diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 879bd4e..ea904eb 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -51,6 +51,7 @@ jobs:
             CONVERTKIT_OAUTH_REFRESH_TOKEN=${{ secrets.CONVERTKIT_OAUTH_REFRESH_TOKEN }}
             CONVERTKIT_OAUTH_ACCESS_TOKEN_NO_DATA=${{ secrets.CONVERTKIT_OAUTH_ACCESS_TOKEN_NO_DATA }}
             CONVERTKIT_OAUTH_REFRESH_TOKEN_NO_DATA=${{ secrets.CONVERTKIT_OAUTH_REFRESH_TOKEN_NO_DATA }}
+            CONVERTKIT_API_KEY=${{ secrets.CONVERTKIT_API_KEY }}
             CONVERTKIT_OAUTH_CLIENT_ID=${{ secrets.CONVERTKIT_OAUTH_CLIENT_ID }}
             CONVERTKIT_OAUTH_CLIENT_SECRET=${{ secrets.CONVERTKIT_OAUTH_CLIENT_SECRET }}
             CONVERTKIT_OAUTH_REDIRECT_URI=${{ secrets.CONVERTKIT_OAUTH_REDIRECT_URI }}
diff --git a/README.md b/README.md
index b2bd177..b8f1ef5 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
-# ConvertKit SDK PHP
+# Kit SDK PHP
 
-The ConvertKit PHP SDK provides convinient access to the ConvertKit API from applications written in the PHP language.
+The Kit PHP SDK provides convinient access to the Kit API from applications written in the PHP language.
 
 It includes a pre-defined set of methods for interacting with the API.
 
@@ -10,6 +10,7 @@ It includes a pre-defined set of methods for interacting with the API.
 |-------------|-------------|--------------------|--------------|
 | 1.x         | v3          | API Key and Secret | 7.4+         |
 | 2.x         | v4          | OAuth              | 8.0+         |
+| 2.2+        | v4          | API Key            | 8.0+         |
 
 Refer to [this guide](MIGRATION.md) for changes when upgrading to the v2 SDK.
 
@@ -41,9 +42,9 @@ If you use Composer, these dependencies should be handled automatically.
 
 ### 2.x (v4 API, OAuth, PHP 8.0+)
 
-First, register your OAuth application in the `OAuth Applications` section at https://app.convertkit.com/account_settings/advanced_settings.
+First, register your OAuth application in the `OAuth Applications` section at https://app.kit.com/account_settings/advanced_settings.
 
-Using the supplied Client ID and secret, redirect the user to ConvertKit to grant your application access to their ConvertKit account.
+Using the supplied Client ID and secret, redirect the user to Kit to grant your application access to their Kit account.
 
 ```php
 // Require the autoloader (if you're using a PHP framework, this may already be done for you).
@@ -59,7 +60,7 @@ $api = new \ConvertKit_API\ConvertKit_API(
 header('Location: '.$api->get_oauth_url('<your_redirect_uri>'));
 ```
 
-Once the user grants your application access to their ConvertKit account, they'll be redirected to your Redirect URI with an authorization code. For example:
+Once the user grants your application access to their Kit account, they'll be redirected to your Redirect URI with an authorization code. For example:
 
 `your-redirect-uri?code=<auth_code>`
 
@@ -118,26 +119,40 @@ $api = new \ConvertKit_API\ConvertKit_API(
 API requests may then be performed:
 
 ```php
-$result = $api->add_subscriber_to_form(12345, 'joe.bloggs@convertkit.com');
+$result = $api->add_subscriber_to_form(12345, 'joe.bloggs@kit.com');
 ```
 
 To determine whether a new entity / relationship was created, or an existing entity / relationship updated, inspect the HTTP code of the last request:
 
 ```php
-$result = $api->add_subscriber_to_form(12345, 'joe.bloggs@convertkit.com');
+$result = $api->add_subscriber_to_form(12345, 'joe.bloggs@kit.com');
 $code = $api->getResponseInterface()->getStatusCode(); // 200 OK if e.g. a subscriber already added to the specified form, 201 Created if the subscriber added to the specified form for the first time.
 ```
 
 The PSR-7 response can be fetched and further inspected, if required - for example, to check if a header exists:
 
 ```php
-$result = $api->add_subscriber_to_form(12345, 'joe.bloggs@convertkit.com');
+$result = $api->add_subscriber_to_form(12345, 'joe.bloggs@kit.com');
 $api->getResponseInterface()->hasHeader('Content-Length'); // Check if the last API request included a `Content-Length` header
 ```
 
+### 2.2+ (v4 API, API Key, PHP 8.0+)
+
+Get your Kit API Key and API Secret [here](https://app.kit.com/account_settings/developer_settings) and set it somewhere in your application.
+
+```php
+// Require the autoloader (if you're using a PHP framework, this may already be done for you).
+require_once 'vendor/autoload.php';
+
+// Initialize the API class.
+$api = new \ConvertKit_API\ConvertKit_API(
+    apiKey: '<your_v4_api_key>'
+);
+```
+
 ### 1.x (v3 API, API Key and Secret, PHP 7.4+)
 
-Get your ConvertKit API Key and API Secret [here](https://app.convertkit.com/account/edit) and set it somewhere in your application.
+Get your Kit API Key and API Secret [here](https://app.kit.com/account_settings/developer_settings) and set it somewhere in your application.
 
 ```php
 // Require the autoloader (if you're using a PHP framework, this may already be done for you).
@@ -149,7 +164,7 @@ $api = new \ConvertKit_API\ConvertKit_API('<your_public_api_key>', '<your_secret
 
 ## Handling Errors
 
-The ConvertKit PHP SDK uses Guzzle for all HTTP API requests.  Errors will be thrown as Guzzle's `ClientException` (for 4xx errors),
+The Kit PHP SDK uses Guzzle for all HTTP API requests.  Errors will be thrown as Guzzle's `ClientException` (for 4xx errors),
 or `ServerException` (for 5xx errors).
 
 ```php
@@ -190,4 +205,4 @@ See the [PHP SDK docs](./docs/classes/ConvertKit_API/ConvertKit_API.md)
 
 See our [contributor guide](CONTRIBUTING.md) for setting up your development environment, testing and submitting a PR.
 
-For ConvertKit, refer to the [deployment guide](DEPLOYMENT.md) on how to publish a new release.
+For Kit, refer to the [deployment guide](DEPLOYMENT.md) on how to publish a new release.
diff --git a/composer.json b/composer.json
index 9b46196..30a377e 100644
--- a/composer.json
+++ b/composer.json
@@ -30,6 +30,11 @@
       "ConvertKit_API\\": "src/"
     }
   },
+  "autoload-dev": {
+    "psr-4": {
+      "": "tests/"
+    }
+  },
   "minimum-stability": "dev",
   "prefer-stable": true,
   "scripts": {
diff --git a/phpunit.xml b/phpunit.xml
index 0d14407..dd54dd6 100644
--- a/phpunit.xml
+++ b/phpunit.xml
@@ -3,6 +3,7 @@
     <testsuites>
         <testsuite name="ConvertKit API Tests">
             <directory>tests</directory>
+            <exclude>tests/ConvertKitAPITest.php</exclude>
         </testsuite>
     </testsuites>
 </phpunit>
diff --git a/src/ConvertKit_API.php b/src/ConvertKit_API.php
index 6fcd9d3..9a8e1b9 100644
--- a/src/ConvertKit_API.php
+++ b/src/ConvertKit_API.php
@@ -62,19 +62,22 @@ class ConvertKit_API
      * @param string  $clientID             OAuth Client ID.
      * @param string  $clientSecret         OAuth Client Secret.
      * @param string  $accessToken          OAuth Access Token.
+     * @param string  $apiKey               API Key.
      * @param boolean $debug                Log requests to debugger.
      * @param string  $debugLogFileLocation Path and filename of debug file to write to.
      */
     public function __construct(
-        string $clientID,
-        string $clientSecret,
+        string $clientID = '',
+        string $clientSecret = '',
         string $accessToken = '',
+        string $apiKey = '',
         bool $debug = false,
         string $debugLogFileLocation = ''
     ) {
         $this->client_id     = $clientID;
         $this->client_secret = $clientSecret;
         $this->access_token  = $accessToken;
+        $this->api_key       = $apiKey;
         $this->debug         = $debug;
 
         // Set the Guzzle client.
@@ -122,22 +125,35 @@ private function create_log(string $message)
             return;
         }
 
-        // Mask the Client ID, Client Secret and Access Token.
-        $message = str_replace(
-            $this->client_id,
-            str_repeat('*', (strlen($this->client_id) - 4)) . substr($this->client_id, - 4),
-            $message
-        );
-        $message = str_replace(
-            $this->client_secret,
-            str_repeat('*', (strlen($this->client_secret) - 4)) . substr($this->client_secret, - 4),
-            $message
-        );
-        $message = str_replace(
-            $this->access_token,
-            str_repeat('*', (strlen($this->access_token) - 4)) . substr($this->access_token, - 4),
-            $message
-        );
+        // Mask the Client ID, Client Secret, Access Token, and API Key.
+        if ($this->client_id) {
+            $message = str_replace(
+                $this->client_id,
+                str_repeat('*', (strlen($this->client_id) - 4)) . substr($this->client_id, - 4),
+                $message
+            );
+        }
+        if ($this->client_secret) {
+            $message = str_replace(
+                $this->client_secret,
+                str_repeat('*', (strlen($this->client_secret) - 4)) . substr($this->client_secret, - 4),
+                $message
+            );
+        }
+        if ($this->access_token) {
+            $message = str_replace(
+                $this->access_token,
+                str_repeat('*', (strlen($this->access_token) - 4)) . substr($this->access_token, - 4),
+                $message
+            );
+        }
+        if ($this->api_key) {
+            $message = str_replace(
+                $this->api_key,
+                str_repeat('*', (strlen($this->api_key) - 4)) . substr($this->api_key, - 4),
+                $message
+            );
+        }
 
         // Mask email addresses that may be contained within the message.
         $message = preg_replace_callback(
@@ -427,7 +443,12 @@ public function get_request_headers(string $type = 'application/json', bool $aut
         }
 
         // Add authorization header and return.
-        $headers['Authorization'] = 'Bearer ' . $this->access_token;
+        if ($this->api_key) {
+            $headers['X-Kit-Api-Key'] = $this->api_key;
+        } else if ($this->access_token) {
+            $headers['Authorization'] = 'Bearer ' . $this->access_token;
+        }
+
         return $headers;
     }
 
diff --git a/src/ConvertKit_API_Traits.php b/src/ConvertKit_API_Traits.php
index 7d72d43..71f2175 100644
--- a/src/ConvertKit_API_Traits.php
+++ b/src/ConvertKit_API_Traits.php
@@ -33,6 +33,13 @@ trait ConvertKit_API_Traits
      */
     protected $access_token = '';
 
+    /**
+     * API Key
+     *
+     * @var string
+     */
+    protected $api_key = '';
+
     /**
      * OAuth Authorization URL
      *
diff --git a/tests/ConvertKitAPIKeyTest.php b/tests/ConvertKitAPIKeyTest.php
new file mode 100644
index 0000000..6558399
--- /dev/null
+++ b/tests/ConvertKitAPIKeyTest.php
@@ -0,0 +1,759 @@
+<?php
+
+use GuzzleHttp\Exception\ClientException;
+use Dotenv\Dotenv;
+use ConvertKit_API\ConvertKit_API;
+
+/**
+ * ConvertKit API Key class tests.
+ */
+class ConvertKitAPIKeyTest extends ConvertKitAPITest
+{
+    /**
+     * Load .env configuration into $_ENV superglobal, and initialize the API
+     * class before each test.
+     *
+     * @since   2.2.0
+     *
+     * @return  void
+     */
+    protected function setUp(): void
+    {
+        // Load environment credentials from root folder.
+        $dotenv = Dotenv::createImmutable(dirname(dirname(__FILE__)));
+        $dotenv->load();
+
+        // Set location where API class will create/write the log file.
+        $this->logFile = dirname(dirname(__FILE__)) . '/src/logs/debug.log';
+
+        // Delete any existing debug log file.
+        $this->deleteLogFile();
+
+        // Setup API instances.
+        $this->api = new ConvertKit_API(
+            apiKey: $_ENV['CONVERTKIT_API_KEY']
+        );
+
+        // Wait a second to avoid hitting a 429 rate limit.
+        sleep(1);
+    }
+
+    /**
+     * Test that debug logging works when enabled and an API call is made.
+     *
+     * @since   2.2.0
+     *
+     * @return  void
+     */
+    public function testDebugEnabled()
+    {
+        // Setup API with debugging enabled.
+        $api = new ConvertKit_API(
+            apiKey: $_ENV['CONVERTKIT_API_KEY'],
+            debug: true
+        );
+
+        $result = $api->get_account();
+
+        // Confirm that the log includes expected data.
+        $this->assertStringContainsString('ck-debug.INFO: GET account', $this->getLogFileContents());
+        $this->assertStringContainsString('ck-debug.INFO: Finish request successfully', $this->getLogFileContents());
+    }
+
+    /**
+     * Test that debug logging works when enabled, a custom debug log file and path is specified
+     * and an API call is made.
+     *
+     * @since   2.2.0
+     *
+     * @return  void
+     */
+    public function testDebugEnabledWithCustomLogFile()
+    {
+        // Define custom log file location.
+        $this->logFile = dirname(dirname(__FILE__)) . '/src/logs/debug-custom.log';
+
+        // Setup API with debugging enabled.
+        $api = new ConvertKit_API(
+            apiKey: $_ENV['CONVERTKIT_API_KEY'],
+            debug: true,
+            debugLogFileLocation: $this->logFile
+        );
+        $result = $api->get_account();
+
+        // Confirm log file exists.
+        $this->assertFileExists($this->logFile);
+
+        // Confirm that the log includes expected data.
+        $this->assertStringContainsString('ck-debug.INFO: GET account', $this->getLogFileContents());
+        $this->assertStringContainsString('ck-debug.INFO: Finish request successfully', $this->getLogFileContents());
+    }
+
+    /**
+     * Test that debug logging works when enabled and an API call is made, with email addresses and credentials
+     * masked in the log file.
+     *
+     * @since   2.2.0
+     *
+     * @return  void
+     */
+    public function testDebugCredentialsAndEmailsAreMasked()
+    {
+        // Setup API with debugging enabled.
+        $api = new ConvertKit_API(
+            apiKey: $_ENV['CONVERTKIT_API_KEY'],
+            debug: true
+        );
+
+        // Create log entries with API Key and Email Address, as if an API method
+        // were to log this sensitive data.
+        $this->callPrivateMethod($api, 'create_log', ['API Key: ' . $_ENV['CONVERTKIT_API_KEY']]);
+        $this->callPrivateMethod($api, 'create_log', ['Email: ' . $_ENV['CONVERTKIT_API_SUBSCRIBER_EMAIL']]);
+
+        // Confirm that the log includes the masked API Key and Email Address.
+        $this->assertStringContainsString(
+            str_repeat(
+                '*',
+                (strlen($_ENV['CONVERTKIT_API_KEY']) - 4)
+            ) . substr($_ENV['CONVERTKIT_API_KEY'], -4),
+            $this->getLogFileContents()
+        );
+        $this->assertStringContainsString(
+            'o****@n********.c**',
+            $this->getLogFileContents()
+        );
+
+        // Confirm that the log does not include the unmasked API Key or Email Address.
+        $this->assertStringNotContainsString($_ENV['CONVERTKIT_API_KEY'], $this->getLogFileContents());
+        $this->assertStringNotContainsString($_ENV['CONVERTKIT_API_SUBSCRIBER_EMAIL'], $this->getLogFileContents());
+    }
+
+    /**
+     * Test that debug logging is not performed when disabled and an API call is made.
+     *
+     * @since   2.2.0
+     *
+     * @return  void
+     */
+    public function testDebugDisabled()
+    {
+        $result = $this->api->get_account();
+        $this->assertEmpty($this->getLogFileContents());
+    }
+
+    /**
+     * Test that calling request_headers() returns the expected array of headers
+     *
+     * @since   2.2.0
+     *
+     * @return  void
+     */
+    public function testRequestHeadersMethod()
+    {
+        $headers = $this->api->get_request_headers();
+        $this->assertArrayHasKey('Accept', $headers);
+        $this->assertArrayHasKey('Content-Type', $headers);
+        $this->assertArrayHasKey('User-Agent', $headers);
+        $this->assertArrayHasKey('X-Kit-Api-Key', $headers);
+        $this->assertEquals($headers['Accept'], 'application/json');
+        $this->assertEquals($headers['Content-Type'], 'application/json; charset=utf-8');
+        $this->assertEquals($headers['User-Agent'], 'ConvertKitPHPSDK/' . $this->api::VERSION . ';PHP/' . phpversion());
+        $this->assertEquals($headers['X-Kit-Api-Key'], $_ENV['CONVERTKIT_API_KEY']);
+    }
+
+    /**
+     * Test that calling request_headers() with a different `type` parameter
+     * returns the expected array of headers
+     *
+     * @since   2.2.0
+     *
+     * @return  void
+     */
+    public function testRequestHeadersMethodWithType()
+    {
+        $headers = $this->api->get_request_headers(
+            type: 'text/html'
+        );
+        $this->assertArrayHasKey('Accept', $headers);
+        $this->assertArrayHasKey('Content-Type', $headers);
+        $this->assertArrayHasKey('User-Agent', $headers);
+        $this->assertArrayHasKey('X-Kit-Api-Key', $headers);
+        $this->assertEquals($headers['Accept'], 'text/html');
+        $this->assertEquals($headers['Content-Type'], 'text/html; charset=utf-8');
+        $this->assertEquals($headers['User-Agent'], 'ConvertKitPHPSDK/' . $this->api::VERSION . ';PHP/' . phpversion());
+        $this->assertEquals($headers['X-Kit-Api-Key'], $_ENV['CONVERTKIT_API_KEY']);
+    }
+
+    /**
+     * Test that calling request_headers() with the `auth` parameter set to false
+     * returns the expected array of headers
+     *
+     * @since   2.2.0
+     *
+     * @return  void
+     */
+    public function testRequestHeadersMethodWithAuthDisabled()
+    {
+        $headers = $this->api->get_request_headers(
+            auth: false
+        );
+        $this->assertArrayHasKey('Accept', $headers);
+        $this->assertArrayHasKey('Content-Type', $headers);
+        $this->assertArrayHasKey('User-Agent', $headers);
+        $this->assertArrayNotHasKey('X-Kit-Api-Key', $headers);
+        $this->assertEquals($headers['Accept'], 'application/json');
+        $this->assertEquals($headers['Content-Type'], 'application/json; charset=utf-8');
+        $this->assertEquals($headers['User-Agent'], 'ConvertKitPHPSDK/' . $this->api::VERSION . ';PHP/' . phpversion());
+    }
+
+    /**
+     * Test that calling request_headers() with a different `type` parameter
+     * and the `auth` parameter set to false returns the expected array of headers
+     *
+     * @since   2.2.0
+     *
+     * @return  void
+     */
+    public function testRequestHeadersMethodWithTypeAndAuthDisabled()
+    {
+        $headers = $this->api->get_request_headers(
+            type: 'text/html',
+            auth: false
+        );
+        $this->assertArrayHasKey('Accept', $headers);
+        $this->assertArrayHasKey('Content-Type', $headers);
+        $this->assertArrayHasKey('User-Agent', $headers);
+        $this->assertArrayNotHasKey('X-Kit-Api-Key', $headers);
+        $this->assertEquals($headers['Accept'], 'text/html');
+        $this->assertEquals($headers['Content-Type'], 'text/html; charset=utf-8');
+        $this->assertEquals($headers['User-Agent'], 'ConvertKitPHPSDK/' . $this->api::VERSION . ';PHP/' . phpversion());
+    }
+
+    /**
+     * Test that create_tags() throws a ClientException when attempting
+     * to create tags, as this is only supported using OAuth.
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testCreateTags()
+    {
+        $this->expectException(ClientException::class);
+        $result = $this->api->create_tags([
+            'Tag Test ' . mt_rand(),
+            'Tag Test ' . mt_rand(),
+        ]);
+    }
+
+    /**
+     * Test that create_tags() throws a ClientException when attempting
+     * to create blank tags, as this is only supported using OAuth.
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testCreateTagsBlank()
+    {
+        $this->expectException(ClientException::class);
+        $result = $this->api->create_tags([
+            '',
+            '',
+        ]);
+    }
+
+    /**
+     * Test that create_tags() throws a ClientException when creating
+     * tags that already exists, as this is only supported using OAuth.
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testCreateTagsThatExist()
+    {
+        $this->expectException(ClientException::class);
+        $result = $this->api->create_tags(
+            [
+                $_ENV['CONVERTKIT_API_TAG_NAME'],
+                $_ENV['CONVERTKIT_API_TAG_NAME_2'],
+            ]
+        );
+    }
+
+    /**
+     * Test that add_subscribers_to_forms() throws a ClientException when
+     * attempting to add subscribers to forms, as this is only supported
+     * using OAuth.
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testAddSubscribersToForms()
+    {
+        // Create subscriber.
+        $emailAddress = $this->generateEmailAddress();
+        $subscriber = $this->api->create_subscriber(
+            email_address: $emailAddress
+        );
+
+        // Set subscriber_id to ensure subscriber is unsubscribed after test.
+        $this->subscriber_ids[] = $subscriber->subscriber->id;
+
+        $this->expectException(ClientException::class);
+
+        // Add subscribers to forms.
+        $result = $this->api->add_subscribers_to_forms(
+            forms_subscribers_ids: [
+                [
+                    'form_id' => (int) $_ENV['CONVERTKIT_API_FORM_ID'],
+                    'subscriber_id' => $subscriber->subscriber->id,
+                ],
+                [
+                    'form_id' => (int) $_ENV['CONVERTKIT_API_FORM_ID_2'],
+                    'subscriber_id' => $subscriber->subscriber->id,
+                ],
+            ]
+        );
+    }
+
+    /**
+     * Test that add_subscribers_to_forms() returns a ClientException
+     * when a referrer URL is specified, as this is only supported
+     * using OAuth.
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testAddSubscribersToFormsWithReferrer()
+    {
+        // Create subscriber.
+        $emailAddress = $this->generateEmailAddress();
+        $subscriber = $this->api->create_subscriber(
+            email_address: $emailAddress
+        );
+
+        // Set subscriber_id to ensure subscriber is unsubscribed after test.
+        $this->subscriber_ids[] = $subscriber->subscriber->id;
+
+        $this->expectException(ClientException::class);
+
+        // Add subscribers to forms.
+        $result = $this->api->add_subscribers_to_forms(
+            forms_subscribers_ids: [
+                [
+                    'form_id' => (int) $_ENV['CONVERTKIT_API_FORM_ID'],
+                    'subscriber_id' => $subscriber->subscriber->id,
+                    'referrer' => 'https://mywebsite.com/bfpromo/',
+                ],
+                [
+                    'form_id' => (int) $_ENV['CONVERTKIT_API_FORM_ID_2'],
+                    'subscriber_id' => $subscriber->subscriber->id,
+                    'referrer' => 'https://mywebsite.com/bfpromo/',
+                ],
+            ]
+        );
+    }
+
+    /**
+     * Test that add_subscribers_to_forms() returns a ClientException
+     * when a referrer URL with UTM parameters is specified, as this is only
+     * supported using OAuth.
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testAddSubscribersToFormsWithReferrerUTMParams()
+    {
+        // Define referrer.
+        $referrerUTMParams = [
+            'utm_source'    => 'facebook',
+            'utm_medium'    => 'cpc',
+            'utm_campaign'  => 'black_friday',
+            'utm_term'      => 'car_owners',
+            'utm_content'   => 'get_10_off',
+        ];
+        $referrer = 'https://mywebsite.com/bfpromo/?' . http_build_query($referrerUTMParams);
+
+        // Create subscriber.
+        $emailAddress = $this->generateEmailAddress();
+        $subscriber = $this->api->create_subscriber(
+            email_address: $emailAddress
+        );
+
+        // Set subscriber_id to ensure subscriber is unsubscribed after test.
+        $this->subscriber_ids[] = $subscriber->subscriber->id;
+
+        $this->expectException(ClientException::class);
+
+        // Add subscribers to forms.
+        $result = $this->api->add_subscribers_to_forms(
+            forms_subscribers_ids: [
+                [
+                    'form_id' => (int) $_ENV['CONVERTKIT_API_FORM_ID'],
+                    'subscriber_id' => $subscriber->subscriber->id,
+                    'referrer' => $referrer,
+                ],
+                [
+                    'form_id' => (int) $_ENV['CONVERTKIT_API_FORM_ID_2'],
+                    'subscriber_id' => $subscriber->subscriber->id,
+                    'referrer' => $referrer,
+                ],
+            ]
+        );
+    }
+
+    /**
+     * Test that add_subscribers_to_forms() returns a ClientException
+     * when invalid Form IDs are specified, as this is only supported
+     * using OAuth.
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testAddSubscribersToFormsWithInvalidFormIDs()
+    {
+        // Create subscriber.
+        $emailAddress = $this->generateEmailAddress();
+        $subscriber = $this->api->create_subscriber(
+            email_address: $emailAddress
+        );
+
+        // Set subscriber_id to ensure subscriber is unsubscribed after test.
+        $this->subscriber_ids[] = $subscriber->subscriber->id;
+
+        $this->expectException(ClientException::class);
+
+        // Add subscribers to forms.
+        $result = $this->api->add_subscribers_to_forms(
+            forms_subscribers_ids: [
+                [
+                    'form_id' => 9999999,
+                    'subscriber_id' => $subscriber->subscriber->id,
+                ],
+                [
+                    'form_id' => 9999999,
+                    'subscriber_id' => $subscriber->subscriber->id,
+                ],
+            ]
+        );
+    }
+
+    /**
+     * Test that add_subscribers_to_forms() returns a ClientException
+     * when invalid Subscriber IDs are specified, as this is only supported
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testAddSubscribersToFormsWithInvalidSubscriberIDs()
+    {
+        // Create subscriber.
+        $emailAddress = $this->generateEmailAddress();
+        $subscriber = $this->api->create_subscriber(
+            email_address: $emailAddress
+        );
+
+        // Set subscriber_id to ensure subscriber is unsubscribed after test.
+        $this->subscriber_ids[] = $subscriber->subscriber->id;
+
+        $this->expectException(ClientException::class);
+
+        // Add subscribers to forms.
+        $result = $this->api->add_subscribers_to_forms(
+            forms_subscribers_ids: [
+                [
+                    'form_id' => (int) $_ENV['CONVERTKIT_API_FORM_ID'],
+                    'subscriber_id' => 999999,
+                ],
+                [
+                    'form_id' => (int) $_ENV['CONVERTKIT_API_FORM_ID_2'],
+                    'subscriber_id' => 999999,
+                ],
+            ]
+        );
+    }
+
+    /**
+     * Test that create_subscribers() returns a ClientException
+     * when attempting to create subscribers, as this is only supported
+     * using OAuth.
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testCreateSubscribers()
+    {
+        $this->expectException(ClientException::class);
+        $subscribers = [
+            [
+                'email_address' => str_replace('@convertkit.com', '-1@convertkit.com', $this->generateEmailAddress()),
+            ],
+            [
+                'email_address' => str_replace('@convertkit.com', '-2@convertkit.com', $this->generateEmailAddress()),
+            ],
+        ];
+        $result = $this->api->create_subscribers($subscribers);
+    }
+
+    /**
+     * Test that create_subscribers() throws a ClientException when no data is specified.
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testCreateSubscribersWithBlankData()
+    {
+        $this->expectException(ClientException::class);
+        $result = $this->api->create_subscribers([
+            [],
+        ]);
+    }
+
+    /**
+     * Test that create_subscribers() throws a ClientException when invalid email addresses
+     * are specified, as this is only supported using OAuth.
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testCreateSubscribersWithInvalidEmailAddresses()
+    {
+        $this->expectException(ClientException::class);
+        $subscribers = [
+            [
+                'email_address' => 'not-an-email-address',
+            ],
+            [
+                'email_address' => 'not-an-email-address-again',
+            ],
+        ];
+        $result = $this->api->create_subscribers($subscribers);
+    }
+
+    /**
+     * Test that create_custom_fields() throws a ClientException
+     * as this is only supported using OAuth.
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testCreateCustomFields()
+    {
+        $this->expectException(ClientException::class);
+        $labels = [
+            'Custom Field ' . mt_rand(),
+            'Custom Field ' . mt_rand(),
+        ];
+        $result = $this->api->create_custom_fields($labels);
+    }
+
+    /**
+     * Test that get_purchases() throws a ClientException
+     * as this is only supported using OAuth.
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testGetPurchases()
+    {
+        $this->expectException(ClientException::class);
+        $result = $this->api->get_purchases();
+    }
+
+    /**
+     * Test that get_purchases() throws a ClientException
+     * when the total count is included, as this is only
+     * supported using OAuth.
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testGetPurchasesWithTotalCount()
+    {
+        $this->expectException(ClientException::class);
+        $result = $this->api->get_purchases(
+            include_total_count: true
+        );
+    }
+
+    /**
+     * Test that get_purchases() throws a ClientException
+     * when pagination parameters and per_page limits are specified,
+     * as this is only supported using OAuth.
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testGetPurchasesPagination()
+    {
+        $this->expectException(ClientException::class);
+        $result = $this->api->get_purchases(
+            per_page: 1
+        );
+    }
+
+    /**
+     * Test that get_purchases() throws a ClientException
+     * when a purchase ID is specified, as this is only
+     * supported using OAuth.
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testGetPurchase()
+    {
+        $this->expectException(ClientException::class);
+        $result = $this->api->get_purchase(12345);
+    }
+
+    /**
+     * Test that get_purchases() throws a ClientException when an invalid
+     * purchase ID is specified, as this is only supported
+     * using OAuth.
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testGetPurchaseWithInvalidID()
+    {
+        $this->expectException(ClientException::class);
+        $this->api->get_purchase(12345);
+    }
+
+    /**
+     * Test that create_purchase() throws a ClientException
+     * as this is only supported using OAuth.
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testCreatePurchase()
+    {
+        $this->expectException(ClientException::class);
+        $purchase = $this->api->create_purchase(
+            // Required fields.
+            email_address: $this->generateEmailAddress(),
+            transaction_id: str_shuffle('wfervdrtgsdewrafvwefds'),
+            currency: 'usd',
+            products: [
+                [
+                    'name' => 'Floppy Disk (512k)',
+                    'sku' => '7890-ijkl',
+                    'pid' => 9999,
+                    'lid' => 7777,
+                    'quantity' => 2,
+                    'unit_price' => 5.00,
+                ],
+                [
+                    'name' => 'Telephone Cord (data)',
+                    'sku' => 'mnop-1234',
+                    'pid' => 5555,
+                    'lid' => 7778,
+                    'quantity' => 1,
+                    'unit_price' => 10.00,
+                ],
+            ],
+            // Optional fields.
+            first_name: 'Tim',
+            status: 'paid',
+            subtotal: 20.00,
+            tax: 2.00,
+            shipping: 2.00,
+            discount: 3.00,
+            total: 21.00,
+            transaction_time: new DateTime('now'),
+        );
+    }
+
+    /**
+     * Test that create_purchase() throws a ClientException when an invalid
+     * email address is specified, as this is only supported using OAuth.
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testCreatePurchaseWithInvalidEmailAddress()
+    {
+        $this->expectException(ClientException::class);
+        $this->api->create_purchase(
+            email_address: 'not-an-email-address',
+            transaction_id: str_shuffle('wfervdrtgsdewrafvwefds'),
+            currency: 'usd',
+            products: [
+                [
+                    'name' => 'Floppy Disk (512k)',
+                    'sku' => '7890-ijkl',
+                    'pid' => 9999,
+                    'lid' => 7777,
+                    'quantity' => 2,
+                    'unit_price' => 5.00,
+                ],
+            ],
+        );
+    }
+
+    /**
+     * Test that create_purchase() throws a ClientException when a blank
+     * transaction ID is specified, as this is only supported using OAuth.
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testCreatePurchaseWithBlankTransactionID()
+    {
+        $this->expectException(ClientException::class);
+        $this->api->create_purchase(
+            email_address: $this->generateEmailAddress(),
+            transaction_id: '',
+            currency: 'usd',
+            products: [
+                [
+                    'name' => 'Floppy Disk (512k)',
+                    'sku' => '7890-ijkl',
+                    'pid' => 9999,
+                    'lid' => 7777,
+                    'quantity' => 2,
+                    'unit_price' => 5.00,
+                ],
+            ],
+        );
+    }
+
+    /**
+     * Test that create_purchase() throws a ClientException when no products
+     * are specified, as this is only supported using OAuth.
+     *
+     * @since   2.2.0
+     *
+     * @return void
+     */
+    public function testCreatePurchaseWithNoProducts()
+    {
+        $this->expectException(ClientException::class);
+        $this->api->create_purchase(
+            email_address: $this->generateEmailAddress(),
+            transaction_id: str_shuffle('wfervdrtgsdewrafvwefds'),
+            currency: 'usd',
+            products: [],
+        );
+    }
+}
diff --git a/tests/ConvertKitAPIOAuthTest.php b/tests/ConvertKitAPIOAuthTest.php
new file mode 100644
index 0000000..829d0a7
--- /dev/null
+++ b/tests/ConvertKitAPIOAuthTest.php
@@ -0,0 +1,457 @@
+<?php
+
+use GuzzleHttp\Exception\ClientException;
+use GuzzleHttp\Exception\ServerException;
+use Dotenv\Dotenv;
+use ConvertKit_API\ConvertKit_API;
+
+/**
+ * ConvertKit API OAuth Key class tests.
+ */
+class ConvertKitAPIOAuthTest extends ConvertKitAPITest
+{
+    /**
+     * Load .env configuration into $_ENV superglobal, and initialize the API
+     * class before each test.
+     *
+     * @since   2.2.0
+     *
+     * @return  void
+     */
+    protected function setUp(): void
+    {
+        // Load environment credentials from root folder.
+        $dotenv = Dotenv::createImmutable(dirname(dirname(__FILE__)));
+        $dotenv->load();
+
+        // Set location where API class will create/write the log file.
+        $this->logFile = dirname(dirname(__FILE__)) . '/src/logs/debug.log';
+
+        // Delete any existing debug log file.
+        $this->deleteLogFile();
+
+        // Setup API.
+        $this->api = new ConvertKit_API(
+            clientID: $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
+            clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET'],
+            accessToken: $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN']
+        );
+
+        // Wait a second to avoid hitting a 429 rate limit.
+        sleep(1);
+    }
+
+    /**
+     * Test that debug logging works when enabled and an API call is made.
+     *
+     * @since   2.2.0
+     *
+     * @return  void
+     */
+    public function testDebugEnabled()
+    {
+        // Setup API with debugging enabled.
+        $api = new ConvertKit_API(
+            clientID: $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
+            clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET'],
+            accessToken: $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN'],
+            debug: true
+        );
+        $result = $api->get_account();
+
+        // Confirm that the log includes expected data.
+        $this->assertStringContainsString('ck-debug.INFO: GET account', $this->getLogFileContents());
+        $this->assertStringContainsString('ck-debug.INFO: Finish request successfully', $this->getLogFileContents());
+    }
+
+    /**
+     * Test that debug logging works when enabled, a custom debug log file and path is specified
+     * and an API call is made.
+     *
+     * @since   2.2.0
+     *
+     * @return  void
+     */
+    public function testDebugEnabledWithCustomLogFile()
+    {
+        // Define custom log file location.
+        $this->logFile = dirname(dirname(__FILE__)) . '/src/logs/debug-custom.log';
+
+        // Setup API with debugging enabled.
+        $api = new ConvertKit_API(
+            clientID: $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
+            clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET'],
+            accessToken: $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN'],
+            debug: true,
+            debugLogFileLocation: $this->logFile
+        );
+        $result = $api->get_account();
+
+        // Confirm log file exists.
+        $this->assertFileExists($this->logFile);
+
+        // Confirm that the log includes expected data.
+        $this->assertStringContainsString('ck-debug.INFO: GET account', $this->getLogFileContents());
+        $this->assertStringContainsString('ck-debug.INFO: Finish request successfully', $this->getLogFileContents());
+    }
+
+    /**
+     * Test that debug logging works when enabled and an API call is made, with email addresses and credentials
+     * masked in the log file.
+     *
+     * @since   2.2.0
+     *
+     * @return  void
+     */
+    public function testDebugCredentialsAndEmailsAreMasked()
+    {
+        // Setup API with debugging enabled.
+        $api = new ConvertKit_API(
+            clientID: $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
+            clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET'],
+            accessToken: $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN'],
+            debug: true
+        );
+
+        // Create log entries with Client ID, Client Secret, Access Token and Email Address, as if an API method
+        // were to log this sensitive data.
+        $this->callPrivateMethod($api, 'create_log', ['Client ID: ' . $_ENV['CONVERTKIT_OAUTH_CLIENT_ID']]);
+        $this->callPrivateMethod($api, 'create_log', ['Client Secret: ' . $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET']]);
+        $this->callPrivateMethod($api, 'create_log', ['Access Token: ' . $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN']]);
+        $this->callPrivateMethod($api, 'create_log', ['Email: ' . $_ENV['CONVERTKIT_API_SUBSCRIBER_EMAIL']]);
+
+        // Confirm that the log includes the masked Client ID, Secret, Access Token and Email Address.
+        $this->assertStringContainsString(
+            str_repeat(
+                '*',
+                (strlen($_ENV['CONVERTKIT_OAUTH_CLIENT_ID']) - 4)
+            ) . substr($_ENV['CONVERTKIT_OAUTH_CLIENT_ID'], -4),
+            $this->getLogFileContents()
+        );
+        $this->assertStringContainsString(
+            str_repeat(
+                '*',
+                (strlen($_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET']) - 4)
+            ) . substr($_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET'], -4),
+            $this->getLogFileContents()
+        );
+        $this->assertStringContainsString(
+            str_repeat(
+                '*',
+                (strlen($_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN']) - 4)
+            ) . substr($_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN'], -4),
+            $this->getLogFileContents()
+        );
+        $this->assertStringContainsString(
+            'o****@n********.c**',
+            $this->getLogFileContents()
+        );
+
+        // Confirm that the log does not include the unmasked Client ID, Secret, Access Token or Email Address.
+        $this->assertStringNotContainsString($_ENV['CONVERTKIT_OAUTH_CLIENT_ID'], $this->getLogFileContents());
+        $this->assertStringNotContainsString($_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET'], $this->getLogFileContents());
+        $this->assertStringNotContainsString($_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN'], $this->getLogFileContents());
+        $this->assertStringNotContainsString($_ENV['CONVERTKIT_API_SUBSCRIBER_EMAIL'], $this->getLogFileContents());
+    }
+
+    /**
+     * Test that debug logging is not performed when disabled and an API call is made.
+     *
+     * @since   2.2.0
+     *
+     * @return  void
+     */
+    public function testDebugDisabled()
+    {
+        $result = $this->api->get_account();
+        $this->assertEmpty($this->getLogFileContents());
+    }
+
+    /**
+     * Test that calling request_headers() returns the expected array of headers
+     *
+     * @since   2.0.0
+     *
+     * @return  void
+     */
+    public function testRequestHeadersMethod()
+    {
+        $headers = $this->api->get_request_headers();
+        $this->assertArrayHasKey('Accept', $headers);
+        $this->assertArrayHasKey('Content-Type', $headers);
+        $this->assertArrayHasKey('User-Agent', $headers);
+        $this->assertArrayHasKey('Authorization', $headers);
+        $this->assertEquals($headers['Accept'], 'application/json');
+        $this->assertEquals($headers['Content-Type'], 'application/json; charset=utf-8');
+        $this->assertEquals($headers['User-Agent'], 'ConvertKitPHPSDK/' . $this->api::VERSION . ';PHP/' . phpversion());
+        $this->assertEquals($headers['Authorization'], 'Bearer ' . $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN']);
+    }
+
+    /**
+     * Test that calling request_headers() with a different `type` parameter
+     * returns the expected array of headers
+     *
+     * @since   2.0.0
+     *
+     * @return  void
+     */
+    public function testRequestHeadersMethodWithType()
+    {
+        $headers = $this->api->get_request_headers(
+            type: 'text/html'
+        );
+        $this->assertArrayHasKey('Accept', $headers);
+        $this->assertArrayHasKey('Content-Type', $headers);
+        $this->assertArrayHasKey('User-Agent', $headers);
+        $this->assertArrayHasKey('Authorization', $headers);
+        $this->assertEquals($headers['Accept'], 'text/html');
+        $this->assertEquals($headers['Content-Type'], 'text/html; charset=utf-8');
+        $this->assertEquals($headers['User-Agent'], 'ConvertKitPHPSDK/' . $this->api::VERSION . ';PHP/' . phpversion());
+        $this->assertEquals($headers['Authorization'], 'Bearer ' . $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN']);
+    }
+
+    /**
+     * Test that calling request_headers() with the `auth` parameter set to false
+     * returns the expected array of headers
+     *
+     * @since   2.0.0
+     *
+     * @return  void
+     */
+    public function testRequestHeadersMethodWithAuthDisabled()
+    {
+        $headers = $this->api->get_request_headers(
+            auth: false
+        );
+        $this->assertArrayHasKey('Accept', $headers);
+        $this->assertArrayHasKey('Content-Type', $headers);
+        $this->assertArrayHasKey('User-Agent', $headers);
+        $this->assertArrayNotHasKey('Authorization', $headers);
+        $this->assertEquals($headers['Accept'], 'application/json');
+        $this->assertEquals($headers['Content-Type'], 'application/json; charset=utf-8');
+        $this->assertEquals($headers['User-Agent'], 'ConvertKitPHPSDK/' . $this->api::VERSION . ';PHP/' . phpversion());
+    }
+
+    /**
+     * Test that calling request_headers() with a different `type` parameter
+     * and the `auth` parameter set to false returns the expected array of headers
+     *
+     * @since   2.0.0
+     *
+     * @return  void
+     */
+    public function testRequestHeadersMethodWithTypeAndAuthDisabled()
+    {
+        $headers = $this->api->get_request_headers(
+            type: 'text/html',
+            auth: false
+        );
+        $this->assertArrayHasKey('Accept', $headers);
+        $this->assertArrayHasKey('Content-Type', $headers);
+        $this->assertArrayHasKey('User-Agent', $headers);
+        $this->assertArrayNotHasKey('Authorization', $headers);
+        $this->assertEquals($headers['Accept'], 'text/html');
+        $this->assertEquals($headers['Content-Type'], 'text/html; charset=utf-8');
+        $this->assertEquals($headers['User-Agent'], 'ConvertKitPHPSDK/' . $this->api::VERSION . ';PHP/' . phpversion());
+    }
+
+    /**
+     * Test that get_oauth_url() returns the correct URL to begin the OAuth process.
+     *
+     * @since   2.0.0
+     *
+     * @return  void
+     */
+    public function testGetOAuthURL()
+    {
+        // Confirm the OAuth URL returned is correct.
+        $this->assertEquals(
+            $this->api->get_oauth_url($_ENV['CONVERTKIT_OAUTH_REDIRECT_URI']),
+            'https://app.convertkit.com/oauth/authorize?' . http_build_query([
+                'client_id' => $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
+                'redirect_uri' => $_ENV['CONVERTKIT_OAUTH_REDIRECT_URI'],
+                'response_type' => 'code',
+            ])
+        );
+    }
+
+    /**
+     * Test that get_access_token() returns the expected data.
+     *
+     * @since   2.0.0
+     *
+     * @return void
+     */
+    public function testGetAccessToken()
+    {
+        // Initialize API.
+        $api = new ConvertKit_API(
+            clientID: $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
+            clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET']
+        );
+
+        // Define response parameters.
+        $params = [
+            'access_token'  => 'example-access-token',
+            'refresh_token' => 'example-refresh-token',
+            'token_type'    => 'Bearer',
+            'created_at'    => strtotime('now'),
+            'expires_in'    => strtotime('+3 days'),
+            'scope'         => 'public',
+        ];
+
+        // Add mock handler for this API request.
+        $api = $this->mockResponse(
+            api: $api,
+            responseBody: $params,
+        );
+
+        // Send request.
+        $result = $api->get_access_token(
+            authCode: 'auth-code',
+            redirectURI: $_ENV['CONVERTKIT_OAUTH_REDIRECT_URI'],
+        );
+
+        // Inspect response.
+        $result = get_object_vars($result);
+        $this->assertIsArray($result);
+        $this->assertArrayHasKey('access_token', $result);
+        $this->assertArrayHasKey('refresh_token', $result);
+        $this->assertArrayHasKey('token_type', $result);
+        $this->assertArrayHasKey('created_at', $result);
+        $this->assertArrayHasKey('expires_in', $result);
+        $this->assertArrayHasKey('scope', $result);
+        $this->assertEquals($result['access_token'], $params['access_token']);
+        $this->assertEquals($result['refresh_token'], $params['refresh_token']);
+        $this->assertEquals($result['created_at'], $params['created_at']);
+        $this->assertEquals($result['expires_in'], $params['expires_in']);
+    }
+
+    /**
+     * Test that a ClientException is thrown when an invalid auth code is supplied
+     * when fetching an access token.
+     *
+     * @since   2.0.0
+     *
+     * @return void
+     */
+    public function testGetAccessTokenWithInvalidAuthCode()
+    {
+        $this->expectException(ClientException::class);
+        $api = new ConvertKit_API(
+            clientID: $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
+            clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET']
+        );
+        $result = $api->get_access_token(
+            authCode: 'not-a-real-auth-code',
+            redirectURI: $_ENV['CONVERTKIT_OAUTH_REDIRECT_URI'],
+        );
+    }
+
+    /**
+     * Test that refresh_token() returns the expected data.
+     *
+     * @since   2.0.0
+     *
+     * @return void
+     */
+    public function testRefreshToken()
+    {
+        // Initialize API.
+        $api = new ConvertKit_API(
+            clientID: $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
+            clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET']
+        );
+
+        // Define response parameters.
+        $params = [
+            'access_token'  => 'new-example-access-token',
+            'refresh_token' => 'new-example-refresh-token',
+            'token_type'    => 'Bearer',
+            'created_at'    => strtotime('now'),
+            'expires_in'    => strtotime('+3 days'),
+            'scope'         => 'public',
+        ];
+
+        // Add mock handler for this API request.
+        $api = $this->mockResponse(
+            api: $api,
+            responseBody: $params,
+        );
+
+        // Send request.
+        $result = $api->refresh_token(
+            refreshToken: 'refresh-token',
+            redirectURI: $_ENV['CONVERTKIT_OAUTH_REDIRECT_URI'],
+        );
+
+        // Inspect response.
+        $result = get_object_vars($result);
+        $this->assertIsArray($result);
+        $this->assertArrayHasKey('access_token', $result);
+        $this->assertArrayHasKey('refresh_token', $result);
+        $this->assertArrayHasKey('token_type', $result);
+        $this->assertArrayHasKey('created_at', $result);
+        $this->assertArrayHasKey('expires_in', $result);
+        $this->assertArrayHasKey('scope', $result);
+        $this->assertEquals($result['access_token'], $params['access_token']);
+        $this->assertEquals($result['refresh_token'], $params['refresh_token']);
+        $this->assertEquals($result['created_at'], $params['created_at']);
+        $this->assertEquals($result['expires_in'], $params['expires_in']);
+    }
+
+    /**
+     * Test that a ServerException is thrown when an invalid refresh token is supplied
+     * when refreshing an access token.
+     *
+     * @since   2.0.0
+     *
+     * @return void
+     */
+    public function testRefreshTokenWithInvalidToken()
+    {
+        $this->expectException(ServerException::class);
+        $api = new ConvertKit_API(
+            clientID: $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
+            clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET']
+        );
+        $result = $api->refresh_token(
+            refreshToken: 'not-a-real-refresh-token',
+            redirectURI: $_ENV['CONVERTKIT_OAUTH_REDIRECT_URI'],
+        );
+    }
+
+    /**
+     * Test that a ClientException is thrown when an invalid access token is supplied.
+     *
+     * @since   1.0.0
+     *
+     * @return void
+     */
+    public function testInvalidAPICredentials()
+    {
+        $this->expectException(ClientException::class);
+        $api = new ConvertKit_API(
+            clientID: 'fakeClientID',
+            clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET'],
+            accessToken: $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN']
+        );
+        $result = $api->get_account();
+
+        $this->expectException(ClientException::class);
+        $api = new ConvertKit_API(
+            clientID: $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
+            clientSecret: 'fakeClientSecret',
+            accessToken: $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN']
+        );
+        $result = $api->get_account();
+
+        $this->expectException(ClientException::class);
+        $api = new ConvertKit_API(
+            clientID: $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
+            clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET'],
+            accessToken: 'fakeAccessToken'
+        );
+        $result = $api->get_account();
+    }
+}
diff --git a/tests/ConvertKitAPITest.php b/tests/ConvertKitAPITest.php
index b425d13..786d85e 100644
--- a/tests/ConvertKitAPITest.php
+++ b/tests/ConvertKitAPITest.php
@@ -69,37 +69,6 @@ class ConvertKitAPITest extends TestCase
      */
     protected $broadcast_ids = [];
 
-    /**
-     * Load .env configuration into $_ENV superglobal, and initialize the API
-     * class before each test.
-     *
-     * @since   1.0.0
-     *
-     * @return  void
-     */
-    protected function setUp(): void
-    {
-        // Load environment credentials from root folder.
-        $dotenv = Dotenv::createImmutable(dirname(dirname(__FILE__)));
-        $dotenv->load();
-
-        // Set location where API class will create/write the log file.
-        $this->logFile = dirname(dirname(__FILE__)) . '/src/logs/debug.log';
-
-        // Delete any existing debug log file.
-        $this->deleteLogFile();
-
-        // Setup API.
-        $this->api = new ConvertKit_API(
-            clientID: $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
-            clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET'],
-            accessToken: $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN']
-        );
-
-        // Wait a second to avoid hitting a 429 rate limit.
-        sleep(1);
-    }
-
     /**
      * Cleanup data from the ConvertKit account on a test pass/fail, such as unsubscribing, deleting custom fields etc
      *
@@ -195,417 +164,6 @@ public function testClientInterfaceInjection()
         $this->assertEquals(200, $this->api->getResponseInterface()->getStatusCode());
     }
 
-    /**
-     * Test that debug logging works when enabled and an API call is made.
-     *
-     * @since   1.2.0
-     *
-     * @return  void
-     */
-    public function testDebugEnabled()
-    {
-        // Setup API with debugging enabled.
-        $api = new ConvertKit_API(
-            clientID: $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
-            clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET'],
-            accessToken: $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN'],
-            debug: true
-        );
-        $result = $api->get_account();
-
-        // Confirm that the log includes expected data.
-        $this->assertStringContainsString('ck-debug.INFO: GET account', $this->getLogFileContents());
-        $this->assertStringContainsString('ck-debug.INFO: Finish request successfully', $this->getLogFileContents());
-    }
-
-    /**
-     * Test that debug logging works when enabled, a custom debug log file and path is specified
-     * and an API call is made.
-     *
-     * @since   1.3.0
-     *
-     * @return  void
-     */
-    public function testDebugEnabledWithCustomLogFile()
-    {
-        // Define custom log file location.
-        $this->logFile = dirname(dirname(__FILE__)) . '/src/logs/debug-custom.log';
-
-        // Setup API with debugging enabled.
-        $api = new ConvertKit_API(
-            clientID: $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
-            clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET'],
-            accessToken: $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN'],
-            debug: true,
-            debugLogFileLocation: $this->logFile
-        );
-        $result = $api->get_account();
-
-        // Confirm log file exists.
-        $this->assertFileExists($this->logFile);
-
-        // Confirm that the log includes expected data.
-        $this->assertStringContainsString('ck-debug.INFO: GET account', $this->getLogFileContents());
-        $this->assertStringContainsString('ck-debug.INFO: Finish request successfully', $this->getLogFileContents());
-    }
-
-    /**
-     * Test that debug logging works when enabled and an API call is made, with email addresses and credentials
-     * masked in the log file.
-     *
-     * @since   2.0.0
-     *
-     * @return  void
-     */
-    public function testDebugCredentialsAndEmailsAreMasked()
-    {
-        // Setup API with debugging enabled.
-        $api = new ConvertKit_API(
-            clientID: $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
-            clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET'],
-            accessToken: $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN'],
-            debug: true
-        );
-
-        // Create log entries with Client ID, Client Secret, Access Token and Email Address, as if an API method
-        // were to log this sensitive data.
-        $this->callPrivateMethod($api, 'create_log', ['Client ID: ' . $_ENV['CONVERTKIT_OAUTH_CLIENT_ID']]);
-        $this->callPrivateMethod($api, 'create_log', ['Client Secret: ' . $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET']]);
-        $this->callPrivateMethod($api, 'create_log', ['Access Token: ' . $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN']]);
-        $this->callPrivateMethod($api, 'create_log', ['Email: ' . $_ENV['CONVERTKIT_API_SUBSCRIBER_EMAIL']]);
-
-        // Confirm that the log includes the masked Client ID, Secret, Access Token and Email Address.
-        $this->assertStringContainsString(
-            str_repeat(
-                '*',
-                (strlen($_ENV['CONVERTKIT_OAUTH_CLIENT_ID']) - 4)
-            ) . substr($_ENV['CONVERTKIT_OAUTH_CLIENT_ID'], -4),
-            $this->getLogFileContents()
-        );
-        $this->assertStringContainsString(
-            str_repeat(
-                '*',
-                (strlen($_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET']) - 4)
-            ) . substr($_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET'], -4),
-            $this->getLogFileContents()
-        );
-        $this->assertStringContainsString(
-            str_repeat(
-                '*',
-                (strlen($_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN']) - 4)
-            ) . substr($_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN'], -4),
-            $this->getLogFileContents()
-        );
-        $this->assertStringContainsString(
-            'o****@n********.c**',
-            $this->getLogFileContents()
-        );
-
-        // Confirm that the log does not include the unmasked Client ID, Secret, Access Token or Email Address.
-        $this->assertStringNotContainsString($_ENV['CONVERTKIT_OAUTH_CLIENT_ID'], $this->getLogFileContents());
-        $this->assertStringNotContainsString($_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET'], $this->getLogFileContents());
-        $this->assertStringNotContainsString($_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN'], $this->getLogFileContents());
-        $this->assertStringNotContainsString($_ENV['CONVERTKIT_API_SUBSCRIBER_EMAIL'], $this->getLogFileContents());
-    }
-
-    /**
-     * Test that debug logging is not performed when disabled and an API call is made.
-     *
-     * @since   1.2.0
-     *
-     * @return  void
-     */
-    public function testDebugDisabled()
-    {
-        $result = $this->api->get_account();
-        $this->assertEmpty($this->getLogFileContents());
-    }
-
-    /**
-     * Test that calling request_headers() returns the expected array of headers
-     *
-     * @since   2.0.0
-     *
-     * @return  void
-     */
-    public function testRequestHeadersMethod()
-    {
-        $headers = $this->api->get_request_headers();
-        $this->assertArrayHasKey('Accept', $headers);
-        $this->assertArrayHasKey('Content-Type', $headers);
-        $this->assertArrayHasKey('User-Agent', $headers);
-        $this->assertArrayHasKey('Authorization', $headers);
-        $this->assertEquals($headers['Accept'], 'application/json');
-        $this->assertEquals($headers['Content-Type'], 'application/json; charset=utf-8');
-        $this->assertEquals($headers['User-Agent'], 'ConvertKitPHPSDK/' . $this->api::VERSION . ';PHP/' . phpversion());
-        $this->assertEquals($headers['Authorization'], 'Bearer ' . $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN']);
-    }
-
-    /**
-     * Test that calling request_headers() with a different `type` parameter
-     * returns the expected array of headers
-     *
-     * @since   2.0.0
-     *
-     * @return  void
-     */
-    public function testRequestHeadersMethodWithType()
-    {
-        $headers = $this->api->get_request_headers(
-            type: 'text/html'
-        );
-        $this->assertArrayHasKey('Accept', $headers);
-        $this->assertArrayHasKey('Content-Type', $headers);
-        $this->assertArrayHasKey('User-Agent', $headers);
-        $this->assertArrayHasKey('Authorization', $headers);
-        $this->assertEquals($headers['Accept'], 'text/html');
-        $this->assertEquals($headers['Content-Type'], 'text/html; charset=utf-8');
-        $this->assertEquals($headers['User-Agent'], 'ConvertKitPHPSDK/' . $this->api::VERSION . ';PHP/' . phpversion());
-        $this->assertEquals($headers['Authorization'], 'Bearer ' . $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN']);
-    }
-
-    /**
-     * Test that calling request_headers() with the `auth` parameter set to false
-     * returns the expected array of headers
-     *
-     * @since   2.0.0
-     *
-     * @return  void
-     */
-    public function testRequestHeadersMethodWithAuthDisabled()
-    {
-        $headers = $this->api->get_request_headers(
-            auth: false
-        );
-        $this->assertArrayHasKey('Accept', $headers);
-        $this->assertArrayHasKey('Content-Type', $headers);
-        $this->assertArrayHasKey('User-Agent', $headers);
-        $this->assertArrayNotHasKey('Authorization', $headers);
-        $this->assertEquals($headers['Accept'], 'application/json');
-        $this->assertEquals($headers['Content-Type'], 'application/json; charset=utf-8');
-        $this->assertEquals($headers['User-Agent'], 'ConvertKitPHPSDK/' . $this->api::VERSION . ';PHP/' . phpversion());
-    }
-
-    /**
-     * Test that calling request_headers() with a different `type` parameter
-     * and the `auth` parameter set to false returns the expected array of headers
-     *
-     * @since   2.0.0
-     *
-     * @return  void
-     */
-    public function testRequestHeadersMethodWithTypeAndAuthDisabled()
-    {
-        $headers = $this->api->get_request_headers(
-            type: 'text/html',
-            auth: false
-        );
-        $this->assertArrayHasKey('Accept', $headers);
-        $this->assertArrayHasKey('Content-Type', $headers);
-        $this->assertArrayHasKey('User-Agent', $headers);
-        $this->assertArrayNotHasKey('Authorization', $headers);
-        $this->assertEquals($headers['Accept'], 'text/html');
-        $this->assertEquals($headers['Content-Type'], 'text/html; charset=utf-8');
-        $this->assertEquals($headers['User-Agent'], 'ConvertKitPHPSDK/' . $this->api::VERSION . ';PHP/' . phpversion());
-    }
-
-    /**
-     * Test that get_oauth_url() returns the correct URL to begin the OAuth process.
-     *
-     * @since   2.0.0
-     *
-     * @return  void
-     */
-    public function testGetOAuthURL()
-    {
-        // Confirm the OAuth URL returned is correct.
-        $this->assertEquals(
-            $this->api->get_oauth_url($_ENV['CONVERTKIT_OAUTH_REDIRECT_URI']),
-            'https://app.convertkit.com/oauth/authorize?' . http_build_query([
-                'client_id' => $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
-                'redirect_uri' => $_ENV['CONVERTKIT_OAUTH_REDIRECT_URI'],
-                'response_type' => 'code',
-            ])
-        );
-    }
-
-    /**
-     * Test that get_access_token() returns the expected data.
-     *
-     * @since   2.0.0
-     *
-     * @return void
-     */
-    public function testGetAccessToken()
-    {
-        // Initialize API.
-        $api = new ConvertKit_API(
-            clientID: $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
-            clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET']
-        );
-
-        // Define response parameters.
-        $params = [
-            'access_token'  => 'example-access-token',
-            'refresh_token' => 'example-refresh-token',
-            'token_type'    => 'Bearer',
-            'created_at'    => strtotime('now'),
-            'expires_in'    => strtotime('+3 days'),
-            'scope'         => 'public',
-        ];
-
-        // Add mock handler for this API request.
-        $api = $this->mockResponse(
-            api: $api,
-            responseBody: $params,
-        );
-
-        // Send request.
-        $result = $api->get_access_token(
-            authCode: 'auth-code',
-            redirectURI: $_ENV['CONVERTKIT_OAUTH_REDIRECT_URI'],
-        );
-
-        // Inspect response.
-        $result = get_object_vars($result);
-        $this->assertIsArray($result);
-        $this->assertArrayHasKey('access_token', $result);
-        $this->assertArrayHasKey('refresh_token', $result);
-        $this->assertArrayHasKey('token_type', $result);
-        $this->assertArrayHasKey('created_at', $result);
-        $this->assertArrayHasKey('expires_in', $result);
-        $this->assertArrayHasKey('scope', $result);
-        $this->assertEquals($result['access_token'], $params['access_token']);
-        $this->assertEquals($result['refresh_token'], $params['refresh_token']);
-        $this->assertEquals($result['created_at'], $params['created_at']);
-        $this->assertEquals($result['expires_in'], $params['expires_in']);
-    }
-
-    /**
-     * Test that a ClientException is thrown when an invalid auth code is supplied
-     * when fetching an access token.
-     *
-     * @since   2.0.0
-     *
-     * @return void
-     */
-    public function testGetAccessTokenWithInvalidAuthCode()
-    {
-        $this->expectException(ClientException::class);
-        $api = new ConvertKit_API(
-            clientID: $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
-            clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET']
-        );
-        $result = $api->get_access_token(
-            authCode: 'not-a-real-auth-code',
-            redirectURI: $_ENV['CONVERTKIT_OAUTH_REDIRECT_URI'],
-        );
-    }
-
-    /**
-     * Test that refresh_token() returns the expected data.
-     *
-     * @since   2.0.0
-     *
-     * @return void
-     */
-    public function testRefreshToken()
-    {
-        // Initialize API.
-        $api = new ConvertKit_API(
-            clientID: $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
-            clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET']
-        );
-
-        // Define response parameters.
-        $params = [
-            'access_token'  => 'new-example-access-token',
-            'refresh_token' => 'new-example-refresh-token',
-            'token_type'    => 'Bearer',
-            'created_at'    => strtotime('now'),
-            'expires_in'    => strtotime('+3 days'),
-            'scope'         => 'public',
-        ];
-
-        // Add mock handler for this API request.
-        $api = $this->mockResponse(
-            api: $api,
-            responseBody: $params,
-        );
-
-        // Send request.
-        $result = $api->refresh_token(
-            refreshToken: 'refresh-token',
-            redirectURI: $_ENV['CONVERTKIT_OAUTH_REDIRECT_URI'],
-        );
-
-        // Inspect response.
-        $result = get_object_vars($result);
-        $this->assertIsArray($result);
-        $this->assertArrayHasKey('access_token', $result);
-        $this->assertArrayHasKey('refresh_token', $result);
-        $this->assertArrayHasKey('token_type', $result);
-        $this->assertArrayHasKey('created_at', $result);
-        $this->assertArrayHasKey('expires_in', $result);
-        $this->assertArrayHasKey('scope', $result);
-        $this->assertEquals($result['access_token'], $params['access_token']);
-        $this->assertEquals($result['refresh_token'], $params['refresh_token']);
-        $this->assertEquals($result['created_at'], $params['created_at']);
-        $this->assertEquals($result['expires_in'], $params['expires_in']);
-    }
-
-    /**
-     * Test that a ServerException is thrown when an invalid refresh token is supplied
-     * when refreshing an access token.
-     *
-     * @since   2.0.0
-     *
-     * @return void
-     */
-    public function testRefreshTokenWithInvalidToken()
-    {
-        $this->expectException(ServerException::class);
-        $api = new ConvertKit_API(
-            clientID: $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
-            clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET']
-        );
-        $result = $api->refresh_token(
-            refreshToken: 'not-a-real-refresh-token',
-            redirectURI: $_ENV['CONVERTKIT_OAUTH_REDIRECT_URI'],
-        );
-    }
-
-    /**
-     * Test that a ClientException is thrown when an invalid access token is supplied.
-     *
-     * @since   1.0.0
-     *
-     * @return void
-     */
-    public function testInvalidAPICredentials()
-    {
-        $this->expectException(ClientException::class);
-        $api = new ConvertKit_API(
-            clientID: 'fakeClientID',
-            clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET'],
-            accessToken: $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN']
-        );
-        $result = $api->get_account();
-
-        $api = new ConvertKit_API(
-            clientID: $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
-            clientSecret: 'fakeClientSecret',
-            accessToken: $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN']
-        );
-        $result = $api->get_account();
-
-        $api = new ConvertKit_API(
-            clientID: $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
-            clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET'],
-            accessToken: 'fakeAccessToken'
-        );
-        $result = $api->get_account();
-    }
 
     /**
      * Test that get_account() returns the expected data.
@@ -5523,7 +5081,7 @@ public function testGetResourceInaccessibleURL()
      *
      * @return  void
      */
-    private function deleteLogFile()
+    public function deleteLogFile()
     {
         if (file_exists($this->logFile)) {
             unlink($this->logFile);
@@ -5537,7 +5095,7 @@ private function deleteLogFile()
      *
      * @return  string
      */
-    private function getLogFileContents()
+    public function getLogFileContents()
     {
         // Return blank string if no log file.
         if (!file_exists($this->logFile)) {
@@ -5557,7 +5115,7 @@ private function getLogFileContents()
      * @param   string $name Method Name.
      * @param   array  $args Method Arguments.
      */
-    private function callPrivateMethod($obj, $name, array $args)
+    public function callPrivateMethod($obj, $name, array $args)
     {
         $class = new \ReflectionClass($obj);
         $method = $class->getMethod($name);
@@ -5578,7 +5136,7 @@ private function callPrivateMethod($obj, $name, array $args)
      *
      * @return  string
      */
-    private function generateEmailAddress($domain = 'kit.com')
+    public function generateEmailAddress($domain = 'kit.com')
     {
         return 'php-sdk-' . date('Y-m-d-H-i-s') . '-php-' . PHP_VERSION_ID . '@' . $domain;
     }
@@ -5591,7 +5149,7 @@ private function generateEmailAddress($domain = 'kit.com')
      * @param   $string Possible HTML.
      * @return  bool
      */
-    private function isHtml($string)
+    public function isHtml($string)
     {
         return preg_match("/<[^<]+>/", $string, $m) != 0;
     }
@@ -5605,7 +5163,7 @@ private function isHtml($string)
      * @param   null|array    $responseBody     Response to return when API call is made.
      * @param   int           $httpCode         HTTP Code to return when API call is made.
      */
-    private function mockResponse(ConvertKit_API $api, $responseBody = null, int $httpCode = 200)
+    public function mockResponse(ConvertKit_API $api, $responseBody = null, int $httpCode = 200)
     {
         // Setup API with a mock Guzzle client, returning the data
         // as if we successfully swapped an auth code for an access token.
@@ -5635,7 +5193,7 @@ private function mockResponse(ConvertKit_API $api, $responseBody = null, int $ht
      *
      * @param   array   $result     API Result.
      */
-    private function assertDataExists($result, $key)
+    public function assertDataExists($result, $key)
     {
         $result = get_object_vars($result);
         $this->assertArrayHasKey($key, $result);
@@ -5649,7 +5207,7 @@ private function assertDataExists($result, $key)
      *
      * @param   array   $result     API Result.
      */
-    private function assertPaginationExists($result)
+    public function assertPaginationExists($result)
     {
         $result = get_object_vars($result);
         $this->assertArrayHasKey('pagination', $result);