diff --git a/src/Internal/Posix/PosixRunner.php b/src/Internal/Posix/PosixRunner.php
index 8a1f441..45aa862 100644
--- a/src/Internal/Posix/PosixRunner.php
+++ b/src/Internal/Posix/PosixRunner.php
@@ -10,6 +10,7 @@ use Amp\Process\Internal\ProcessStatus;
 use Amp\Process\ProcessException;
 use Revolt\EventLoop;
 
+/** @internal */
 final class PosixRunner implements ProcessRunner
 {
     private const FD_SPEC = [
diff --git a/src/Internal/ProcHolder.php b/src/Internal/ProcHolder.php
index de143b2..b4f5c18 100644
--- a/src/Internal/ProcHolder.php
+++ b/src/Internal/ProcHolder.php
@@ -2,6 +2,7 @@
 
 namespace Amp\Process\Internal;
 
+/** @internal */
 final class ProcHolder
 {
     private ProcessRunner $runner;
diff --git a/src/Internal/ProcessHandle.php b/src/Internal/ProcessHandle.php
index 28229ae..d8f50a1 100644
--- a/src/Internal/ProcessHandle.php
+++ b/src/Internal/ProcessHandle.php
@@ -6,6 +6,7 @@ use Amp\ByteStream\ReadableResourceStream;
 use Amp\ByteStream\WritableResourceStream;
 use Amp\DeferredFuture;
 
+/** @internal */
 abstract class ProcessHandle
 {
     /** @var resource */
diff --git a/src/Internal/ProcessRunner.php b/src/Internal/ProcessRunner.php
index 79ba52f..cb668c0 100644
--- a/src/Internal/ProcessRunner.php
+++ b/src/Internal/ProcessRunner.php
@@ -4,6 +4,7 @@ namespace Amp\Process\Internal;
 
 use Amp\Process\ProcessException;
 
+/** @internal */
 interface ProcessRunner
 {
     /**
diff --git a/src/Internal/ProcessStatus.php b/src/Internal/ProcessStatus.php
index 9989ff6..5cae0bf 100644
--- a/src/Internal/ProcessStatus.php
+++ b/src/Internal/ProcessStatus.php
@@ -2,6 +2,7 @@
 
 namespace Amp\Process\Internal;
 
+/** @internal */
 final class ProcessStatus
 {
     public const STARTING = 0;