diff --git a/src/session/controller_itp.ml b/src/session/controller_itp.ml
index c522ab49e74d876adaef806ad24a51e718581a89..ea1615c44a3bf77ecf00f8a8443e243dc074205f 100644
--- a/src/session/controller_itp.ml
+++ b/src/session/controller_itp.ml
@@ -1151,11 +1151,14 @@ let create_rem_list =
   Buffer.contents b
 
 
+exception CannotRunBisectionOn of proofAttemptID
+
+
 let bisect_proof_attempt ~callback_tr ~callback_pa ~notification ~removed c pa_id =
   let ses = c.controller_session in
   let pa = get_proof_attempt_node ses pa_id in
   if not (proof_is_complete pa) then
-    invalid_arg "bisect: proof attempt should be valid";
+    raise (CannotRunBisectionOn pa_id); (* proof attempt should be valid *)
   let goal_id = pa.parent in
   let prover = pa.prover in
   let limit = { pa.limit with
@@ -1221,12 +1224,12 @@ let bisect_proof_attempt ~callback_tr ~callback_pa ~notification ~removed c pa_i
      *)
     let rem = create_rem_list rem in
     let callback st =
-      callback_tr "remove" [rem] st;
       begin match st with
       | TSscheduled ->
          Debug.dprintf
            debug
-           "[Bisect] transformation 'remove' scheduled@."
+           "[Bisect] transformation 'remove' scheduled@.";
+         callback_tr "remove" [rem] st;
       | TSfailed(_,exn) ->
          (* may happen if removing a type or a lsymbol that is used
 later on. We do has if proof fails. *)
@@ -1244,6 +1247,7 @@ later on. We do has if proof fails. *)
          Debug.dprintf
            debug
            "[Bisect] transformation 'remove' succeeds@.";
+         callback_tr "remove" [rem] st;
          match get_sub_tasks ses trid with
          | [pn] ->
             let limit = { limit with Call_provers.limit_time = !timelimit; } in
diff --git a/src/session/controller_itp.mli b/src/session/controller_itp.mli
index 85bd76b4a91a2ccc08f6880e45ad984f3ce28d31..21e7c44fae2ccfcf89be457af7e37a4d1137dbdf 100644
--- a/src/session/controller_itp.mli
+++ b/src/session/controller_itp.mli
@@ -343,6 +343,7 @@ val replay:
 
  *)
 
+exception CannotRunBisectionOn of proofAttemptID
 
 val bisect_proof_attempt:
   callback_tr:(string -> string list -> transformation_status -> unit) ->
@@ -350,5 +351,23 @@ val bisect_proof_attempt:
   notification:notifier ->
   removed:notifier ->
   controller -> proofAttemptID -> unit
-
+  (** [bisect_proof_attempt ~callback_tr ~callback_pa ~notification
+                            ~removed cont id] runs a bisection process
+      based on the proof attempt [id] of the session managed by [cont].
+
+      The proof attempt [id] must be a successful one, otherwise,
+      exception [CannotRunBisectionOn id] is raised.
+
+      Bisection tries to remove from the context the largest number of
+      definitions and axioms, using the `remove` transformation (bound
+      to [Cut.remove_list]). It proceeeds by dichotomy of the
+      context. Note that there is no garantee that the removed data at
+      the end is globally maximal. During that process, [callback_tr]
+      is called each time the `remove` transformation is added to the session,
+      [callback_pa] is called each time the prover is called on a
+      reduced task, [notification] is called when a proof node is
+      created or modified, and [removed] is called when a node is
+      removed.
+
+   *)
 end
diff --git a/src/session/itp_server.ml b/src/session/itp_server.ml
index 8406dc29b574b5ffbf2ce5148a719128a028cde2..39b748530bd02ea7309805fbc41d09bd142dd6c9 100644
--- a/src/session/itp_server.ml
+++ b/src/session/itp_server.ml
@@ -1197,6 +1197,11 @@ end
         (Message
            (Information
               "for bisection please select some proof attempt"))
+       | C.CannotRunBisectionOn _ ->
+          P.notify
+            (Message
+               (Error
+                  "for bisection please select a successful proof attempt"))
 
 
   (* ----------------- run strategy -------------------- *)