diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 904c4111..b2a4af71 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -16,6 +16,7 @@
   "lambda_calc.ml": 7,
   "levenshtein.ml": 11,
   "memo_fib.ml": 75025,
+  "merge_sort.ml": 44,
   "module_use.ml": 3,
   "mutable_record.ml": 10,
   "option_match.ml": 5,
diff --git a/lib/ocaml/baseline/merge_sort.ml b/lib/ocaml/baseline/merge_sort.ml
new file mode 100644
index 00000000..cfcc30d6
--- /dev/null
+++ b/lib/ocaml/baseline/merge_sort.ml
@@ -0,0 +1,28 @@
+let rec split lst =
+  match lst with
+  | [] -> ([], [])
+  | [x] -> ([x], [])
+  | x :: y :: rest ->
+    let (a, b) = split rest in
+    (x :: a, y :: b)
+
+let rec merge xs ys =
+  match xs with
+  | [] -> ys
+  | x :: xs' ->
+    match ys with
+    | [] -> xs
+    | y :: ys' ->
+      if x <= y then x :: merge xs' (y :: ys')
+      else y :: merge (x :: xs') ys'
+
+let rec sort lst =
+  match lst with
+  | [] -> []
+  | [x] -> [x]
+  | _ ->
+    let (a, b) = split lst in
+    merge (sort a) (sort b)
+;;
+
+List.fold_left (+) 0 (sort [3;1;4;1;5;9;2;6;5;3;5])
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 3ee58e54..17e0c98c 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,12 @@ _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 — merge_sort.ml baseline (user-implemented
+  mergesort, sorted sum = 44). Stress-tests `let (a, b) = split rest
+  in (x :: a, y :: b)` (let-tuple destructuring inside a recursive
+  match arm), nested match-in-match for the merge merge step, and
+  the (op) operator section `(+)` as fold accumulator. 26 baseline
+  programs total.
 - 2026-05-09 Phase 4 — top-level `let f (a, b) = body` tuple-param
   decl (+3 tests, 559 total). parse-decl-let (which lives outside
   the ocaml-parse scope and lacks parse-pattern access) uses a