diff --git a/HACKING.md b/HACKING.md
index a9462d3df06..6672dfb4455 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -121,6 +121,9 @@ install parallel`.
 There is also a `make ci` target (best run as e.g. `make -j16 ci`) which does a full build
 and test run.
 
+Some of our tests are expect tests run using a custom tool called `flexpect`.
+Corrected outputs can be promoted using `make promote`.
+
 ## Running only part of the upstream testsuite
 
 This can be done from the `_runtest` directory after it has been initialised by a previous `make runtest-upstream`.
diff --git a/Makefile.in b/Makefile.in
index 562193dcea6..91b82665f79 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -267,6 +267,21 @@ runtest:
 	  ARCH=`grep '^ARCH=' ocaml/Makefile.config | cut -d'=' -f2` \
 	  $(dune) runtest --root=. --profile=release --build-dir=_build2
 
+# Only needed for running the test tools by hand; runtest will take care of
+# building them using Dune
+.PHONY: test-tools
+test-tools: stage1
+	PATH=$(stage1_prefix)/bin:$$PATH \
+	  ARCH=`grep '^ARCH=' ocaml/Makefile.config | cut -d'=' -f2` \
+	  $(dune) build @middle_end/flambda2/tests/tools/all \
+	    --root=. --profile=release --build-dir=_build2
+
+.PHONY: promote
+promote:
+	PATH=$(stage1_prefix)/bin:$$PATH \
+	  ARCH=`grep '^ARCH=' ocaml/Makefile.config | cut -d'=' -f2` \
+	  $(dune) promote --root=. --build-dir=_build2
+
 # The following horror will be removed when work to allow the testsuite to
 # run on an installed tree (led by David Allsopp) is completed.
 # stage2 needs to have been built first.
diff --git a/middle_end/flambda2/compare/compare.ml b/middle_end/flambda2/compare/compare.ml
index d3d732c60cf..43e226c17ab 100644
--- a/middle_end/flambda2/compare/compare.ml
+++ b/middle_end/flambda2/compare/compare.ml
@@ -351,12 +351,13 @@ and subst_pattern env (pattern : Bound_symbols.Pattern.t) :
 and subst_static_const env (static_const : Static_const_or_code.t) :
     Static_const_or_code.t =
   match static_const with
-  | Code code -> Code (subst_code env code)
+  | Code code -> Static_const_or_code.create_code (subst_code env code)
   | Static_const (Block (tag, mut, fields)) ->
     let fields = List.map (subst_field env) fields in
-    Static_const (Block (tag, mut, fields))
+    Static_const_or_code.create_static_const (Block (tag, mut, fields))
   | Static_const (Set_of_closures set_of_closures) ->
-    Static_const (Set_of_closures (subst_set_of_closures env set_of_closures))
+    Static_const_or_code.create_static_const
+      (Set_of_closures (subst_set_of_closures env set_of_closures))
   | _ -> static_const
 
 and subst_code env (code : Code.t) : Code.t =
@@ -391,7 +392,7 @@ and subst_params_and_body env params_and_body =
   Function_params_and_body.pattern_match params_and_body
     ~f:(fun
          ~return_continuation
-         exn_continuation
+         ~exn_continuation
          params
          ~body
          ~my_closure
@@ -401,7 +402,7 @@ and subst_params_and_body env params_and_body =
        ->
       let body = subst_expr env body in
       let dbg = Function_params_and_body.debuginfo params_and_body in
-      Function_params_and_body.create ~return_continuation exn_continuation
+      Function_params_and_body.create ~return_continuation ~exn_continuation
         params ~dbg ~body ~my_closure ~free_names_of_body ~my_depth)
 
 and subst_let_cont env (let_cont_expr : Let_cont_expr.t) =
@@ -1126,18 +1127,18 @@ and static_consts env (const1 : Static_const_or_code.t)
     (const2 : Static_const_or_code.t) : Static_const_or_code.t Comparison.t =
   match const1, const2 with
   | Code code1, Code code2 ->
-    codes env code1 code2
-    |> Comparison.map ~f:(fun code1' : Static_const_or_code.t -> Code code1')
+    codes env code1 code2 |> Comparison.map ~f:Static_const_or_code.create_code
   | ( Static_const (Block (tag1, mut1, fields1)),
       Static_const (Block (tag2, mut2, fields2)) ) ->
     blocks env (tag1, mut1, fields1) (tag2, mut2, fields2)
     |> Comparison.map
          ~f:(fun (tag1', mut1', fields1') : Static_const_or_code.t ->
-           Static_const (Block (tag1', mut1', fields1')))
+           Static_const_or_code.create_static_const
+             (Block (tag1', mut1', fields1')))
   | Static_const (Set_of_closures set1), Static_const (Set_of_closures set2) ->
     sets_of_closures env set1 set2
     |> Comparison.map ~f:(fun set1' : Static_const_or_code.t ->
-           Static_const (Set_of_closures set1'))
+           Static_const_or_code.create_static_const (Set_of_closures set1'))
   | _, _ ->
     if Static_const_or_code.equal const1 const2
     then Equivalent
@@ -1149,7 +1150,7 @@ and codes env (code1 : Code.t) (code2 : Code.t) =
       params_and_body2
       ~f:(fun
            ~return_continuation
-           exn_continuation
+           ~exn_continuation
            params
            ~body1
            ~body2
@@ -1160,8 +1161,8 @@ and codes env (code1 : Code.t) (code2 : Code.t) =
         |> Comparison.map ~f:(fun body1' ->
                let dbg = Function_params_and_body.debuginfo params_and_body1 in
                Function_params_and_body.create ~return_continuation
-                 exn_continuation params ~dbg ~body:body1' ~my_closure ~my_depth
-                 ~free_names_of_body:Unknown))
+                 ~exn_continuation params ~dbg ~body:body1' ~my_closure
+                 ~my_depth ~free_names_of_body:Unknown))
   in
 
   let bodies_or_deleted env body1 body2 : _ Or_deleted.t Comparison.t =
@@ -1260,25 +1261,23 @@ and let_cont_exprs env (let_cont1 : Let_cont.t) (let_cont2 : Let_cont.t) :
   | _, _ -> Different { approximant = subst_let_cont env let_cont1 }
 
 and cont_handlers env handler1 handler2 =
-  log Continuation_handler.print handler1 handler2 (fun () ->
-      Continuation_handler.pattern_match_pair handler1 handler2
-        ~f:(fun params ~handler1:expr1 ~handler2:expr2 ->
-          exprs env expr1 expr2
-          |> Comparison.map ~f:(fun handler ->
-                 Continuation_handler.create params ~handler
-                   ~free_names_of_handler:Unknown
-                   ~is_exn_handler:
-                     (Continuation_handler.is_exn_handler handler2))
-          |> Comparison.add_condition
-               ~cond:
-                 (Bool.equal
-                    (Continuation_handler.is_exn_handler handler1)
-                    (Continuation_handler.is_exn_handler handler2))
-               ~approximant:(fun () -> subst_cont_handler env handler1))
-      |> function
-      | Ok comp -> comp
-      | Error _ ->
-        Comparison.Different { approximant = subst_cont_handler env handler1 })
+  Flambda.Continuation_handler.pattern_match_pair handler1 handler2
+    ~f:(fun params ~handler1:expr1 ~handler2:expr2 ->
+      exprs env expr1 expr2
+      |> Comparison.map ~f:(fun handler ->
+             Continuation_handler.create params ~handler
+               ~free_names_of_handler:Unknown
+               ~is_exn_handler:(Continuation_handler.is_exn_handler handler2))
+      |> Comparison.add_condition
+           ~cond:
+             (Bool.equal
+                (Continuation_handler.is_exn_handler handler1)
+                (Continuation_handler.is_exn_handler handler2))
+           ~approximant:(fun () -> subst_cont_handler env handler1))
+  |> function
+  | Ok comp -> comp
+  | Error _ ->
+    Comparison.Different { approximant = subst_cont_handler env handler1 }
 
 let flambda_units u1 u2 =
   let ret_cont = Continuation.create ~sort:Toplevel_return () in
diff --git a/middle_end/flambda2/compare/dune b/middle_end/flambda2/compare/dune
index 2f066afcc66..25a387cb5cc 100644
--- a/middle_end/flambda2/compare/dune
+++ b/middle_end/flambda2/compare/dune
@@ -8,9 +8,21 @@
    -principal
    -nostdlib
    -open
-   Flambda2_terms
+   Flambda2_algorithms
    -open
-   Flambda2_algorithms))
+   Flambda2_bound_identifiers
+   -open
+   Flambda2_identifiers
+   -open
+   Flambda2_kinds
+   -open
+   Flambda2_nominal
+   -open
+   Flambda2_numbers
+   -open
+   Flambda2_term_basics
+   -open
+   Flambda2_terms))
  (ocamlopt_flags
   (:standard -O3))
  (libraries stdlib ocamlcommon flambda2_algorithms flambda2_terms))
diff --git a/middle_end/flambda2/flambda2.mli b/middle_end/flambda2/flambda2.mli
index 8b08975cd54..4ab6e60f353 100644
--- a/middle_end/flambda2/flambda2.mli
+++ b/middle_end/flambda2/flambda2.mli
@@ -27,3 +27,12 @@ val lambda_to_cmm :
   module_block_size_in_words:int ->
   module_initializer:Lambda.lambda ->
   Cmm.phrase list
+
+val symbol_for_global :
+  ?comp_unit:Flambda2_identifiers.Compilation_unit.t ->
+  Ident.t ->
+  Flambda2_identifiers.Symbol.t
+
+val get_global_info :
+  Flambda2_identifiers.Compilation_unit.t ->
+  Flambda2_cmx.Flambda_cmx_format.t option
diff --git a/middle_end/flambda2/tests/dune b/middle_end/flambda2/tests/dune
index 3247c4cf56b..d16db2d93a3 100644
--- a/middle_end/flambda2/tests/dune
+++ b/middle_end/flambda2/tests/dune
@@ -1,14 +1,12 @@
-(executable
- (name meet_test)
+(tests
+ (names meet_test)
  (modes native)
- (flags (:standard -principal -nostdlib -cclib "-I ../../runtime"))
- (libraries ocamloptcomp ocamloptcommon ocamlmiddleend flambda ocamlcommon runtimeopt stdlib)
- (modules meet_test))
+ (flags (:standard -principal -nostdlib))
+ (libraries
+  stdlib runtime_native ocamloptcomp ocamlcommon
+  flambda2_bound_identifiers flambda2_cmx flambda2_identifiers flambda2_kinds
+  flambda2_nominal flambda2_numbers flambda2_term_basics flambda2_types))
 
 (include_subdirs no)
 
-(alias
-  (name world)
-  (deps
-    meet_test.exe
-  ))
+(dirs mlexamples tools)
diff --git a/middle_end/flambda2/tests/meet_test.ml b/middle_end/flambda2/tests/meet_test.ml
index d6710603e4e..692127fc583 100644
--- a/middle_end/flambda2/tests/meet_test.ml
+++ b/middle_end/flambda2/tests/meet_test.ml
@@ -1,16 +1,23 @@
+open Flambda2_bound_identifiers
+open Flambda2_cmx
+open Flambda2_identifiers
+open Flambda2_kinds
+open Flambda2_nominal
+open Flambda2_numbers
+open Flambda2_term_basics
+open Flambda2_types
 module K = Flambda_kind
 module T = Flambda2_types
 module TE = T.Typing_env
 module TEE = T.Typing_env_extension
 
-let resolver _ = None
-
-let get_imported_names () = Name.Set.empty
-
-let get_imported_code () = Exported_code.empty
+let create_env () =
+  let resolver _ = None in
+  let get_imported_names () = Name.Set.empty in
+  TE.create ~resolver ~get_imported_names
 
 let test_meet_chains_two_vars () =
-  let env = TE.create ~resolver ~get_imported_names ~get_imported_code in
+  let env = create_env () in
   let var1 = Variable.create "var1" in
   let var1' = Bound_var.create var1 Name_mode.normal in
   let env = TE.add_definition env (Bound_name.var var1') K.value in
@@ -43,7 +50,7 @@ let test_meet_chains_two_vars () =
     Format.eprintf "Final situation:@ %a\n%!" TE.print env
 
 let test_meet_chains_three_vars () =
-  let env = TE.create ~resolver ~get_imported_names ~get_imported_code in
+  let env = create_env () in
   let var1 = Variable.create "var1" in
   let var1' = Bound_var.create var1 Name_mode.normal in
   let env = TE.add_definition env (Bound_name.var var1') K.value in
@@ -81,7 +88,7 @@ let test_meet_chains_three_vars () =
     Format.eprintf "Final situation:@ %a\n%!" TE.print env
 
 let meet_variants_don't_lose_aliases () =
-  let env = TE.create ~resolver ~get_imported_names ~get_imported_code in
+  let env = create_env () in
   let define env v =
     let v' = Bound_var.create v Name_mode.normal in
     TE.add_definition env (Bound_name.var v') K.value
@@ -136,7 +143,7 @@ let test_meet_two_blocks () =
     TE.add_definition env (Bound_name.var v') K.value
   in
   let defines env l = List.fold_left define env l in
-  let env = TE.create ~resolver ~get_imported_names ~get_imported_code in
+  let env = create_env () in
   let block1 = Variable.create "block1" in
   let field1 = Variable.create "field1" in
   let block2 = Variable.create "block2" in
diff --git a/middle_end/flambda2/tests/mlexamples/dune b/middle_end/flambda2/tests/mlexamples/dune
new file mode 100644
index 00000000000..bfdc36393d3
--- /dev/null
+++ b/middle_end/flambda2/tests/mlexamples/dune
@@ -0,0 +1,6 @@
+(rule
+ (alias runtest)
+ (action
+  (progn
+   (run ../tools/flexpect.exe tests0.flt)
+   (diff? tests0.flt tests0.flt.corrected))))
diff --git a/middle_end/flambda2/tests/tools/dune b/middle_end/flambda2/tests/tools/dune
new file mode 100644
index 00000000000..d8a07048e34
--- /dev/null
+++ b/middle_end/flambda2/tests/tools/dune
@@ -0,0 +1,7 @@
+(executables
+  (names flexpect fldiff parseflambda)
+  (modes native)
+  (flags (:standard -principal -nostdlib))
+  (libraries
+   stdlib runtime_native ocamlcommon ocamloptcomp
+   flambda2 flambda2_compare flambda2_parser flambda2_terms))
diff --git a/middle_end/flambda2/tests/tools/fldiff.ml b/middle_end/flambda2/tests/tools/fldiff.ml
new file mode 100644
index 00000000000..bcf58a77829
--- /dev/null
+++ b/middle_end/flambda2/tests/tools/fldiff.ml
@@ -0,0 +1,26 @@
+open Import
+
+let parse_flambda file =
+  match
+    Parse_flambda.parse ~symbol_for_global:Flambda2.symbol_for_global file
+  with
+  | Ok unit -> unit
+  | Error e ->
+    begin
+      match e with
+      | Parsing_error (msg, loc) ->
+        Format.eprintf "%a:@.Syntax error: %s@." Location.print_loc loc msg
+      | Lexing_error (error, loc) ->
+        Format.eprintf "%a:@.Lex error: %a@." Location.print_loc loc
+          Flambda_lex.pp_error error
+    end;
+    exit 1
+
+let _ =
+  let file1 = Sys.argv.(1) in
+  let file2 = Sys.argv.(2) in
+  let unit1 = parse_flambda file1 in
+  let unit2 = parse_flambda file2 in
+  Format.printf "%a@."
+    (Compare.Comparison.print Flambda_unit.print)
+    (Compare.flambda_units unit1 unit2)
diff --git a/middle_end/flambda2/tests/tools/flexpect.ml b/middle_end/flambda2/tests/tools/flexpect.ml
new file mode 100644
index 00000000000..926ef97a14e
--- /dev/null
+++ b/middle_end/flambda2/tests/tools/flexpect.ml
@@ -0,0 +1,147 @@
+open Import
+
+(* Dune expects that on failure we [exit 0] but write a corrected file. *)
+(* CR lmaurer: Make this an argument. *)
+let exit_normally_on_failure = true
+
+let symbol_for_global = Flambda2.symbol_for_global
+
+let get_global_info = Flambda2.get_global_info
+
+let check_invariants program =
+  try () (* Flambda_unit.invariant program *)
+  with exn ->
+    Format.eprintf "Program which failed invariant check:@ %a\n%!"
+      Flambda_unit.print program;
+    raise exn
+
+module Outcome = struct
+  type t =
+    | Success
+    | Failure
+    | Error
+
+  let to_exit_code = function
+    | Success -> 0
+    | Failure when exit_normally_on_failure -> 0
+    | Failure -> 1
+    | Error -> 2
+end
+
+module Test_outcome = struct
+  type t =
+    | Pass
+    | Fail of { corrected : Fexpr.expect_test_spec }
+end
+
+let dump_error (e : Parse_flambda.error) =
+  match e with
+  | Parsing_error (msg, loc) ->
+    Format.eprintf "%a:@.Syntax error: %s@." Location.print_loc loc msg
+  | Lexing_error (error, loc) ->
+    Format.eprintf "%a:@.Lex error: %a@." Location.print_loc loc
+      Flambda_lex.pp_error error
+
+let run_expect_test ~symbol_for_global ~get_global_info ~extension ~filename
+    ({ before; after = expected } : Fexpr.expect_test_spec) : Test_outcome.t =
+  let comp_unit = Parse_flambda.make_compilation_unit ~extension ~filename () in
+  Compilation_unit.set_current comp_unit;
+  let module_ident = Compilation_unit.get_persistent_ident comp_unit in
+  let before_fl =
+    Fexpr_to_flambda.conv ~symbol_for_global ~module_ident before
+  in
+  check_invariants before_fl;
+  let { Simplify.unit = actual_fl; _ } =
+    Simplify.run ~symbol_for_global ~get_global_info ~round:0 before_fl
+  in
+  let expected_fl =
+    Fexpr_to_flambda.conv ~symbol_for_global ~module_ident expected
+  in
+  match Compare.flambda_units actual_fl expected_fl with
+  | Equivalent -> Pass
+  | Different { approximant = actual' } ->
+    let actual_fexpr = Flambda_to_fexpr.conv actual' in
+    Fail { corrected = { before; after = actual_fexpr } }
+
+let show_diff a b =
+  let command_line = Filename.quote_command "diff" ["-u"; a; b] in
+  let _exit_code = Sys.command command_line in
+  ()
+
+let save_corrected ~desc ~print ~orig_filename corrected =
+  let corrected_filename = orig_filename ^ ".corrected" in
+  Format.eprintf "Saving corrected %s as %s@." desc corrected_filename;
+  let corrected_out = open_out corrected_filename in
+  Misc.try_finally
+    ~always:(fun () -> close_out corrected_out)
+    (fun () ->
+      let ppf = corrected_out |> Format.formatter_of_out_channel in
+      print ppf corrected;
+      Format.pp_print_flush ppf ());
+  if false then show_diff orig_filename corrected_filename
+
+let run_flt_file filename : Outcome.t =
+  match Parse_flambda.parse_expect_test_spec filename with
+  | Ok test_spec -> begin
+    match
+      run_expect_test ~symbol_for_global ~get_global_info ~extension:".flt"
+        ~filename test_spec
+    with
+    | Pass ->
+      Format.eprintf "PASS@.";
+      Success
+    | Fail { corrected } ->
+      Format.eprintf "FAIL@.";
+      save_corrected corrected ~desc:"test" ~print:Print_fexpr.expect_test_spec
+        ~orig_filename:filename;
+      Failure
+  end
+  | Error e ->
+    dump_error e;
+    Error
+
+let run_mdflx_file filename : Outcome.t =
+  match Parse_flambda.parse_markdown_doc filename with
+  | Ok doc ->
+    let all_passed = ref true in
+    let corrected_doc =
+      List.map
+        (fun (node : Fexpr.markdown_node) : Fexpr.markdown_node ->
+          match node with
+          | Text _ -> node
+          | Expect test_spec -> (
+            match
+              run_expect_test test_spec ~symbol_for_global ~get_global_info
+                ~extension:".mdflx" ~filename
+            with
+            | Pass ->
+              Format.eprintf "PASS@.";
+              node
+            | Fail { corrected } ->
+              all_passed := false;
+              Format.eprintf "FAIL@.";
+              Expect corrected))
+        doc
+    in
+    if !all_passed
+    then Outcome.Success
+    else begin
+      save_corrected corrected_doc ~desc:"document"
+        ~print:Print_fexpr.markdown_doc ~orig_filename:filename;
+      Failure
+    end
+  | Error e ->
+    dump_error e;
+    Error
+
+let _ =
+  let file = Sys.argv.(1) in
+  let ext = Filename.extension file in
+  let outcome =
+    match ext with
+    | ".flt" -> run_flt_file file
+    | ".mdflx" -> run_mdflx_file file
+    | _ ->
+      Misc.fatal_errorf "Unrecognized extension %s; expected .flt or .mdflx" ext
+  in
+  exit (outcome |> Outcome.to_exit_code)
diff --git a/middle_end/flambda2/tests/tools/import.ml b/middle_end/flambda2/tests/tools/import.ml
new file mode 100644
index 00000000000..900c1eaf80a
--- /dev/null
+++ b/middle_end/flambda2/tests/tools/import.ml
@@ -0,0 +1,5 @@
+include Flambda2_compare
+include Flambda2_identifiers
+include Flambda2_terms
+include Flambda2_parser
+include Flambda2_simplify
diff --git a/middle_end/flambda2/tests/tools/parseflambda.ml b/middle_end/flambda2/tests/tools/parseflambda.ml
new file mode 100644
index 00000000000..b4f55e75e80
--- /dev/null
+++ b/middle_end/flambda2/tests/tools/parseflambda.ml
@@ -0,0 +1,49 @@
+open Import
+
+let symbol_for_global = Flambda2.symbol_for_global
+
+let get_global_info = Flambda2.get_global_info
+
+let check_invariants program =
+  try () (* Flambda_unit.invariant program *)
+  with exn ->
+    Format.eprintf "Program which failed invariant check:@ %a\n%!"
+      Flambda_unit.print program;
+    raise exn
+
+let parse_flambda filename =
+  match Parse_flambda.parse_fexpr filename with
+  | Ok unit ->
+    let comp_unit =
+      Parse_flambda.make_compilation_unit ~extension:".fl" ~filename ()
+    in
+    Compilation_unit.set_current comp_unit;
+    Format.printf "%a@.@." Print_fexpr.flambda_unit unit;
+    let module_ident = Compilation_unit.get_persistent_ident comp_unit in
+    let fl2 = Fexpr_to_flambda.conv ~symbol_for_global ~module_ident unit in
+    Format.printf "flambda:@.%a@.@." Flambda_unit.print fl2;
+    check_invariants fl2;
+    let { Simplify.unit = fl2'; _ } =
+      Simplify.run ~symbol_for_global ~get_global_info ~round:1 fl2
+    in
+    Format.printf "simplify:@.%a@." Flambda_unit.print fl2';
+    let fl3 = Flambda_to_fexpr.conv fl2' in
+    Format.printf "back to fexpr:@.%a@." Print_fexpr.flambda_unit fl3;
+    fl3
+  | Error e ->
+    begin
+      match e with
+      | Parsing_error (msg, loc) ->
+        Format.eprintf "%a:@.Syntax error: %s@." Location.print_loc loc msg
+      | Lexing_error (error, loc) ->
+        Format.eprintf "%a:@.Lex error: %a@." Location.print_loc loc
+          Flambda_lex.pp_error error
+    end;
+    exit 1
+
+let _ =
+  let file = Sys.argv.(1) in
+  let ext = Filename.extension file in
+  match ext with
+  | ".fl" -> parse_flambda file
+  | _ -> Misc.fatal_errorf "Unrecognized extension %s" ext