diff --git a/distributed/client.py b/distributed/client.py
index f017612bd5..3667bb46fb 100644
--- a/distributed/client.py
+++ b/distributed/client.py
@@ -151,11 +151,11 @@
 
 
 class FutureCancelledError(CancelledError):
-    key: str
+    key: Key
     reason: str
     msg: str | None
 
-    def __init__(self, key: str, reason: str | None, msg: str | None = None):
+    def __init__(self, key: Key, reason: str | None, msg: str | None = None):
         self.key = key
         self.reason = reason if reason else "unknown"
         self.msg = msg
@@ -163,9 +163,10 @@ def __init__(self, key: str, reason: str | None, msg: str | None = None):
     def __str__(self) -> str:
         result = f"{self.key} cancelled for reason: {self.reason}."
         if self.msg:
-            result = "\n".join([result, self.msg])
+            result += f"\n{self.msg}"
         return result
 
+    # Workaround to tblib <3.2.1
     def __reduce__(self):
         return self.__class__, (self.key, self.reason, self.msg)
 
@@ -295,21 +296,35 @@ class Future(TaskRef, Generic[_T]):
     Client:  Creates futures
     """
 
+    key: Key
+    _client: Client | None
+    _input_state: FutureState | None
+    _state: FutureState | None
+    _id: tuple[str, int]
+    _cleared: bool
+
     _is_finalizing: staticmethod[[], bool] = staticmethod(sys.is_finalizing)
+    _cb_executor: ClassVar[ThreadPoolExecutor | None] = None
+    _cb_executor_pid: ClassVar[int | None] = None
 
-    _cb_executor = None
-    _cb_executor_pid = None
-    _counter = itertools.count()
-    # Make sure this stays unique even across multiple processes or hosts
-    _uid = uuid.uuid4().hex
+    # Generate _id of Future instances as (_uid, next(_counter))
+    # Make sure _uid stays unique even across multiple processes or hosts
+    _uid: ClassVar[str] = uuid.uuid4().hex
+    _counter: ClassVar[itertools.count[int]] = itertools.count()
 
-    def __init__(self, key, client=None, state=None, _id=None):
+    def __init__(
+        self,
+        key: Key,
+        client: Client | None = None,
+        state: FutureState | None = None,
+        _id: tuple[str, int] | None = None,
+    ):
         self.key = key
-        self._cleared = False
         self._client = client
-        self._id = _id or (Future._uid, next(Future._counter))
         self._input_state = state
         self._state = None
+        self._id = _id or (Future._uid, next(Future._counter))
+        self._cleared = False
         self._bind_late()
 
     @property
@@ -651,7 +666,10 @@ class FutureState:
 
     __slots__ = ("_event", "key", "status", "type", "exception", "traceback")
 
-    def __init__(self, key: str):
+    key: Key
+    status: Literal["pending", "cancelled", "finished", "lost", "error"]
+
+    def __init__(self, key: Key):
         self._event = None
         self.key = key
         self.exception = None