diff --git a/lib/Worker/Internal/Job.php b/lib/Worker/Internal/Job.php
index 9d71766..cc2029a 100644
--- a/lib/Worker/Internal/Job.php
+++ b/lib/Worker/Internal/Job.php
@@ -4,6 +4,7 @@ namespace Amp\Parallel\Worker\Internal;
 
 use Amp\Parallel\Worker\Task;
 
+/** @internal */
 class Job {
     /** @var string */
     private $id;
diff --git a/lib/Worker/Internal/PooledWorker.php b/lib/Worker/Internal/PooledWorker.php
index 775c57b..b9364b5 100644
--- a/lib/Worker/Internal/PooledWorker.php
+++ b/lib/Worker/Internal/PooledWorker.php
@@ -6,6 +6,7 @@ use Amp\Parallel\Worker\Task;
 use Amp\Parallel\Worker\Worker;
 use Amp\Promise;
 
+/** @internal */
 class PooledWorker implements Worker {
     /** @var callable */
     private $push;
diff --git a/lib/Worker/Internal/TaskFailure.php b/lib/Worker/Internal/TaskFailure.php
index 6203998..1df67e6 100644
--- a/lib/Worker/Internal/TaskFailure.php
+++ b/lib/Worker/Internal/TaskFailure.php
@@ -7,6 +7,7 @@ use Amp\Parallel\Worker\TaskError;
 use Amp\Parallel\Worker\TaskException;
 use Amp\Promise;
 
+/** @internal */
 class TaskFailure extends TaskResult {
     const PARENT_EXCEPTION = 0;
     const PARENT_ERROR = 1;
diff --git a/lib/Worker/Internal/TaskResult.php b/lib/Worker/Internal/TaskResult.php
index 2083fb3..f505629 100644
--- a/lib/Worker/Internal/TaskResult.php
+++ b/lib/Worker/Internal/TaskResult.php
@@ -4,6 +4,7 @@ namespace Amp\Parallel\Worker\Internal;
 
 use Amp\Promise;
 
+/** @internal */
 abstract class TaskResult {
     /** @var string Task identifier. */
     private $id;
diff --git a/lib/Worker/Internal/TaskSuccess.php b/lib/Worker/Internal/TaskSuccess.php
index 383a987..e39dc82 100644
--- a/lib/Worker/Internal/TaskSuccess.php
+++ b/lib/Worker/Internal/TaskSuccess.php
@@ -5,6 +5,7 @@ namespace Amp\Parallel\Worker\Internal;
 use Amp\Promise;
 use Amp\Success;
 
+/** @internal */
 class TaskSuccess extends TaskResult {
     /** @var mixed Result of task. */
     private $result;