite- prefix
-                    if (0 === strpos($key, 'x-appwrite-')) {
+                    if (str_starts_with($key, 'x-appwrite-')) {
                         return false;
                     }
                 }
             }
+            return true;
         }
-        return true;
     }
 
     /**
diff --git a/src/Appwrite/Utopia/Response.php b/src/Appwrite/Utopia/Response.php
index 6d8c671d89..a71e9a1e8d 100644
--- a/src/Appwrite/Utopia/Response.php
+++ b/src/Appwrite/Utopia/Response.php
@@ -743,7 +743,7 @@ class Response extends SwooleResponse
     public function json($data): void
     {
         if (!is_array($data) && !$data instanceof \stdClass) {
-            throw new \Exception('Invalid JSON input var');
+            throw new \Exception('Response body is not a valid JSON object.');
         }
 
         $this
diff --git a/tests/e2e/Services/Functions/FunctionsCustomServerTest.php b/tests/e2e/Services/Functions/FunctionsCustomServerTest.php
index 1e07c3b8e0..e4f7ced2d2 100644
--- a/tests/e2e/Services/Functions/FunctionsCustomServerTest.php
+++ b/tests/e2e/Services/Functions/FunctionsCustomServerTest.php
@@ -1350,23 +1350,7 @@ class FunctionsCustomServerTest extends Scope
         $deploymentId = $deployment['body']['$id'] ?? '';
         $this->assertEquals(202, $deployment['headers']['status-code']);
 
-        // Poll until deployment is built
-        while (true) {
-            $deployment = $this->client->call(Client::METHOD_GET, '/functions/' . $function['body']['$id'] . '/deployments/' . $deploymentId, [
-                'content-type' => 'application/json',
-                'x-appwrite-project' => $this->getProject()['$id'],
-                'x-appwrite-key' => $this->getProject()['apiKey'],
-            ]);
-
-            if (
-                $deployment['headers']['status-code'] >= 400
-                || \in_array($deployment['body']['status'], ['ready', 'failed'])
-            ) {
-                break;
-            }
-
-            \sleep(1);
-        }
+        $this->awaitDeploymentIsBuilt($function['body']['$id'], $deploymentId, checkForSuccess: false);
 
         $deployment = $this->client->call(Client::METHOD_PATCH, '/functions/' . $functionId . '/deployments/' . $deploymentId, array_merge([
             'content-type' => 'application/json',
@@ -1438,16 +1422,6 @@ class FunctionsCustomServerTest extends Scope
 
         $this->assertEquals(201, $function['headers']['status-code']);
 
-        $variable = $this->client->call(Client::METHOD_POST, '/functions/' . $functionId . '/variables', array_merge([
-            'content-type' => 'application/json',
-            'x-appwrite-project' => $this->getProject()['$id'],
-        ], $this->getHeaders()), [
-            'key' => 'CUSTOM_VARIABLE',
-            'value' => 'variable',
-        ]);
-
-        $this->assertEquals(201, $variable['headers']['status-code']);
-
         $deployment = $this->client->call(Client::METHOD_POST, '/functions/' . $functionId . '/deployments', array_merge([
             'content-type' => 'multipart/form-data',
             'x-appwrite-project' => $this->getProject()['$id'],
@@ -1460,22 +1434,7 @@ class FunctionsCustomServerTest extends Scope
         $deploymentId = $deployment['body']['$id'] ?? '';
         $this->assertEquals(202, $deployment['headers']['status-code']);
 
-        while (true) {
-            $deployment = $this->client->call(Client::METHOD_GET, '/functions/' . $function['body']['$id'] . '/deployments/' . $deploymentId, [
-                'content-type' => 'application/json',
-                'x-appwrite-project' => $this->getProject()['$id'],
-                'x-appwrite-key' => $this->getProject()['apiKey'],
-            ]);
-
-            if (
-                $deployment['headers']['status-code'] >= 400
-                || \in_array($deployment['body']['status'], ['ready', 'failed'])
-            ) {
-                break;
-            }
-
-            \sleep(1);
-        }
+        $this->awaitDeploymentIsBuilt($function['body']['$id'], $deploymentId, checkForSuccess: false);
 
         $deployment = $this->client->call(Client::METHOD_PATCH, '/functions/' . $functionId . '/deployments/' . $deploymentId, array_merge([
             'content-type' => 'application/json',
diff --git a/tests/unit/Functions/Validator/HeadersTest.php b/tests/unit/Functions/Validator/HeadersTest.php
index b06a71cb37..57baa1949c 100644
--- a/tests/unit/Functions/Validator/HeadersTest.php
+++ b/tests/unit/Functions/Validator/HeadersTest.php
@@ -44,5 +44,56 @@ class HeadersTest extends TestCase
             'header/////Key' => 'headerValue',
         ];
         $this->assertEquals($this->object->isValid($headers), false);
+
+        $headers = [
+            'Content-Type' => 'application/json',
+            'X-Custom-Header' => 'value'
+        ];
+        $this->assertEquals($this->object->isValid($headers), true);
+
+        $headers = [
+            'X-Custom-Header_With-Hyphens_and_Underscores' => 'value'
+        ];
+        $this->assertFalse($this->object->isValid($headers));
+
+        $headers = [
+            'X-Header-123' => 'value'
+        ];
+        $this->assertTrue($this->object->isValid($headers));
+
+        $headers = [
+            'X-Header<>' => 'value'
+        ];
+        $this->assertFalse($this->object->isValid($headers));
+
+        $headers = [
+            'X Header' => 'value'
+        ];
+        $this->assertFalse($this->object->isValid($headers));
+
+        $headers = [
+            '' => 'value'
+        ];
+        $this->assertFalse($this->object->isValid($headers));
+
+        $headers = [
+            null => 'value',
+        ];
+        $this->assertFalse($this->object->isValid($headers));
+
+        $headers = [
+            'X-Header' => null,
+        ];
+        $this->assertTrue($this->object->isValid($headers));
+
+        $headers = [
+            true => 'value',
+        ];
+        $this->assertFalse($this->object->isValid($headers));
+
+        $headers = [
+            'a' => 'b',
+        ];
+        $this->assertTrue($this->object->isValid($headers));
     }
 }