diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index b076242f..1b08ac96 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -16,6 +16,7 @@
   "frequency.ml": 5,
   "fizzbuzz.ml": 57,
   "list_ops.ml": 30,
+  "json_pretty.ml": 24,
   "lambda_calc.ml": 7,
   "levenshtein.ml": 11,
   "memo_fib.ml": 75025,
diff --git a/lib/ocaml/baseline/json_pretty.ml b/lib/ocaml/baseline/json_pretty.ml
new file mode 100644
index 00000000..861866c8
--- /dev/null
+++ b/lib/ocaml/baseline/json_pretty.ml
@@ -0,0 +1,20 @@
+type json =
+  | JNull
+  | JBool of bool
+  | JInt of int
+  | JStr of string
+  | JList of json list
+
+let rec to_string j =
+  match j with
+  | JNull -> "null"
+  | JBool b -> if b then "true" else "false"
+  | JInt n -> string_of_int n
+  | JStr s -> "\"" ^ s ^ "\""
+  | JList xs ->
+    "[" ^ String.concat "," (List.map to_string xs) ^ "]"
+
+;;
+
+let j = JList [JInt 1; JBool true; JNull; JStr "hi"; JList [JInt 2; JInt 3]] in
+String.length (to_string j)
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 7ac4126c..840fbdc9 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 — json_pretty.ml baseline (recursive ADT
+  serialization). Defines a JSON-like ADT (JNull / JBool / JInt /
+  JStr / JList) and recursively pretty-prints to a string, then
+  measures length. Tests algebraic data types with five constructors
+  (one nullary, three single-arg, one list-arg), recursive `match`
+  with five arms, `String.concat "," (List.map ...)`, and string
+  concatenation. `[1,true,null,"hi",[2,3]]` → 24 chars. 34 baseline
+  programs total.
 - 2026-05-09 Phase 5.1 — shuffle.ml baseline (Fisher-Yates with
   deterministic Random.init seed). In-place swap loop using `for i =
   n - 1 downto 1` and `a.(i) <- a.(j)`. Sum is invariant under