diff --git a/src/PostProcessor/FirestoreRequestParamProcessor.php b/src/PostProcessor/FirestoreRequestParamProcessor.php
new file mode 100644
index 000000000..e7fcc527d
--- /dev/null
+++ b/src/PostProcessor/FirestoreRequestParamProcessor.php
@@ -0,0 +1,129 @@
+<?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.
+ */
+declare(strict_types=1);
+
+namespace Google\PostProcessor;
+
+use Microsoft\PhpParser\Node\MethodDeclaration;
+use LogicException;
+use ParseError;
+
+class FirestoreRequestParamProcessor implements ProcessorInterface
+{
+    use PostProcessorTrait;
+
+    // Line to insert the PHP doc param above
+    private const DATABASE_PHPDOC_INSERT_AT =
+        '     *     @type int $timeoutMillis';
+
+    // PHPdoc param to insert
+    private const DATABASE_PHPDOC_PARAM = <<<'EOL'
+     *     @type string $database
+     *           Set the database of the call, to be added as a routing header
+EOL;
+
+    // Line to insert the request param code above
+    private const DATABASE_REQUEST_PARAM_INSERT_AT =
+        '        return $this->startApiCall(\'Listen\', null, $callOptions);';
+
+    // Request param code to insert
+    private const DATABASE_REQUEST_PARAM_CODE = <<<'EOL'
+$requestParamHeaders = [];
+if (isset($callOptions['database'])) {
+    $requestParamHeaders['database'] = $callOptions['database'];
+}
+$requestParams = new \Google\ApiCore\RequestParamsHeaderDescriptor($requestParamHeaders);
+$callOptions['headers'] = isset($callOptions['headers']) ? array_merge($requestParams->getHeader(), $callOptions['headers']) : $requestParams->getHeader();
+EOL;
+
+    public static function run(string $inputDir): void
+    {
+        $firestoreClientFile = $inputDir . '/src/V1/Client/FirestoreClient.php';
+        if (file_exists($firestoreClientFile)) {
+            self::inject($firestoreClientFile);
+        }
+    }
+
+    private static function inject(string $classFile): void
+    {
+        // The class to update
+        $classContent = file_get_contents($classFile);
+        $processor = new FirestoreRequestParamProcessor($classContent);
+
+        $processor->addDatabaseRequestParamToListenMethod();
+
+        // Write the new contents to the class file.
+        file_put_contents($classFile, $processor->getContents());
+        print("Fragment written to $classFile\n");
+    }
+
+    /**
+     * @throws LogicException
+     * @throws ParseError
+     */
+    public function addDatabaseRequestParamToListenMethod(): void
+    {
+        $contents = $this->classNode->getFileContents();
+        $listenMethod = $this->getMethodDeclaration('listen');
+
+        // update PHPDoc
+        $phpdoc = $listenMethod->getDocCommentText();
+        if (false === strpos($phpdoc, self::DATABASE_PHPDOC_PARAM)) {
+            $newLines = explode(PHP_EOL, self::DATABASE_PHPDOC_PARAM);
+            $newLines[] = self::DATABASE_PHPDOC_INSERT_AT;
+
+
+            $newPhpdoc = str_replace(self::DATABASE_PHPDOC_INSERT_AT, implode(PHP_EOL, $newLines), $phpdoc);
+            $contents = str_replace($phpdoc, $newPhpdoc, $contents);
+        }
+
+        // Update param code
+        $methodText = $listenMethod->compoundStatementOrSemicolon->getText();
+        // indent each line 8 spaces
+        $indent = str_repeat(' ', 8);
+        $newLines = array_map(
+            fn ($line) => $indent . $line,
+            explode(PHP_EOL, self::DATABASE_REQUEST_PARAM_CODE)
+        );
+        if (false === strpos($methodText, $newLines[0])) {
+            $newLines[] = self::DATABASE_REQUEST_PARAM_INSERT_AT;
+
+            $newMethodText = str_replace(
+                self::DATABASE_REQUEST_PARAM_INSERT_AT,
+                implode(PHP_EOL, $newLines), $methodText
+            );
+            $contents = str_replace($methodText, $newMethodText, $contents);
+        }
+
+        $this->classNode = self::fromCode($contents);
+    }
+
+    private function getMethodDeclaration(string $insertBeforeMethod): MethodDeclaration
+    {
+        foreach ($this->classNode->getDescendantNodes() as $childNode) {
+            if ($childNode instanceof MethodDeclaration) {
+                if ($childNode->getName() === $insertBeforeMethod) {
+                    return $childNode;
+                }
+            }
+        }
+
+        throw new LogicException(
+            'Provided contents does not contain method ' . $insertBeforeMethod
+        );
+    }
+}
diff --git a/src/PostProcessor/FragmentInjectionProcessor.php b/src/PostProcessor/FragmentInjectionProcessor.php
index 3c1e1afcb..02da3a238 100644
--- a/src/PostProcessor/FragmentInjectionProcessor.php
+++ b/src/PostProcessor/FragmentInjectionProcessor.php
@@ -18,17 +18,14 @@
 
 namespace Google\PostProcessor;
 
-use Microsoft\PhpParser\Node\Statement\ClassDeclaration;
 use Microsoft\PhpParser\Node\MethodDeclaration;
-use Microsoft\PhpParser\Parser;
 use Microsoft\PhpParser\PositionUtilities;
-use Microsoft\PhpParser\DiagnosticsProvider;
 use LogicException;
 use ParseError;
 
 class FragmentInjectionProcessor implements ProcessorInterface
 {
-    private ClassDeclaration $classNode;
+    use PostProcessorTrait;
 
     public static function run(string $inputDir): void
     {
@@ -62,28 +59,6 @@ private static function inject(string $fragmentFile, string $classFile): void
         print("Fragment written to $classFile\n");
     }
 
-    private static function fromCode(string $contents): ClassDeclaration
-    {
-        $parser = new Parser();
-        $astNode = $parser->parseSourceFile($contents);
-        if ($errors = DiagnosticsProvider::getDiagnostics($astNode)) {
-            throw new ParseError('Provided contents contains a PHP syntax error');
-        }
-
-        foreach ($astNode->getDescendantNodes() as $childNode) {
-            if ($childNode instanceof ClassDeclaration) {
-                return $childNode;
-            }
-        }
-
-        throw new LogicException('Provided contents does not contain a PHP class');
-    }
-
-    public function __construct(string $contents)
-    {
-        $this->classNode = self::fromCode($contents);
-    }
-
     /**
      * @throws LogicException
      * @throws ParseError
@@ -100,11 +75,6 @@ public function insert(string $newContent, ?string $insertBeforeMethod = null):
         $this->classNode = self::fromCode($contents);
     }
 
-    public function getContents(): string
-    {
-        return $this->classNode->getFileContents();
-    }
-
     private function getLineNumberFromPosition(int $startPosition): int
     {
         return PositionUtilities::getLineCharacterPositionFromPosition(
diff --git a/src/PostProcessor/PostProcessor.php b/src/PostProcessor/PostProcessor.php
index caa50d4a2..0610045b5 100644
--- a/src/PostProcessor/PostProcessor.php
+++ b/src/PostProcessor/PostProcessor.php
@@ -45,6 +45,9 @@ private function execute(): void
 
     private static function loadProcessors(array $opts): Vector
     {
-        return Vector::new([FragmentInjectionProcessor::class]);
+        return Vector::new([
+            FragmentInjectionProcessor::class,
+            FirestoreRequestParamProcessor::class,
+        ]);
     }
 }
diff --git a/src/PostProcessor/PostProcessorTrait.php b/src/PostProcessor/PostProcessorTrait.php
new file mode 100644
index 000000000..9c8021662
--- /dev/null
+++ b/src/PostProcessor/PostProcessorTrait.php
@@ -0,0 +1,59 @@
+<?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.
+ */
+declare(strict_types=1);
+
+namespace Google\PostProcessor;
+
+use Microsoft\PhpParser\Node\Statement\ClassDeclaration;
+use Microsoft\PhpParser\Parser;
+use Microsoft\PhpParser\DiagnosticsProvider;
+use LogicException;
+use ParseError;
+
+trait PostProcessorTrait
+{
+    private ClassDeclaration $classNode;
+
+    public function __construct(string $contents)
+    {
+        $this->classNode = self::fromCode($contents);
+    }
+
+    public function getContents(): string
+    {
+        return $this->classNode->getFileContents();
+    }
+
+    public static abstract function run(string $inputDir): void;
+
+    private static function fromCode(string $contents): ClassDeclaration
+    {
+        $parser = new Parser();
+        $astNode = $parser->parseSourceFile($contents);
+        if ($errors = DiagnosticsProvider::getDiagnostics($astNode)) {
+            throw new ParseError('Provided contents contains a PHP syntax error');
+        }
+
+        foreach ($astNode->getDescendantNodes() as $childNode) {
+            if ($childNode instanceof ClassDeclaration) {
+                return $childNode;
+            }
+        }
+
+        throw new LogicException('Provided contents does not contain a PHP class');
+    }
+}
\ No newline at end of file
diff --git a/tests/Unit/PostProcessor/FirestoreRequestParamProcessorTest.php b/tests/Unit/PostProcessor/FirestoreRequestParamProcessorTest.php
new file mode 100644
index 000000000..e45825c4c
--- /dev/null
+++ b/tests/Unit/PostProcessor/FirestoreRequestParamProcessorTest.php
@@ -0,0 +1,103 @@
+<?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.
+ */
+declare(strict_types=1);
+
+namespace Google\Generator\Tests\Unit\PostProcessor;
+
+use PHPUnit\Framework\TestCase;
+use Google\PostProcessor\FirestoreRequestParamProcessor;
+use ParseError;
+
+final class FirestoreRequestParamProcessorTest extends TestCase
+{
+    private static $classContents = <<<EOL
+<?php
+namespace Google\Cloud\Firestore\V1\Client;
+
+final class FirestoreClient
+{
+    use GapicClientTrait;
+
+    /** The name of the service. */
+    private const SERVICE_NAME = 'google.firestore.v1.Firestore';
+
+    private static function getClientDefaults()
+    {
+        return [
+            'serviceName' => self::SERVICE_NAME,
+        ];
+    }
+
+    public function __construct(array \$options = [])
+    {
+        \$clientOptions = \$this->buildClientOptions(\$options);
+        \$this->setClientOptions(\$clientOptions);
+    }
+
+    /**
+     * Listens to changes. This method is only available via gRPC or WebChannel
+     * (not REST).
+     *
+     * @example samples/V1/FirestoreClient/listen.php
+     *
+     * @param array \$callOptions {
+     *     Optional.
+     *
+     *     @type int \$timeoutMillis
+     *           Timeout to use for this call.
+     * }
+     *
+     * @return BidiStream
+     *
+     * @throws ApiException Thrown if the API call fails.
+     */
+    public function listen(array \$callOptions = []): BidiStream
+    {
+        return \$this->startApiCall('Listen', null, \$callOptions);
+    }
+}
+EOL;
+
+    public function testFirestoreRequestParamProcessor()
+    {
+        $firestorePostProcessor = new FirestoreRequestParamProcessor(self::$classContents);
+
+        // Insert the function before this one
+        $firestorePostProcessor->addDatabaseRequestParamToListenMethod();
+        $newClassContents = $firestorePostProcessor->getContents();
+
+        $this->assertStringContainsString('@type string $database', $newClassContents);
+        $this->assertStringContainsString('new \Google\ApiCore\RequestParamsHeaderDescriptor', $newClassContents);
+
+        // Insert again and ensure it's not added twice
+        $firestorePostProcessor->addDatabaseRequestParamToListenMethod();
+        $newClassContents = $firestorePostProcessor->getContents();
+        $this->assertEquals(1, substr_count($newClassContents, '@type string $database'));
+        $this->assertEquals(1, substr_count($newClassContents, 'new \Google\ApiCore\RequestParamsHeaderDescriptor'));
+    }
+
+    public function testFirestoreRequestParamDoesNotContainSyntaxErrors()
+    {
+        $firestorePostProcessor = new FirestoreRequestParamProcessor(self::$classContents);
+
+        $codeString = $firestorePostProcessor->getContents();
+
+        // This will throw a ParseError if the syntax is invalid
+        $tokens = token_get_all("<?php " . $codeString);
+        $this->assertNotNull($tokens);
+    }
+}