diff --git a/tests/Integration/APITest.php b/tests/Integration/APITest.php
index 026d51c..61a8850 100644
--- a/tests/Integration/APITest.php
+++ b/tests/Integration/APITest.php
@@ -184,19 +184,39 @@ public function tearDown(): void
 	 * (never as a thrown exception). We accept both return and throw so
 	 * that any input-validation code that throws still counts.
 	 *
+	 * Where the code validates arguments before performing an API request, specify
+	 * $expected, to assert that validation produced the error and not the API. A
+	 * returned WP_Error does not satisfy $expected, so a test that expects an
+	 * exception fails if the validation is removed.
+	 *
 	 * @since   2.0.5
+	 * @since   2.7.0 Added the $expected parameter.
 	 *
-	 * @param   callable $fn Callable that should fail.
+	 * @param   callable    $fn       Callable that should fail.
+	 * @param   string|null $expected Expected exception class name.
 	 * @return  void
 	 */
-	protected function assertApiError(callable $fn): void
+	protected function assertApiError(callable $fn, string|null $expected = null): void
 	{
 		try {
 			$result = $fn();
 		} catch (\Throwable $e) {
+			if ( ! is_null($expected)) {
+				$this->assertInstanceOf($expected, $e);
+				return;
+			}
+
 			$this->assertTrue(true, 'Callable threw an exception as expected.');
 			return;
 		}
+
+		// An exception was expected, so a returned WP_Error isn't the error we asked to assert.
+		if ( ! is_null($expected)) {
+			$this->fail(
+				sprintf('Expected %s to be thrown, but the callable returned instead.', $expected)
+			);
+		}
+
 		$this->assertInstanceOf(\WP_Error::class, $result);
 	}
 
@@ -1681,6 +1701,26 @@ public function testGetAllPostsWithInvalidPostsPerRequestParameter()
 		$this->assertEquals('get_all_posts(): the posts_per_request parameter must be equal to or less than 50.', $result->get_error_message());
 	}
 
+	/**
+	 * Test that get_resource() returns a WP_Error when an invalid URL is specified.
+	 *
+	 * Overrides the version in TestsTrait: the PHP SDK validates the URL and throws an
+	 * InvalidArgumentException, whereas WordPress Libraries passes the URL to
+	 * wp_remote_get(), which returns a WP_Error.
+	 *
+	 * @since   2.7.0
+	 *
+	 * @return  void
+	 */
+	public function testGetResourceInvalidURL()
+	{
+		$this->assertApiError(
+			function () {
+				return $this->api->get_resource('not-a-url');
+			}
+		);
+	}
+
 	/**
 	 * Test that the `get_post()` function returns expected data.
 	 *
diff --git a/tests/Integration/TestsTrait.php b/tests/Integration/TestsTrait.php
index 4becfdd..e0083a2 100644
--- a/tests/Integration/TestsTrait.php
+++ b/tests/Integration/TestsTrait.php
@@ -1,7 +1,7 @@
 <?php
 
 /**
- * Holds tests for the Kit API.
+ * Test methods in ConvertKit_API_Traits that interact with the API.
  */
 trait TestsTrait
 {
@@ -4925,8 +4925,8 @@ public function testUnsubscribeByEmail()
             email_address: $emailAddress
         );
 
-        // Wait a moment to ensure subscriber is created.
-        sleep(3);
+        // Wait until the subscriber can be found by their email address.
+        $this->waitForSubscriber($emailAddress);
 
         // Unsubscribe.
         $this->assertNull($this->api->unsubscribe_by_email($emailAddress));
@@ -6161,12 +6161,15 @@ public function testCreateWebhookWithEventParameter()
      */
     public function testCreateWebhookWithInvalidEvent()
     {
-        $this->assertApiError(function () {
-            return $this->api->create_webhook(
-                url: 'https://webhook.site/' . str_shuffle('wfervdrtgsdewrafvwefds'),
-                event: 'invalid.event'
-            );
-        });
+        $this->assertApiError(
+            function () {
+                return $this->api->create_webhook(
+                    url: 'https://webhook.site/' . str_shuffle('wfervdrtgsdewrafvwefds'),
+                    event: 'invalid.event'
+                );
+            },
+            \InvalidArgumentException::class
+        );
     }
 
     /**
@@ -6900,9 +6903,12 @@ public function testGetResourceLegacyLandingPage()
      */
     public function testGetResourceInvalidURL()
     {
-        $this->assertApiError(function () {
-            return $this->api->get_resource('not-a-url');
-        });
+        $this->assertApiError(
+            function () {
+                return $this->api->get_resource('not-a-url');
+            },
+            \InvalidArgumentException::class
+        );
     }
 
     /**
@@ -6938,6 +6944,70 @@ public function generateEmailAddress($domain = 'kit.com')
         return 'php-sdk-' . date('Y-m-d-H-i-s') . '-php-' . PHP_VERSION_ID . '@' . $domain;
     }
 
+    /**
+     * Repeatedly invokes the given callback until it returns a truthy value, or the
+     * maximum number of attempts is reached.
+     *
+     * Use this to wrap API checks that can be flaky due to eventual consistency at Kit's
+     * end. List endpoints typically reflect a write within ~30 seconds, and can take up
+     * to 5 minutes, so reading back immediately after a write is not reliable.
+     *
+     * @since   2.7.0
+     *
+     * @see     https://developers.kit.com/api-reference/eventual-consistency
+     *
+     * @param   callable $callback Callback to invoke. Should return the value to use, or
+     *                             false / null when the check has not yet succeeded.
+     * @param   integer  $attempts Maximum number of attempts.
+     * @param   integer  $delay    Seconds to wait between attempts.
+     * @return  mixed              Value returned by the callback, or false if all attempts are exhausted.
+     */
+    public function retryUntil(callable $callback, $attempts = 20, $delay = 5)
+    {
+        for ($i = 0; $i < $attempts; $i++) {
+            $result = $callback();
+
+            if ($result) {
+                return $result;
+            }
+
+            // Don't sleep after the final attempt.
+            if ($i < ($attempts - 1)) {
+                sleep($delay);
+            }
+        }
+
+        return false;
+    }
+
+    /**
+     * Waits for the given email address to be queryable by get_subscriber_id(), returning
+     * the Subscriber ID, and failing the test if it never becomes queryable.
+     *
+     * @since   2.7.0
+     *
+     * @param   string $emailAddress Email Address.
+     * @return  integer              Subscriber ID.
+     */
+    public function waitForSubscriber($emailAddress)
+    {
+        $subscriberID = $this->retryUntil(
+            function () use ($emailAddress) {
+                $subscriberID = $this->api->get_subscriber_id($emailAddress);
+
+                // WP Libraries returns a WP_Error on failure; keep retrying unless we have an ID.
+                return is_numeric($subscriberID) ? (int) $subscriberID : false;
+            }
+        );
+
+        $this->assertNotFalse(
+            $subscriberID,
+            sprintf('Subscriber %s was not returned by the API in time.', $emailAddress)
+        );
+
+        return $subscriberID;
+    }
+
     /**
      * Checks if string is html.
      *