diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 953cb99a..b7f41169 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -35,6 +35,7 @@
   "fizzbuzz.ml": 57,
   "flatten_tree.ml": 28,
   "list_ops.ml": 30,
+  "luhn.ml": 2,
   "mat_mul.ml": 621,
   "max_path_tree.ml": 11,
   "mod_inverse.ml": 27,
diff --git a/lib/ocaml/baseline/luhn.ml b/lib/ocaml/baseline/luhn.ml
new file mode 100644
index 00000000..21f3ce72
--- /dev/null
+++ b/lib/ocaml/baseline/luhn.ml
@@ -0,0 +1,20 @@
+let luhn s =
+  let n = String.length s in
+  let total = ref 0 in
+  for i = 0 to n - 1 do
+    let d = Char.code s.[n - 1 - i] - Char.code '0' in
+    let v = if i mod 2 = 1 then
+      let dd = d * 2 in
+      if dd > 9 then dd - 9 else dd
+    else d
+    in
+    total := !total + v
+  done;
+  !total mod 10 = 0
+
+;;
+
+(if luhn "79927398713" then 1 else 0)
++ (if luhn "79927398710" then 1 else 0)
++ (if luhn "4532015112830366" then 1 else 0)
++ (if luhn "1234567890123456" then 1 else 0)
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 0e36da8e..f29b437d 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,16 @@ _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 — luhn.ml baseline (Luhn check digit, 2/4
+  inputs valid). Walks digits right-to-left, doubles every other
+  starting from the second-from-right; if doubled value > 9
+  subtract 9. Sum must be divisible by 10. Tests:
+    79927398713      ✓ valid
+    79927398710      ✗
+    4532015112830366 ✓ valid (real Visa test number)
+    1234567890123456 ✗
+  Sum = 2. Tests right-to-left index walk + Char.code '0' arithmetic
+  + nested if-then-else. 75 baseline programs total.
 - 2026-05-09 Phase 5.1 — triangle.ml baseline (Pascal-shape min path
   sum, 2+3+5+1 = 11). Bottom-up DP over the triangle:
     2