diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 37132b62..7a5bb436 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -59,6 +59,7 @@
   "merge_sort.ml": 44,
   "merge_two.ml": 441,
   "module_use.ml": 3,
+  "monotonic.ml": 4,
   "newton_sqrt.ml": 1414,
   "mutable_record.ml": 10,
   "option_match.ml": 5,
diff --git a/lib/ocaml/baseline/monotonic.ml b/lib/ocaml/baseline/monotonic.ml
new file mode 100644
index 00000000..50020301
--- /dev/null
+++ b/lib/ocaml/baseline/monotonic.ml
@@ -0,0 +1,25 @@
+let is_monotonic xs =
+  match xs with
+  | [] -> true
+  | [_] -> true
+  | _ ->
+    let inc = ref true in
+    let dec = ref true in
+    let rec walk prev rest =
+      match rest with
+      | [] -> ()
+      | h :: t ->
+        if h < prev then inc := false;
+        if h > prev then dec := false;
+        walk h t
+    in
+    (match xs with h :: t -> walk h t | [] -> ());
+    !inc || !dec
+
+;;
+
+(if is_monotonic [1;2;3;4] then 1 else 0) +
+(if is_monotonic [4;3;2;1] then 1 else 0) +
+(if is_monotonic [1;2;1] then 1 else 0) +
+(if is_monotonic [5;5;5] then 1 else 0) +
+(if is_monotonic [] then 1 else 0)
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 9bd0f9ae..e73aeb8b 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,17 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-09 Phase 5.1 — monotonic.ml baseline (monotonicity check,
+  4/5 inputs monotonic). Tracks two bool refs (inc, dec). Each pair
+  of consecutive elements: if `h < prev` clear `inc`, if `h > prev`
+  clear `dec`. Empty list and singleton are vacuously true. Five
+  test cases:
+    [1;2;3;4]   inc only        true
+    [4;3;2;1]   dec only        true
+    [1;2;1]     neither          false
+    [5;5;5]     both (constant)  true
+    []           empty            true (vacuous)
+  Sum = 4. 98 baseline programs total.
 - 2026-05-09 Phase 5.1 — majority_vote.ml baseline (Boyer-Moore
   majority, [3;3;4;2;4;4;2;4;4] → 4). O(n) time / O(1) space:
   candidate-and-count refs; on match increment, on mismatch