diff --git a/src/analysis_and_optimization/Optimize.ml b/src/analysis_and_optimization/Optimize.ml
index 52779e5725..f9e0e4542f 100644
--- a/src/analysis_and_optimization/Optimize.ml
+++ b/src/analysis_and_optimization/Optimize.ml
@@ -279,7 +279,8 @@ let rec inline_function_expression propto adt fim (Expr.Fixed.{pattern; _} as e)
             match suffix with
             | FnLpdf propto' when propto' && propto ->
                 ( Fun_kind.FnLpdf true
-                , Utils.with_unnormalized_suffix fname |> Option.value_exn )
+                , Utils.with_unnormalized_suffix fname
+                  |> Option.value ~default:fname )
             | FnLpdf _ -> (Fun_kind.FnLpdf false, fname)
             | _ -> (suffix, fname) in
           match Map.find fim fname' with
diff --git a/src/frontend/add_missing_messages.py b/src/frontend/add_missing_messages.py
index cf1c1a3bf5..11518ac86a 100755
--- a/src/frontend/add_missing_messages.py
+++ b/src/frontend/add_missing_messages.py
@@ -6,9 +6,9 @@
 import sys
 parse = re.compile('File "(.*)", line (\d+).*\nError: this sentence.*\nNo sentence that leads to this state exists in "(.*)".')
 
-parser = sys.argv[1]
-new_messages = sys.argv[2]
-old_messages = sys.argv[3]
+parser = (Sys.get_argv ())[1]
+new_messages = (Sys.get_argv ())[2]
+old_messages = (Sys.get_argv ())[3]
 
 p = subprocess.run("menhir {} --compare-errors {} --compare-errors {}".format(
     parser, new_messages, old_messages),
diff --git a/src/frontend/strip_redundant_parser_state.py b/src/frontend/strip_redundant_parser_state.py
index fac60cecf1..a936a927d0 100755
--- a/src/frontend/strip_redundant_parser_state.py
+++ b/src/frontend/strip_redundant_parser_state.py
@@ -24,7 +24,7 @@ def strip_lines(s):
 
 
 if __name__ == "__main__":
-    with open(sys.argv[1]) as f:
+    with open((Sys.get_argv())[1]) as f:
         messages = f.read()
     curr = strip_redundant_parser_states(messages)
     while messages != curr:
diff --git a/src/stan_math_backend/Stan_math_code_gen.ml b/src/stan_math_backend/Stan_math_code_gen.ml
index 7e9043a53c..b0ff74a951 100644
--- a/src/stan_math_backend/Stan_math_code_gen.ml
+++ b/src/stan_math_backend/Stan_math_code_gen.ml
@@ -34,7 +34,8 @@ let stanc_args_to_print =
         && not (is_prefix ~prefix:"--filename-in-msg" x)
         || is_prefix ~prefix:"--o" x) in
   (* Ignore the "--o" arg, the stan file and the binary name (bin/stanc). *)
-  Array.to_list (Sys.get_argv ()) |> List.tl_exn
+  Array.to_list (Sys.get_argv ())
+  |> List.tl_exn
   |> List.filter ~f:sans_model_and_hpp_paths
   |> String.concat ~sep:" "
 
diff --git a/src/stanc/stanc.ml b/src/stanc/stanc.ml
index 5c8f061b34..37b12bc11f 100644
--- a/src/stanc/stanc.ml
+++ b/src/stanc/stanc.ml
@@ -214,8 +214,9 @@ let options =
 let print_deprecated_arg_warning =
   (* is_prefix is used to also cover the --include-paths=... *)
   let arg_is_used arg =
-    Array.mem ~equal:(fun x y -> String.is_prefix ~prefix:x y) (Sys.get_argv ()) arg
-  in
+    Array.mem
+      ~equal:(fun x y -> String.is_prefix ~prefix:x y)
+      (Sys.get_argv ()) arg in
   if arg_is_used "--allow_undefined" then
     eprintf
       "--allow_undefined is deprecated and will be removed in Stan 2.32.0. \
diff --git a/test/integration/dune b/test/integration/dune
index a53efd44bc..682d5ced41 100644
--- a/test/integration/dune
+++ b/test/integration/dune
@@ -1,7 +1,7 @@
 (executable
  (name run_bin_on_args)
  (public_name run_bin_on_args)
- (libraries core_kernel core_kernel.caml_unix))
+ (libraries core_kernel core_kernel.caml_unix core_unix.sys_unix))
 
 (cram
  (applies_to :whole_subtree)
diff --git a/test/integration/good/code-gen/chop_summary.py b/test/integration/good/code-gen/chop_summary.py
index affcbc0055..842e32639d 100644
--- a/test/integration/good/code-gen/chop_summary.py
+++ b/test/integration/good/code-gen/chop_summary.py
@@ -1,7 +1,7 @@
 import sys
 import re
 
-with open(sys.argv[1]) as f:
+with open((Sys.get_argv ())[1]) as f:
     lines = f.readlines()[6:-6]
     for l in lines:
         cols = re.split("\s+", l)[0:7]
diff --git a/test/integration/run_bin_on_args.ml b/test/integration/run_bin_on_args.ml
index e1bc672740..3acfaf35ac 100644
--- a/test/integration/run_bin_on_args.ml
+++ b/test/integration/run_bin_on_args.ml
@@ -1,4 +1,4 @@
-open Core_kernel
+open Core
 module Unix = Caml_unix
 
 let maybe_convert_cmd_to_windows cmd =
@@ -9,7 +9,10 @@ let maybe_convert_cmd_to_windows cmd =
   let path =
     String.prefix cmd
       (String.substr_index_exn ~pattern cmd + String.length pattern) in
-  if Sys.file_exists (to_windows path) then to_windows cmd else cmd
+  match Sys_unix.file_exists (to_windows path) with
+  | `Yes -> to_windows cmd
+  | `No -> cmd
+  | `Unknown -> cmd
 
 let run_capturing_output cmd =
   let noflags = Array.create ~len:0 "" in
@@ -21,8 +24,10 @@ let run_capturing_output cmd =
   String.concat ~sep:"\n" (List.concat out)
 
 let () =
-  let binary = Sys.argv.(1) in
-  let dirs = Array.(sub Sys.argv ~pos:2 ~len:(length Sys.argv - 2)) in
+  let binary = (Sys.get_argv ()).(1) in
+  let dirs =
+    Array.(sub (Sys.get_argv ()) ~pos:2 ~len:(length (Sys.get_argv ()) - 2))
+  in
   Array.stable_sort ~compare:String.compare dirs ;
   Array.iter dirs ~f:(fun arg ->
       let cmd = binary ^ " " ^ arg in
diff --git a/test/stancjs/run_js_on_args.ml b/test/stancjs/run_js_on_args.ml
index c5b979d154..e8f474bb79 100644
--- a/test/stancjs/run_js_on_args.ml
+++ b/test/stancjs/run_js_on_args.ml
@@ -1,4 +1,4 @@
-open Core_kernel
+open Core
 module Unix = Caml_unix
 
 let run_capturing_output cmd =
@@ -10,7 +10,9 @@ let run_capturing_output cmd =
   String.concat ~sep:"\n" (List.concat out)
 
 let () =
-  let files = Array.(sub Sys.argv ~pos:1 ~len:(length Sys.argv - 1)) in
+  let files =
+    Array.(sub (Sys.get_argv ()) ~pos:1 ~len:(length (Sys.get_argv ()) - 1))
+  in
   Array.stable_sort ~compare:String.compare files ;
   Array.iter files ~f:(fun arg ->
       let cmd = "node " ^ arg in
diff --git a/test/unit/Ast_to_Mir_tests.ml b/test/unit/Ast_to_Mir_tests.ml
index 025a5f46f7..8732a13e3e 100644
--- a/test/unit/Ast_to_Mir_tests.ml
+++ b/test/unit/Ast_to_Mir_tests.ml
@@ -1,6 +1,6 @@
 open Frontend
 open Middle
-open Core_kernel
+open Core
 open Ast_to_Mir
 
 let%expect_test "Operator-assign example" =
diff --git a/test/unit/Dataflow_utils.ml b/test/unit/Dataflow_utils.ml
index 6a4e50d808..c9f7aa0c31 100644
--- a/test/unit/Dataflow_utils.ml
+++ b/test/unit/Dataflow_utils.ml
@@ -1,8 +1,8 @@
 open Frontend
 open Middle
 open Analysis_and_optimization.Dataflow_utils
-open Core_kernel
-open Core_kernel.Poly
+open Core
+open Core.Poly
 open Analysis_and_optimization.Dataflow_types
 
 let mir_of_string s =
diff --git a/test/unit/Debug_data_generation_tests.ml b/test/unit/Debug_data_generation_tests.ml
index f033ca7d67..f577c48ffe 100644
--- a/test/unit/Debug_data_generation_tests.ml
+++ b/test/unit/Debug_data_generation_tests.ml
@@ -1,5 +1,5 @@
 open Analysis_and_optimization
-open Core_kernel
+open Core
 open Frontend
 open Debug_data_generation
 
diff --git a/test/unit/Dependence_analysis.ml b/test/unit/Dependence_analysis.ml
index e2d6b3b131..fc475f71c3 100644
--- a/test/unit/Dependence_analysis.ml
+++ b/test/unit/Dependence_analysis.ml
@@ -1,4 +1,4 @@
-open Core_kernel
+open Core
 open Frontend
 open Analysis_and_optimization.Dependence_analysis
 open Middle
diff --git a/test/unit/Desugar_test.ml b/test/unit/Desugar_test.ml
index b17e52a58f..6004819418 100644
--- a/test/unit/Desugar_test.ml
+++ b/test/unit/Desugar_test.ml
@@ -1,4 +1,4 @@
-open Core_kernel
+open Core
 open Analysis_and_optimization
 
 let to_mir s =
diff --git a/test/unit/Factor_graph.ml b/test/unit/Factor_graph.ml
index 024ae3bae1..e5b7833a95 100644
--- a/test/unit/Factor_graph.ml
+++ b/test/unit/Factor_graph.ml
@@ -1,6 +1,6 @@
 open Frontend
 open Analysis_and_optimization.Factor_graph
-open Core_kernel
+open Core
 open Analysis_and_optimization.Dataflow_types
 
 let reject_example =
diff --git a/test/unit/Optimize.ml b/test/unit/Optimize.ml
index 6a2b263751..b7ff5cc14a 100644
--- a/test/unit/Optimize.ml
+++ b/test/unit/Optimize.ml
@@ -1,4 +1,4 @@
-open Core_kernel
+open Core
 open Frontend
 open Analysis_and_optimization.Optimize
 open Middle
diff --git a/test/unit/Parse_tests.ml b/test/unit/Parse_tests.ml
index 86bccfea68..401c0b9721 100644
--- a/test/unit/Parse_tests.ml
+++ b/test/unit/Parse_tests.ml
@@ -1,4 +1,4 @@
-open Core_kernel
+open Core
 open Frontend
 
 let print_ast_of_string s =
diff --git a/test/unit/Semantic_check_tests.ml b/test/unit/Semantic_check_tests.ml
index 25ec49ec80..93435e06d2 100644
--- a/test/unit/Semantic_check_tests.ml
+++ b/test/unit/Semantic_check_tests.ml
@@ -1,4 +1,4 @@
-open Core_kernel
+open Core
 open Frontend
 open Frontend_utils