diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 03ab6707..e338cc68 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -105,6 +105,7 @@
   "poly_stack.ml": 5,
   "pow_mod.ml": 738639,
   "prime_factors.ml": 17,
+  "pythagorean.ml": 16,
   "queens.ml": 2,
   "quicksort.ml": 44,
   "roman.ml": 44,
diff --git a/lib/ocaml/baseline/pythagorean.ml b/lib/ocaml/baseline/pythagorean.ml
new file mode 100644
index 00000000..3710c0c2
--- /dev/null
+++ b/lib/ocaml/baseline/pythagorean.ml
@@ -0,0 +1,19 @@
+let rec gcd a b = if b = 0 then a else gcd b (a mod b)
+
+let count_primitive_triples n =
+  let c = ref 0 in
+  for m = 2 to 50 do
+    let kk = ref 1 in
+    while !kk < m do
+      if (m - !kk) mod 2 = 1 && gcd m !kk = 1 then begin
+        let h = m * m + !kk * !kk in
+        if h <= n then c := !c + 1
+      end;
+      kk := !kk + 1
+    done
+  done;
+  !c
+
+;;
+
+count_primitive_triples 100
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index e121528e..ef2b5f7b 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.
+- 2026-05-10 Phase 5.1 — pythagorean.ml baseline (count primitive
+  Pythagorean triples with hypotenuse ≤ 100 = 16). Uses Euclid's
+  formula: for coprime m > k of opposite parity, the triple
+  (m² - k², 2mk, m² + k²) is primitive Pythagorean. The 16 triples
+  are (3,4,5), (5,12,13), (8,15,17), (7,24,25), (20,21,29),
+  (9,40,41), ..., (65,72,97). 134 baseline programs total.
 - 2026-05-10 Phase 5.1 — harshad.ml baseline (count Niven/Harshad
   numbers ≤ 100 = 33). A Harshad number is divisible by its digit
   sum. All single-digit numbers qualify trivially (9). Plus 10, 12,