diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 140edeea..037e7084 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -24,6 +24,7 @@
   "grep_count.ml": 3,
   "hanoi.ml": 1023,
   "fizzbuzz.ml": 57,
+  "flatten_tree.ml": 28,
   "list_ops.ml": 30,
   "mat_mul.ml": 621,
   "json_pretty.ml": 24,
diff --git a/lib/ocaml/baseline/flatten_tree.ml b/lib/ocaml/baseline/flatten_tree.ml
new file mode 100644
index 00000000..8572dd4b
--- /dev/null
+++ b/lib/ocaml/baseline/flatten_tree.ml
@@ -0,0 +1,17 @@
+type 'a tree = Leaf of 'a | Node of 'a tree list
+
+let rec flatten t =
+  match t with
+  | Leaf x -> [x]
+  | Node ts -> List.concat (List.map flatten ts)
+
+;;
+
+let t = Node [
+  Leaf 1;
+  Node [Leaf 2; Leaf 3];
+  Node [Node [Leaf 4]; Leaf 5; Leaf 6];
+  Leaf 7
+]
+in
+List.fold_left (+) 0 (flatten t)
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 377f0165..e185bb34 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,13 @@ _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 — flatten_tree.ml baseline (parametric ADT
+  flatten, sum 1..7 = 28). Defines `type 'a tree = Leaf of 'a |
+  Node of 'a tree list` then `flatten` recursively expands using
+  `List.concat (List.map flatten ts)`. Tree has 3 levels of
+  nesting; flattens to [1;2;3;4;5;6;7]. Tests parametric ADT, mutual
+  recursion via map+self, List.concat from runtime. 55 baseline
+  programs total.
 - 2026-05-09 Phase 5.1 — gcd_lcm.ml baseline (Euclidean gcd + lcm,
   12 + 12 + 36 = 60). Two-line baseline: `let rec gcd a b = if b = 0
   then a else gcd b (a mod b)` + `let lcm a b = a * b / gcd a b`.