diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 07296ad2..f2b22765 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -122,6 +122,7 @@
   "sum_squares.ml": 385,
   "tree_depth.ml": 4,
   "triangle.ml": 11,
+  "triangle_div.ml": 120,
   "twosum.ml": 5,
   "unique_set.ml": 9,
   "validate.ml": 417,
diff --git a/lib/ocaml/baseline/triangle_div.ml b/lib/ocaml/baseline/triangle_div.ml
new file mode 100644
index 00000000..20c079db
--- /dev/null
+++ b/lib/ocaml/baseline/triangle_div.ml
@@ -0,0 +1,26 @@
+let count_divisors n =
+  let c = ref 0 in
+  let i = ref 1 in
+  while !i * !i <= n do
+    if n mod !i = 0 then begin
+      c := !c + 1;
+      if !i * !i <> n then c := !c + 1
+    end;
+    i := !i + 1
+  done;
+  !c
+
+let first_triangle_with_divs target =
+  let t = ref 0 in
+  let n = ref 0 in
+  let found = ref false in
+  while not !found do
+    n := !n + 1;
+    t := !t + !n;
+    if count_divisors !t > target then found := true
+  done;
+  !t
+
+;;
+
+first_triangle_with_divs 10
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 4de83aaf..312d14ae 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,11 @@ _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.
+- 2026-05-10 Phase 5.1 — triangle_div.ml baseline (first triangle
+  number with > 10 divisors = 120). PE12 with target 10. T(15) = 120
+  has 16 divisors {1,2,3,4,5,6,8,10,12,15,20,24,30,40,60,120} —
+  first to break 10. Real PE12 uses target 500 (answer 76576500);
+  10 stays well under our budget. 126 baseline programs total.
 - 2026-05-10 Phase 5.1 — perfect.ml baseline (count perfect numbers
   ≤ 500 = 3). Perfect numbers = those where d(n) = n. Three under
   500: 6, 28, 496. (8128 is the next.) Same div_sum machinery as