diff --git a/src/tef/trace_tef.ml b/src/tef/trace_tef.ml
index c06c73d..7fea2ae 100644
--- a/src/tef/trace_tef.ml
+++ b/src/tef/trace_tef.ml
@@ -70,6 +70,7 @@ module Writer = struct
       | `File path -> open_out path, true
       | `File_append path ->
         open_out_gen [ Open_creat; Open_wronly; Open_append ] 0o644 path, true
+      | `Output oc -> oc, false
     in
     let pid =
       if !Mock_.enabled then
diff --git a/src/tef/trace_tef.mli b/src/tef/trace_tef.mli
index 03866f0..6c73f13 100644
--- a/src/tef/trace_tef.mli
+++ b/src/tef/trace_tef.mli
@@ -47,7 +47,7 @@ module Internal_ : sig
 
   val collector_jsonl :
     finally:(unit -> unit) ->
-    out:[ `File_append of string ] ->
+    out:[ `File_append of string | `Output of out_channel ] ->
     unit ->
     Trace_core.collector