diff --git a/src/fuchsia/fcollector.ml b/src/fuchsia/fcollector.ml
index c2fc367..35c7925 100644
--- a/src/fuchsia/fcollector.ml
+++ b/src/fuchsia/fcollector.ml
@@ -5,6 +5,19 @@ module Int_map = Map.Make (Int)
 
 let pid = Unix.getpid ()
 
+module Mock_ = struct
+  let enabled = ref false
+  let now = ref 0
+
+  (* used to mock timing *)
+  let get_now_ns () : float =
+    let x = !now in
+    incr now;
+    float_of_int x *. 1000.
+
+  let get_tid_ () : int = 3
+end
+
 (** Thread-local stack of span info *)
 module Span_info_stack : sig
   type t
@@ -408,3 +421,10 @@ let create ~out () : collector =
       ()
   in
   (module Coll)
+
+module Internal_ = struct
+  let mock_all_ () =
+    Mock_.enabled := true;
+    Sub.Private_.get_now_ns_ := Some Mock_.get_now_ns;
+    Sub.Private_.get_tid_ := Some Mock_.get_tid_
+end
diff --git a/src/fuchsia/fcollector.mli b/src/fuchsia/fcollector.mli
index 780b3f1..2ba03e9 100644
--- a/src/fuchsia/fcollector.mli
+++ b/src/fuchsia/fcollector.mli
@@ -1,3 +1,11 @@
 open Trace_core
 
 val create : out:Bg_thread.out -> unit -> collector
+
+(**/**)
+
+module Internal_ : sig
+  val mock_all_ : unit -> unit
+end
+
+(**/**)
diff --git a/src/fuchsia/trace_fuchsia.ml b/src/fuchsia/trace_fuchsia.ml
index 102a744..f6b2b14 100644
--- a/src/fuchsia/trace_fuchsia.ml
+++ b/src/fuchsia/trace_fuchsia.ml
@@ -34,5 +34,6 @@ let with_setup ?out () f =
   Fun.protect ~finally:Trace_core.shutdown f
 
 module Internal_ = struct
+  let mock_all_ = Fcollector.Internal_.mock_all_
   let on_tracing_error = on_tracing_error
 end
diff --git a/src/fuchsia/trace_fuchsia.mli b/src/fuchsia/trace_fuchsia.mli
index eaf1847..d28111a 100644
--- a/src/fuchsia/trace_fuchsia.mli
+++ b/src/fuchsia/trace_fuchsia.mli
@@ -45,6 +45,9 @@ val with_setup : ?out:[ output | `Env ] -> unit -> (unit -> 'a) -> 'a
 
 module Internal_ : sig
   val on_tracing_error : (string -> unit) ref
+
+  val mock_all_ : unit -> unit
+  (** use fake, deterministic timestamps, TID, PID *)
 end
 
 (**/**)