diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 8c6e86e7..805525cc 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -52,6 +52,7 @@
   "mutable_record.ml": 10,
   "option_match.ml": 5,
   "palindrome.ml": 4,
+  "pancake_sort.ml": 910,
   "pascal.ml": 252,
   "pi_leibniz.ml": 314,
   "prefix_sum.ml": 66,
diff --git a/lib/ocaml/baseline/pancake_sort.ml b/lib/ocaml/baseline/pancake_sort.ml
new file mode 100644
index 00000000..23bade9a
--- /dev/null
+++ b/lib/ocaml/baseline/pancake_sort.ml
@@ -0,0 +1,33 @@
+let pancake_sort xs =
+  let a = Array.of_list xs in
+  let n = Array.length a in
+  let flips = ref 0 in
+  let flip k =
+    let lo = ref 0 and hi = ref k in
+    while !lo < !hi do
+      let tmp = a.(!lo) in
+      a.(!lo) <- a.(!hi);
+      a.(!hi) <- tmp;
+      lo := !lo + 1;
+      hi := !hi - 1
+    done;
+    flips := !flips + 1
+  in
+  let find_max k =
+    let m = ref 0 in
+    for i = 1 to k do
+      if a.(i) > a.(!m) then m := i
+    done;
+    !m
+  in
+  for size = n downto 2 do
+    let mi = find_max (size - 1) in
+    if mi <> size - 1 then begin
+      if mi > 0 then flip mi;
+      flip (size - 1)
+    end
+  done;
+  !flips * 100 + a.(0) + a.(n - 1)
+;;
+
+pancake_sort [3; 1; 4; 1; 5; 9; 2; 6]
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 93229db5..615a22a8 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,15 @@ _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 — pancake_sort.ml baseline (in-place pancake
+  sort, 9 flips → 910). Each pass finds the max in [0..size-1],
+  flips it to position 0 (if needed), then flips the size-prefix to
+  push max to the end. Inner `flip k` reverses prefix [0..k] using
+  two pointer-refs lo/hi. Inner `find_max k` walks 1..k tracking
+  the max-position. `[3;1;4;1;5;9;2;6]` sorts in 9 flips, sum
+  ends[1, 9] adds 10 → 9*100 + 10 = 910. Tests two inner functions
+  closing over the same Array, ref-based two-pointer flip, plus
+  downto loop. 77 baseline programs total.
 - 2026-05-09 Phase 5.1 — fib_mod.ml baseline (Fibonacci mod prime,
   fib(100) mod 1000003 = 391360). Iterative two-ref Fibonacci with
   modular reduction at every step to keep intermediate values