diff --git a/src/core/collector.ml b/src/core/collector.ml
index 41832d1..650b806 100644
--- a/src/core/collector.ml
+++ b/src/core/collector.ml
@@ -94,6 +94,12 @@ module type S = sig
   val counter_float : data:(string * user_data) list -> string -> float -> unit
   (** Float counter. *)
 
+  val extension_event : extension_event -> unit
+  (** Handle an extension event.
+      A collector {b MUST} simple ignore events it doesn't know,
+      and return [()] silently.
+      @since NEXT_RELEASE *)
+
   val shutdown : unit -> unit
   (** Shutdown collector, possibly waiting for it to finish sending data. *)
 end
diff --git a/src/core/trace_core.ml b/src/core/trace_core.ml
index 841004a..a1f7bd4 100644
--- a/src/core/trace_core.ml
+++ b/src/core/trace_core.ml
@@ -168,6 +168,13 @@ let shutdown () =
   | None -> ()
   | Some (module C) -> C.shutdown ()
 
+type extension_event = Types.extension_event = ..
+
+let[@inline] extension_event ev =
+  match A.get collector with
+  | None -> ()
+  | Some (module C) -> C.extension_event ev
+
 module Internal_ = struct
   module Atomic_ = Atomic_
 end
diff --git a/src/core/trace_core.mli b/src/core/trace_core.mli
index 735b7c6..b685a08 100644
--- a/src/core/trace_core.mli
+++ b/src/core/trace_core.mli
@@ -214,3 +214,16 @@ val set_current_level : Level.t -> unit
 val shutdown : unit -> unit
 (** [shutdown ()] shutdowns the current collector, if one was installed,
     and waits for it to terminate before returning. *)
+
+(** {2 Extensions} *)
+
+type extension_event = Types.extension_event = ..
+(** Extension event
+  @since NEXT_RELEASE *)
+
+val extension_event : extension_event -> unit
+(** Trigger an extension event, whose meaning depends on
+    the library that defines it. Some collectors will
+    simply ignore it. This does nothing if no collector
+    is setup.
+    @since NEXT_RELEASE *)
diff --git a/src/core/types.ml b/src/core/types.ml
index 4e7f6f2..e4a2f10 100644
--- a/src/core/types.ml
+++ b/src/core/types.ml
@@ -23,3 +23,8 @@ type explicit_span = {
         of the span, to the end of the span. *)
 }
 (** Explicit span, with collector-specific metadata *)
+
+type extension_event = ..
+(** An extension event, used to add features that are backend specific
+    or simply not envisioned by [trace].
+  @since NEXT_RELEASE *)
diff --git a/src/fuchsia/fcollector.ml b/src/fuchsia/fcollector.ml
index 893d066..789de37 100644
--- a/src/fuchsia/fcollector.ml
+++ b/src/fuchsia/fcollector.ml
@@ -363,6 +363,8 @@ struct
       encode out ~name ~ty:ty_thread ~kid:tls.tid
         ~args:[ "process", `Kid pid ]
         ())
+
+  let extension_event _ = ()
 end
 
 let create ~out () : collector =
diff --git a/src/subscriber/callbacks.ml b/src/subscriber/callbacks.ml
index 46c33ea..4bca092 100644
--- a/src/subscriber/callbacks.ml
+++ b/src/subscriber/callbacks.ml
@@ -104,6 +104,11 @@ module type S = sig
     span ->
     unit
   (** Exit a manual span *)
+
+  val on_extension_event :
+    st -> time_ns:float -> tid:int -> extension_event -> unit
+  (** Extension event
+      @since NEXT_RELEASE *)
 end
 
 type 'st t = (module S with type st = 'st)
@@ -137,6 +142,8 @@ module Dummy = struct
   let on_exit_manual_span _ ~time_ns:_ ~tid:_ ~name:_ ~data:_ ~flavor:_
       ~trace_id:_ _ =
     ()
+
+  let on_extension_event _ ~time_ns:_ ~tid:_ _ = ()
 end
 
 (** Dummy callbacks, ignores all events. *)
diff --git a/src/subscriber/subscriber.ml b/src/subscriber/subscriber.ml
index 46c100c..7230c3a 100644
--- a/src/subscriber/subscriber.ml
+++ b/src/subscriber/subscriber.ml
@@ -96,6 +96,12 @@ open struct
         span;
       CB2.on_exit_manual_span s2 ~time_ns ~tid ~name ~data ~flavor ~trace_id
         span
+
+    let on_extension_event
+        ( Sub { st = s1; callbacks = (module CB1) },
+          Sub { st = s2; callbacks = (module CB2) } ) ~time_ns ~tid ev : unit =
+      CB1.on_extension_event s1 ~time_ns ~tid ev;
+      CB2.on_extension_event s2 ~time_ns ~tid ev
   end
 end
 
diff --git a/src/subscriber/trace_subscriber.ml b/src/subscriber/trace_subscriber.ml
index fe13910..6d0535c 100644
--- a/src/subscriber/trace_subscriber.ml
+++ b/src/subscriber/trace_subscriber.ml
@@ -177,6 +177,11 @@ let collector (Sub { st; callbacks = (module CB) } : Subscriber.t) : collector =
       let time_ns = now_ns () in
       CB.on_shutdown st ~time_ns
 
+    let extension_event ev =
+      let tid = tid_ () in
+      let time_ns = now_ns () in
+      CB.on_extension_event st ~time_ns ~tid ev
+
     let () =
       (* init code *)
       let time_ns = now_ns () in
diff --git a/src/tef/trace_tef.ml b/src/tef/trace_tef.ml
index 7002cbf..b29d37f 100644
--- a/src/tef/trace_tef.ml
+++ b/src/tef/trace_tef.ml
@@ -354,6 +354,8 @@ let subscriber_ ~finally ~out ~(mode : [ `Single | `Jsonl ]) () : Sub.t =
       let time_us = time_ns *. 1e-3 in
       B_queue.push self.events
       @@ E_exit_manual_span { tid; id = trace_id; name; time_us; data; flavor }
+
+    let on_extension_event _ ~time_ns:_ ~tid:_ _ev = ()
   end in
   let events = B_queue.create () in
   let t_write =