diff --git a/beets/util/pipeline.py b/beets/util/pipeline.py
index d797dea13..3a31175b6 100644
--- a/beets/util/pipeline.py
+++ b/beets/util/pipeline.py
@@ -30,11 +30,12 @@ up a bottleneck stage by dividing its work among multiple threads.
 To do so, pass an iterable of coroutines to the Pipeline constructor
 in place of any single coroutine.
 """
+from __future__ import annotations
 
 import queue
 import sys
 from threading import Lock, Thread
-from typing import Callable, Generator, Optional, Union
+from typing import Callable, Generator
 
 from typing_extensions import TypeVar, TypeVarTuple, Unpack
 
@@ -152,15 +153,20 @@ def multiple(messages):
     return MultiMessage(messages)
 
 
-A = TypeVarTuple("A")
-T = TypeVar("T")
+# Arguments of the function (omitting the task)
+Args = TypeVarTuple("Args")
+# Task as an additional argument to the function
+Task = TypeVar("Task")
+# Return type of the function (should normally be task but sadly
+# we cant enforce this with the current stage functions without
+# a refactor)
 R = TypeVar("R")
 
 
 def stage(
     func: Callable[
-        [Unpack[A], T],
-        Optional[R],
+        [Unpack[Args], Task],
+        R | None,
     ],
 ):
     """Decorate a function to become a simple stage.
@@ -176,8 +182,8 @@ def stage(
     [3, 4, 5]
     """
 
-    def coro(*args: Unpack[A]) -> Generator[Union[R, T, None], T, R]:
-        task = None
+    def coro(*args: Unpack[Args]) -> Generator[R | Task | None, Task, None]:
+        task: R | Task | None = None
         while True:
             task = yield task
             task = func(*(args + (task,)))
@@ -185,7 +191,7 @@ def stage(
     return coro
 
 
-def mutator_stage(func: Callable[[Unpack[A], T], R]):
+def mutator_stage(func: Callable[[Unpack[Args], Task], R]):
     """Decorate a function that manipulates items in a coroutine to
     become a simple stage.
 
@@ -200,7 +206,7 @@ def mutator_stage(func: Callable[[Unpack[A], T], R]):
     [{'x': True}, {'a': False, 'x': True}]
     """
 
-    def coro(*args: Unpack[A]) -> Generator[Union[T, None], T, None]:
+    def coro(*args: Unpack[Args]) -> Generator[Task | None, Task, None]:
         task = None
         while True:
             task = yield task
@@ -419,9 +425,7 @@ class Pipeline:
         for i in range(1, queue_count):
             for coro in self.stages[i]:
                 threads.append(
-                    MiddlePipelineThread(
-                        coro, queues[i - 1], queues[i], threads
-                    )
+                    MiddlePipelineThread(coro, queues[i - 1], queues[i], threads)
                 )
 
         # Last stage.