diff --git a/src/Google/Ads/GoogleAds/Lib/V20/AdsAssistantHeaderMiddleware.php b/src/Google/Ads/GoogleAds/Lib/V20/AdsAssistantHeaderMiddleware.php
new file mode 100644
index 0000000000..5b3b426003
--- /dev/null
+++ b/src/Google/Ads/GoogleAds/Lib/V20/AdsAssistantHeaderMiddleware.php
@@ -0,0 +1,52 @@
+<?php
+
+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+namespace Google\Ads\GoogleAds\Lib\V20;
+
+use Google\ApiCore\Call;
+use Google\ApiCore\AgentHeader;
+use Google\ApiCore\Middleware\MiddlewareInterface;
+
+/**
+ * Appends ads assistant metadata to the x-goog-api-client header.
+ */
+class AdsAssistantHeaderMiddleware implements MiddlewareInterface
+{
+    private $nextHandler;
+    private $adsAssistant;
+
+    public function __construct(callable $nextHandler, string $adsAssistant)
+    {
+        $this->nextHandler = $nextHandler;
+        $this->adsAssistant = $adsAssistant;
+    }
+
+    public function __invoke(Call $call, array $options)
+    {
+        // Check if the agent header exists to avoid errors
+        if (isset($options['headers'][AgentHeader::AGENT_HEADER_KEY][0])) {
+            $options['headers'][AgentHeader::AGENT_HEADER_KEY][0] .= sprintf(
+                ' gaada/%s',
+                $this->adsAssistant
+            );
+        }
+        
+        $next = $this->nextHandler;
+        return $next($call, $options);
+    }
+}
\ No newline at end of file
diff --git a/src/Google/Ads/GoogleAds/Lib/V20/GoogleAdsGapicClientTrait.php b/src/Google/Ads/GoogleAds/Lib/V20/GoogleAdsGapicClientTrait.php
index 54b21e5a6f..f1d2ea0a08 100644
--- a/src/Google/Ads/GoogleAds/Lib/V20/GoogleAdsGapicClientTrait.php
+++ b/src/Google/Ads/GoogleAds/Lib/V20/GoogleAdsGapicClientTrait.php
@@ -38,7 +38,7 @@ trait GoogleAdsGapicClientTrait
     private static $UNARY_MIDDLEWARES = 'unary-middlewares';
     private static $STREAMING_MIDDLEWARES = 'streaming-middlewares';
     private static $ADS_ASSISTANT_HEADER_NAME = 'google-ads-api-assistant';
-
+    
     private $developerToken = null;
     private $loginCustomerId = null;
     private $linkedCustomerId = null;
@@ -79,9 +79,6 @@ protected function modifyClientOptions(array &$options)
 
     /**
      * Adds a FixedHeaderMiddleware to a callable.
-     *
-     * @param callable $callable the callable to add to
-     * @return callable the modified callable
      */
     private function addFixedHeaderMiddleware(callable &$callable)
     {
@@ -115,6 +112,11 @@ protected function modifyUnaryCallable(callable &$callable)
             /** @var GoogleAdsMiddlewareAbstract $unaryMiddleware */
             $callable = $unaryMiddleware->withNextHandler($callable);
         }
+
+        // Prepend the AdsAssistantHeaderMiddleware
+        if (!is_null($this->adsAssistant)) {
+            $callable = new AdsAssistantHeaderMiddleware($callable, $this->adsAssistant);
+        }
     }
 
     /**
@@ -129,6 +131,11 @@ protected function modifyStreamingCallable(callable &$callable)
             /** @var GoogleAdsMiddlewareAbstract $streamingMiddleware */
             $callable = $streamingMiddleware->withNextHandler($callable);
         }
+
+        // Prepend the AdsAssistantHeaderMiddleware
+        if (!is_null($this->adsAssistant)) {
+            $callable = new AdsAssistantHeaderMiddleware($callable, $this->adsAssistant);
+        }
     }
 
     public function getResponseMetadata(): ?GoogleAdsResponseMetadata
diff --git a/src/Google/Ads/GoogleAds/Lib/V21/AdsAssistantHeaderMiddleware.php b/src/Google/Ads/GoogleAds/Lib/V21/AdsAssistantHeaderMiddleware.php
new file mode 100644
index 0000000000..3a00c685d4
--- /dev/null
+++ b/src/Google/Ads/GoogleAds/Lib/V21/AdsAssistantHeaderMiddleware.php
@@ -0,0 +1,52 @@
+<?php
+
+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+namespace Google\Ads\GoogleAds\Lib\V21;
+
+use Google\ApiCore\Call;
+use Google\ApiCore\AgentHeader;
+use Google\ApiCore\Middleware\MiddlewareInterface;
+
+/**
+ * Appends ads assistant metadata to the x-goog-api-client header.
+ */
+class AdsAssistantHeaderMiddleware implements MiddlewareInterface
+{
+    private $nextHandler;
+    private $adsAssistant;
+
+    public function __construct(callable $nextHandler, string $adsAssistant)
+    {
+        $this->nextHandler = $nextHandler;
+        $this->adsAssistant = $adsAssistant;
+    }
+
+    public function __invoke(Call $call, array $options)
+    {
+        // Check if the agent header exists to avoid errors
+        if (isset($options['headers'][AgentHeader::AGENT_HEADER_KEY][0])) {
+            $options['headers'][AgentHeader::AGENT_HEADER_KEY][0] .= sprintf(
+                ' gaada/%s',
+                $this->adsAssistant
+            );
+        }
+        
+        $next = $this->nextHandler;
+        return $next($call, $options);
+    }
+}
\ No newline at end of file
diff --git a/src/Google/Ads/GoogleAds/Lib/V21/GoogleAdsGapicClientTrait.php b/src/Google/Ads/GoogleAds/Lib/V21/GoogleAdsGapicClientTrait.php
index 6252ac90a7..53e27faf17 100644
--- a/src/Google/Ads/GoogleAds/Lib/V21/GoogleAdsGapicClientTrait.php
+++ b/src/Google/Ads/GoogleAds/Lib/V21/GoogleAdsGapicClientTrait.php
@@ -79,9 +79,6 @@ protected function modifyClientOptions(array &$options)
 
     /**
      * Adds a FixedHeaderMiddleware to a callable.
-     *
-     * @param callable $callable the callable to add to
-     * @return callable the modified callable
      */
     private function addFixedHeaderMiddleware(callable &$callable)
     {
@@ -115,6 +112,11 @@ protected function modifyUnaryCallable(callable &$callable)
             /** @var GoogleAdsMiddlewareAbstract $unaryMiddleware */
             $callable = $unaryMiddleware->withNextHandler($callable);
         }
+
+        // Prepend the AdsAssistantHeaderMiddleware
+        if (!is_null($this->adsAssistant)) {
+            $callable = new AdsAssistantHeaderMiddleware($callable, $this->adsAssistant);
+        }
     }
 
     /**
@@ -129,6 +131,11 @@ protected function modifyStreamingCallable(callable &$callable)
             /** @var GoogleAdsMiddlewareAbstract $streamingMiddleware */
             $callable = $streamingMiddleware->withNextHandler($callable);
         }
+
+        // Prepend the AdsAssistantHeaderMiddleware
+        if (!is_null($this->adsAssistant)) {
+            $callable = new AdsAssistantHeaderMiddleware($callable, $this->adsAssistant);
+        }
     }
 
     public function getResponseMetadata(): ?GoogleAdsResponseMetadata
diff --git a/src/Google/Ads/GoogleAds/Lib/V22/AdsAssistantHeaderMiddleware.php b/src/Google/Ads/GoogleAds/Lib/V22/AdsAssistantHeaderMiddleware.php
new file mode 100644
index 0000000000..ff9f3bdc7b
--- /dev/null
+++ b/src/Google/Ads/GoogleAds/Lib/V22/AdsAssistantHeaderMiddleware.php
@@ -0,0 +1,52 @@
+<?php
+
+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+namespace Google\Ads\GoogleAds\Lib\V22;
+
+use Google\ApiCore\Call;
+use Google\ApiCore\AgentHeader;
+use Google\ApiCore\Middleware\MiddlewareInterface;
+
+/**
+ * Appends ads assistant metadata to the x-goog-api-client header.
+ */
+class AdsAssistantHeaderMiddleware implements MiddlewareInterface
+{
+    private $nextHandler;
+    private $adsAssistant;
+
+    public function __construct(callable $nextHandler, string $adsAssistant)
+    {
+        $this->nextHandler = $nextHandler;
+        $this->adsAssistant = $adsAssistant;
+    }
+
+    public function __invoke(Call $call, array $options)
+    {
+        // Check if the agent header exists to avoid errors
+        if (isset($options['headers'][AgentHeader::AGENT_HEADER_KEY][0])) {
+            $options['headers'][AgentHeader::AGENT_HEADER_KEY][0] .= sprintf(
+                ' gaada/%s',
+                $this->adsAssistant
+            );
+        }
+        
+        $next = $this->nextHandler;
+        return $next($call, $options);
+    }
+}
\ No newline at end of file
diff --git a/src/Google/Ads/GoogleAds/Lib/V22/GoogleAdsGapicClientTrait.php b/src/Google/Ads/GoogleAds/Lib/V22/GoogleAdsGapicClientTrait.php
index a32f4dce01..439f5e20c6 100644
--- a/src/Google/Ads/GoogleAds/Lib/V22/GoogleAdsGapicClientTrait.php
+++ b/src/Google/Ads/GoogleAds/Lib/V22/GoogleAdsGapicClientTrait.php
@@ -82,9 +82,6 @@ protected function modifyClientOptions(array &$options)
 
     /**
      * Adds a FixedHeaderMiddleware to a callable.
-     *
-     * @param callable $callable the callable to add to
-     * @return callable the modified callable
      */
     private function addFixedHeaderMiddleware(callable &$callable)
     {
@@ -121,6 +118,11 @@ protected function modifyUnaryCallable(callable &$callable)
             /** @var GoogleAdsMiddlewareAbstract $unaryMiddleware */
             $callable = $unaryMiddleware->withNextHandler($callable);
         }
+
+        // Prepend the AdsAssistantHeaderMiddleware
+        if (!is_null($this->adsAssistant)) {
+            $callable = new AdsAssistantHeaderMiddleware($callable, $this->adsAssistant);
+        }
     }
 
     /**
@@ -135,6 +137,11 @@ protected function modifyStreamingCallable(callable &$callable)
             /** @var GoogleAdsMiddlewareAbstract $streamingMiddleware */
             $callable = $streamingMiddleware->withNextHandler($callable);
         }
+
+        // Prepend the AdsAssistantHeaderMiddleware
+        if (!is_null($this->adsAssistant)) {
+            $callable = new AdsAssistantHeaderMiddleware($callable, $this->adsAssistant);
+        }
     }
 
     public function getResponseMetadata(): ?GoogleAdsResponseMetadata
diff --git a/src/Google/Ads/GoogleAds/Lib/V23/AdsAssistantHeaderMiddleware.php b/src/Google/Ads/GoogleAds/Lib/V23/AdsAssistantHeaderMiddleware.php
new file mode 100644
index 0000000000..e16b4affac
--- /dev/null
+++ b/src/Google/Ads/GoogleAds/Lib/V23/AdsAssistantHeaderMiddleware.php
@@ -0,0 +1,52 @@
+<?php
+
+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+namespace Google\Ads\GoogleAds\Lib\V23;
+
+use Google\ApiCore\Call;
+use Google\ApiCore\AgentHeader;
+use Google\ApiCore\Middleware\MiddlewareInterface;
+
+/**
+ * Appends ads assistant metadata to the x-goog-api-client header.
+ */
+class AdsAssistantHeaderMiddleware implements MiddlewareInterface
+{
+    private $nextHandler;
+    private $adsAssistant;
+
+    public function __construct(callable $nextHandler, string $adsAssistant)
+    {
+        $this->nextHandler = $nextHandler;
+        $this->adsAssistant = $adsAssistant;
+    }
+
+    public function __invoke(Call $call, array $options)
+    {
+        // Check if the agent header exists to avoid errors
+        if (isset($options['headers'][AgentHeader::AGENT_HEADER_KEY][0])) {
+            $options['headers'][AgentHeader::AGENT_HEADER_KEY][0] .= sprintf(
+                ' gaada/%s',
+                $this->adsAssistant
+            );
+        }
+        
+        $next = $this->nextHandler;
+        return $next($call, $options);
+    }
+}
\ No newline at end of file
diff --git a/src/Google/Ads/GoogleAds/Lib/V23/GoogleAdsGapicClientTrait.php b/src/Google/Ads/GoogleAds/Lib/V23/GoogleAdsGapicClientTrait.php
index 666cc71511..cdfbcd11eb 100644
--- a/src/Google/Ads/GoogleAds/Lib/V23/GoogleAdsGapicClientTrait.php
+++ b/src/Google/Ads/GoogleAds/Lib/V23/GoogleAdsGapicClientTrait.php
@@ -82,9 +82,6 @@ protected function modifyClientOptions(array &$options)
 
     /**
      * Adds a FixedHeaderMiddleware to a callable.
-     *
-     * @param callable $callable the callable to add to
-     * @return callable the modified callable
      */
     private function addFixedHeaderMiddleware(callable &$callable)
     {
@@ -103,6 +100,7 @@ private function addFixedHeaderMiddleware(callable &$callable)
             if (!is_null($this->adsAssistant)) {
                 $headers[self::$ADS_ASSISTANT_HEADER_NAME] = [$this->adsAssistant];
             }
+
             $callable = new FixedHeaderMiddleware($callable, $headers);
         }
         return $callable;
@@ -120,6 +118,11 @@ protected function modifyUnaryCallable(callable &$callable)
             /** @var GoogleAdsMiddlewareAbstract $unaryMiddleware */
             $callable = $unaryMiddleware->withNextHandler($callable);
         }
+
+        // Prepend the AdsAssistantHeaderMiddleware
+        if (!is_null($this->adsAssistant)) {
+            $callable = new AdsAssistantHeaderMiddleware($callable, $this->adsAssistant);
+        }
     }
 
     /**
@@ -134,6 +137,11 @@ protected function modifyStreamingCallable(callable &$callable)
             /** @var GoogleAdsMiddlewareAbstract $streamingMiddleware */
             $callable = $streamingMiddleware->withNextHandler($callable);
         }
+
+        // Prepend the AdsAssistantHeaderMiddleware
+        if (!is_null($this->adsAssistant)) {
+            $callable = new AdsAssistantHeaderMiddleware($callable, $this->adsAssistant);
+        }
     }
 
     public function getResponseMetadata(): ?GoogleAdsResponseMetadata
diff --git a/tests/Google/Ads/GoogleAds/Lib/V20/AdsAssistantHeaderMiddlewareTest.php b/tests/Google/Ads/GoogleAds/Lib/V20/AdsAssistantHeaderMiddlewareTest.php
new file mode 100644
index 0000000000..f176d59424
--- /dev/null
+++ b/tests/Google/Ads/GoogleAds/Lib/V20/AdsAssistantHeaderMiddlewareTest.php
@@ -0,0 +1,48 @@
+<?php
+
+namespace Google\Ads\GoogleAds\Lib\V21;
+
+use Google\ApiCore\Call;
+use Google\ApiCore\AgentHeader;
+use PHPUnit\Framework\TestCase;
+
+class AdsAssistantHeaderMiddlewareTest extends TestCase
+{
+    public function testInvokeAppendsHeader()
+    {
+        $adsAssistant = 'test-assistant-123';
+        $initialHeader = 'gl-php/8.4.0 gccl/1.0.0';
+        $expectedHeader = $initialHeader . ' gaada/' . $adsAssistant;
+
+        // 1. Create a dummy "next handler" to capture the result
+        $nextHandlerCalled = false;
+        $capturedOptions = [];
+        $nextHandler = function (Call $call, array $options) use (&$nextHandlerCalled, &$capturedOptions) {
+            $nextHandlerCalled = true;
+            $capturedOptions = $options;
+            return "final-result";
+        };
+
+        // 2. Instantiate the middleware
+        $middleware = new AdsAssistantHeaderMiddleware($nextHandler, $adsAssistant);
+
+        // 3. Prepare mock Call and Options
+        $call = $this->createMock(Call::class);
+        $options = [
+            'headers' => [
+                AgentHeader::AGENT_HEADER_KEY => [$initialHeader]
+            ]
+        ];
+
+        // 4. Run it
+        $result = $middleware($call, $options);
+
+        // 5. Assertions
+        $this->assertTrue($nextHandlerCalled, 'The next handler was not called.');
+        $this->assertEquals("final-result", $result);
+        
+        $actualHeader = $capturedOptions['headers'][AgentHeader::AGENT_HEADER_KEY][0];
+        $this->assertEquals($expectedHeader, $actualHeader);
+        $this->assertStringContainsString('gaada/test-assistant-123', $actualHeader);
+    }
+}
\ No newline at end of file
diff --git a/tests/Google/Ads/GoogleAds/Lib/V21/AdsAssistantHeaderMiddlewareTest.php b/tests/Google/Ads/GoogleAds/Lib/V21/AdsAssistantHeaderMiddlewareTest.php
new file mode 100644
index 0000000000..f176d59424
--- /dev/null
+++ b/tests/Google/Ads/GoogleAds/Lib/V21/AdsAssistantHeaderMiddlewareTest.php
@@ -0,0 +1,48 @@
+<?php
+
+namespace Google\Ads\GoogleAds\Lib\V21;
+
+use Google\ApiCore\Call;
+use Google\ApiCore\AgentHeader;
+use PHPUnit\Framework\TestCase;
+
+class AdsAssistantHeaderMiddlewareTest extends TestCase
+{
+    public function testInvokeAppendsHeader()
+    {
+        $adsAssistant = 'test-assistant-123';
+        $initialHeader = 'gl-php/8.4.0 gccl/1.0.0';
+        $expectedHeader = $initialHeader . ' gaada/' . $adsAssistant;
+
+        // 1. Create a dummy "next handler" to capture the result
+        $nextHandlerCalled = false;
+        $capturedOptions = [];
+        $nextHandler = function (Call $call, array $options) use (&$nextHandlerCalled, &$capturedOptions) {
+            $nextHandlerCalled = true;
+            $capturedOptions = $options;
+            return "final-result";
+        };
+
+        // 2. Instantiate the middleware
+        $middleware = new AdsAssistantHeaderMiddleware($nextHandler, $adsAssistant);
+
+        // 3. Prepare mock Call and Options
+        $call = $this->createMock(Call::class);
+        $options = [
+            'headers' => [
+                AgentHeader::AGENT_HEADER_KEY => [$initialHeader]
+            ]
+        ];
+
+        // 4. Run it
+        $result = $middleware($call, $options);
+
+        // 5. Assertions
+        $this->assertTrue($nextHandlerCalled, 'The next handler was not called.');
+        $this->assertEquals("final-result", $result);
+        
+        $actualHeader = $capturedOptions['headers'][AgentHeader::AGENT_HEADER_KEY][0];
+        $this->assertEquals($expectedHeader, $actualHeader);
+        $this->assertStringContainsString('gaada/test-assistant-123', $actualHeader);
+    }
+}
\ No newline at end of file
diff --git a/tests/Google/Ads/GoogleAds/Lib/V22/AdsAssistantHeaderMiddlewareTest.php b/tests/Google/Ads/GoogleAds/Lib/V22/AdsAssistantHeaderMiddlewareTest.php
new file mode 100644
index 0000000000..f176d59424
--- /dev/null
+++ b/tests/Google/Ads/GoogleAds/Lib/V22/AdsAssistantHeaderMiddlewareTest.php
@@ -0,0 +1,48 @@
+<?php
+
+namespace Google\Ads\GoogleAds\Lib\V21;
+
+use Google\ApiCore\Call;
+use Google\ApiCore\AgentHeader;
+use PHPUnit\Framework\TestCase;
+
+class AdsAssistantHeaderMiddlewareTest extends TestCase
+{
+    public function testInvokeAppendsHeader()
+    {
+        $adsAssistant = 'test-assistant-123';
+        $initialHeader = 'gl-php/8.4.0 gccl/1.0.0';
+        $expectedHeader = $initialHeader . ' gaada/' . $adsAssistant;
+
+        // 1. Create a dummy "next handler" to capture the result
+        $nextHandlerCalled = false;
+        $capturedOptions = [];
+        $nextHandler = function (Call $call, array $options) use (&$nextHandlerCalled, &$capturedOptions) {
+            $nextHandlerCalled = true;
+            $capturedOptions = $options;
+            return "final-result";
+        };
+
+        // 2. Instantiate the middleware
+        $middleware = new AdsAssistantHeaderMiddleware($nextHandler, $adsAssistant);
+
+        // 3. Prepare mock Call and Options
+        $call = $this->createMock(Call::class);
+        $options = [
+            'headers' => [
+                AgentHeader::AGENT_HEADER_KEY => [$initialHeader]
+            ]
+        ];
+
+        // 4. Run it
+        $result = $middleware($call, $options);
+
+        // 5. Assertions
+        $this->assertTrue($nextHandlerCalled, 'The next handler was not called.');
+        $this->assertEquals("final-result", $result);
+        
+        $actualHeader = $capturedOptions['headers'][AgentHeader::AGENT_HEADER_KEY][0];
+        $this->assertEquals($expectedHeader, $actualHeader);
+        $this->assertStringContainsString('gaada/test-assistant-123', $actualHeader);
+    }
+}
\ No newline at end of file
diff --git a/tests/Google/Ads/GoogleAds/Lib/V23/AdsAssistantHeaderMiddlewareTest.php b/tests/Google/Ads/GoogleAds/Lib/V23/AdsAssistantHeaderMiddlewareTest.php
new file mode 100644
index 0000000000..f176d59424
--- /dev/null
+++ b/tests/Google/Ads/GoogleAds/Lib/V23/AdsAssistantHeaderMiddlewareTest.php
@@ -0,0 +1,48 @@
+<?php
+
+namespace Google\Ads\GoogleAds\Lib\V21;
+
+use Google\ApiCore\Call;
+use Google\ApiCore\AgentHeader;
+use PHPUnit\Framework\TestCase;
+
+class AdsAssistantHeaderMiddlewareTest extends TestCase
+{
+    public function testInvokeAppendsHeader()
+    {
+        $adsAssistant = 'test-assistant-123';
+        $initialHeader = 'gl-php/8.4.0 gccl/1.0.0';
+        $expectedHeader = $initialHeader . ' gaada/' . $adsAssistant;
+
+        // 1. Create a dummy "next handler" to capture the result
+        $nextHandlerCalled = false;
+        $capturedOptions = [];
+        $nextHandler = function (Call $call, array $options) use (&$nextHandlerCalled, &$capturedOptions) {
+            $nextHandlerCalled = true;
+            $capturedOptions = $options;
+            return "final-result";
+        };
+
+        // 2. Instantiate the middleware
+        $middleware = new AdsAssistantHeaderMiddleware($nextHandler, $adsAssistant);
+
+        // 3. Prepare mock Call and Options
+        $call = $this->createMock(Call::class);
+        $options = [
+            'headers' => [
+                AgentHeader::AGENT_HEADER_KEY => [$initialHeader]
+            ]
+        ];
+
+        // 4. Run it
+        $result = $middleware($call, $options);
+
+        // 5. Assertions
+        $this->assertTrue($nextHandlerCalled, 'The next handler was not called.');
+        $this->assertEquals("final-result", $result);
+        
+        $actualHeader = $capturedOptions['headers'][AgentHeader::AGENT_HEADER_KEY][0];
+        $this->assertEquals($expectedHeader, $actualHeader);
+        $this->assertStringContainsString('gaada/test-assistant-123', $actualHeader);
+    }
+}
\ No newline at end of file