diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index ae92cffc..b86add6d 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -16,6 +16,7 @@
   "factorial.ml": 3628800,
   "fraction.ml": 7,
   "frequency.ml": 5,
+  "grep_count.ml": 3,
   "hanoi.ml": 1023,
   "fizzbuzz.ml": 57,
   "list_ops.ml": 30,
diff --git a/lib/ocaml/baseline/grep_count.ml b/lib/ocaml/baseline/grep_count.ml
new file mode 100644
index 00000000..7b341f57
--- /dev/null
+++ b/lib/ocaml/baseline/grep_count.ml
@@ -0,0 +1,17 @@
+let rec str_contains s sub i =
+  let nl = String.length s in
+  let sl = String.length sub in
+  if i + sl > nl then false
+  else if String.sub s i sl = sub then true
+  else str_contains s sub (i + 1)
+
+let count_matching needle text =
+  let lines = String.split_on_char '\n' text in
+  List.fold_left (fun acc line ->
+    if str_contains line needle 0 then acc + 1
+    else acc
+  ) 0 lines
+
+;;
+
+count_matching "fox" "the quick brown fox\nfox runs fast\nthe dog\nfoxes are clever"
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index f4747065..23cb7bd0 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 — grep_count.ml baseline (substring-aware
+  line filter, 3 lines match). Defines a recursive `str_contains`
+  that walks the haystack with `String.sub` slices to find a needle
+  substring (real OCaml's `String.contains` only takes a char).
+  Splits text on `'\n'` then folds with the contains predicate. Test
+  text has 4 lines, 3 contain 'fox' (incl 'foxes'). 43 baseline
+  programs total.
 - 2026-05-09 Phase 5.1 — pretty_table.ml baseline (Buffer + Printf
   width specifiers, total length 64). Builds a 4-row scoreboard via
   Buffer + `Printf.sprintf "%-10s %4d\n"`. Each row is exactly 16