diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 367541d2..6a61cd43 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -14,6 +14,7 @@
   "factorial.ml": 3628800,
   "fraction.ml": 7,
   "frequency.ml": 5,
+  "hanoi.ml": 1023,
   "fizzbuzz.ml": 57,
   "list_ops.ml": 30,
   "json_pretty.ml": 24,
diff --git a/lib/ocaml/baseline/hanoi.ml b/lib/ocaml/baseline/hanoi.ml
new file mode 100644
index 00000000..a57cc1f5
--- /dev/null
+++ b/lib/ocaml/baseline/hanoi.ml
@@ -0,0 +1,11 @@
+let rec hanoi n from to_ via =
+  if n = 0 then 0
+  else
+    let a = hanoi (n - 1) from via to_ in
+    let b = 1 in
+    let c = hanoi (n - 1) via to_ from in
+    a + b + c
+
+;;
+
+hanoi 10 1 3 2
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 7ede6105..3e799773 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,14 @@ _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 — hanoi.ml baseline (Tower of Hanoi move
+  count, n=10 → 1023). Classic doubly-recursive solution returning
+  the number of moves: `hanoi n from to via = hanoi (n-1) from via
+  to + 1 + hanoi (n-1) via to from`. Counts to 2^10 - 1 = 1023 for
+  n=10, exercising tail-position addition + 4-arg recursion +
+  conditional base case. (Uses `to_` instead of `to` to avoid
+  collision with the `to` keyword in for-loops — OCaml conventional
+  workaround.) 37 baseline programs total.
 - 2026-05-09 Phase 5.1 — validate.ml baseline (Either-based input
   validation, 3 errors × 100 + 117 sum = 417). validate_int returns
   `Left msg` on empty / non-digit, `Right (int_of_string s)` on a