diff --git a/src/fib/fiber.ml b/src/fib/fiber.ml
index ebfd3319..7007aa17 100644
--- a/src/fib/fiber.ml
+++ b/src/fib/fiber.ml
@@ -9,22 +9,26 @@ type cancel_callback = Exn_bt.t -> unit
 let prom_of_fut : 'a Fut.t -> 'a Fut.promise =
   Fut.Private_.unsafe_promise_of_fut
 
-type 'a t = {
-  id: Handle.t;  (** unique identifier for this fiber *)
-  state: 'a state A.t;  (** Current state in the lifetime of the fiber *)
-  res: 'a Fut.t;
-  runner: Runner.t;
-}
+module Private_ = struct
+  type 'a t = {
+    id: Handle.t;  (** unique identifier for this fiber *)
+    state: 'a state A.t;  (** Current state in the lifetime of the fiber *)
+    res: 'a Fut.t;
+    runner: Runner.t;
+  }
 
-and 'a state =
-  | Alive of {
-      children: children;
-      on_cancel: cancel_callback list;
-    }
-  | Terminating_or_done of 'a Exn_bt.result A.t
+  and 'a state =
+    | Alive of {
+        children: children;
+        on_cancel: cancel_callback list;
+      }
+    | Terminating_or_done of 'a Exn_bt.result A.t
 
-and children = any FM.t
-and any = Any : _ t -> any [@@unboxed]
+  and children = any FM.t
+  and any = Any : _ t -> any [@@unboxed]
+end
+
+include Private_
 
 let[@inline] res self = self.res
 let[@inline] peek self = Fut.peek self.res
@@ -178,6 +182,11 @@ let spawn_ ~on (f : _ -> 'a) : 'a t =
 
 let[@inline] spawn_top ~on f : _ t = spawn_ ~on f
 
+let[@inline] self () : any =
+  match Task_local_storage.get k_current_fiber with
+  | None -> failwith "Fiber.self: must be run from inside a fiber."
+  | Some f -> f
+
 let spawn_link ~protect f : _ t =
   match Task_local_storage.get k_current_fiber with
   | None -> failwith "Fiber.spawn_link: must be run from inside a fiber."
@@ -217,6 +226,11 @@ let with_cancel_callback (self : _ t) cb (k : unit -> 'a) : 'a =
   add_cancel_cb_ self cb;
   Fun.protect k ~finally:(fun () -> remove_top_cancel_cb_ self)
 
+let with_self_cancel_callback cb (k : unit -> 'a) : 'a =
+  let (Any self) = self () in
+  add_cancel_cb_ self cb;
+  Fun.protect k ~finally:(fun () -> remove_top_cancel_cb_ self)
+
 let[@inline] await self = Fut.await self.res
 
 module Suspend_ = Moonpool.Private.Suspend_
diff --git a/src/fib/fiber.mli b/src/fib/fiber.mli
index 5b01948a..694d3a92 100644
--- a/src/fib/fiber.mli
+++ b/src/fib/fiber.mli
@@ -9,7 +9,24 @@
       of structured concurrency
 *)
 
-type 'a t
+(**/**)
+
+module Private_ : sig
+  type 'a state
+
+  type 'a t = private {
+    id: Handle.t;  (** unique identifier for this fiber *)
+    state: 'a state Atomic.t;  (** Current state in the lifetime of the fiber *)
+    res: 'a Fut.t;
+    runner: Runner.t;
+  }
+  (** Type definition, exposed so that {!any} can be unboxed.
+      Please do not rely on that. *)
+end
+
+(**/**)
+
+type 'a t = 'a Private_.t
 (** A fiber returning a value of type ['a]. *)
 
 val res : 'a t -> 'a Fut.t
@@ -20,6 +37,14 @@ type 'a callback = 'a Exn_bt.result -> unit
 
 type cancel_callback = Exn_bt.t -> unit
 
+(** Type erased fiber *)
+type any = Any : _ t -> any [@@unboxed]
+
+val self : unit -> any
+(** [self ()] is the current fiber.
+    Must be run from inside a fiber.
+    @raise Failure if not run from inside a fiber. *)
+
 val peek : 'a t -> 'a Fut.or_error option
 (** Peek inside the future result *)
 
@@ -50,6 +75,10 @@ val with_cancel_callback : _ t -> cancel_callback -> (unit -> 'a) -> 'a
     [cb()] is called. If [e] returns without the fiber being cancelled,
     this callback is removed. *)
 
+val with_self_cancel_callback : cancel_callback -> (unit -> 'a) -> 'a
+(** [with_self_cancel_callback cb f] calls [f()] in a scope where
+    [cb] is added to the cancel callbacks of the current fiber *)
+
 val on_result : 'a t -> 'a callback -> unit
 (** Wait for fiber to be done and call the callback
     with the result. If the fiber is done already then the