diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 3453f55b..367541d2 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -35,5 +35,6 @@
   "sieve.ml": 15,
   "sum_squares.ml": 385,
   "unique_set.ml": 9,
+  "validate.ml": 417,
   "word_count.ml": 3
 }
diff --git a/lib/ocaml/baseline/validate.ml b/lib/ocaml/baseline/validate.ml
new file mode 100644
index 00000000..5065bbcb
--- /dev/null
+++ b/lib/ocaml/baseline/validate.ml
@@ -0,0 +1,24 @@
+let validate_int s =
+  if String.length s = 0 then Left "empty"
+  else
+    let rec all_digits i =
+      if i >= String.length s then true
+      else
+        let c = s.[i] in
+        if c >= '0' && c <= '9' then all_digits (i + 1)
+        else false
+    in
+    if all_digits 0 then Right (int_of_string s)
+    else Left ("not a number: " ^ s)
+
+let process inputs =
+  List.fold_left (fun (errs, vals) s ->
+    match validate_int s with
+    | Left _ -> (errs + 1, vals)
+    | Right v -> (errs, vals + v)
+  ) (0, 0) inputs
+
+;;
+
+let (errs, sum) = process ["12"; "abc"; "5"; ""; "100"; "x"] in
+errs * 100 + sum
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 4f7654f3..7ede6105 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 — validate.ml baseline (Either-based input
+  validation, 3 errors × 100 + 117 sum = 417). validate_int returns
+  `Left msg` on empty / non-digit, `Right (int_of_string s)` on a
+  digit-only string. process folds inputs with a tuple accumulator
+  `(errs, sum)`, branching on the result. ["12"; "abc"; "5"; "";
+  "100"; "x"] → (3, 117) → 417. Exercises Either constructors used
+  bare (no qualification), char range comparison, tuple-pattern
+  destructuring on let-binding, recursive helper inside if-else. 36
+  baseline programs total.
 - 2026-05-09 Phase 5.1 — word_freq.ml baseline (Map.Make on String,
   count distinct words → 8). Defines a StringOrd module + applies
   Map.Make to it. Folds the input through SMap.find_opt + SMap.add to