diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index b767248a..777c5e41 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -34,6 +34,7 @@
   "grid_paths.ml": 210,
   "group_consec.ml": 53,
   "hailstone.ml": 111,
+  "hamming.ml": 4,
   "hanoi.ml": 1023,
   "hist.ml": 75,
   "int_sqrt.ml": 1027,
diff --git a/lib/ocaml/baseline/hamming.ml b/lib/ocaml/baseline/hamming.ml
new file mode 100644
index 00000000..7d59106f
--- /dev/null
+++ b/lib/ocaml/baseline/hamming.ml
@@ -0,0 +1,13 @@
+let hamming s t =
+  if String.length s <> String.length t then -1
+  else begin
+    let d = ref 0 in
+    for i = 0 to String.length s - 1 do
+      if s.[i] <> t.[i] then d := !d + 1
+    done;
+    !d
+  end
+
+;;
+
+hamming "karolin" "kathrin" + hamming "1011101" "1001001" + hamming "abc" "abcd"
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 3e1b5c6f..6ef1d29a 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 — hamming.ml baseline (Hamming distance,
+  3 + 2 + (-1) = 4). Counts position-wise differences in equal-length
+  strings; returns -1 sentinel for length mismatch.
+    karolin vs kathrin   3 (positions 2,3,4)
+    1011101 vs 1001001   2 (positions 2,4)
+    abc     vs abcd     -1 (length mismatch)
+  Sum = 4. 91 baseline programs total.
 - 2026-05-09 Phase 5.1 — xor_cipher.ml baseline (XOR roll-key
   encryption, round-trip → 601). For each character, XOR with the
   corresponding key char (key cycled via `i mod kn`). Encrypts