diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 8f60dd64..452170a2 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -79,6 +79,7 @@
   "shuffle.ml": 55,
   "simpson_int.ml": 10000,
   "stable_unique.ml": 46,
+  "subseq_check.ml": 3,
   "subset_sum.ml": 8,
   "tic_tac_toe.ml": 1,
   "word_freq.ml": 8,
diff --git a/lib/ocaml/baseline/subseq_check.ml b/lib/ocaml/baseline/subseq_check.ml
new file mode 100644
index 00000000..9ae39287
--- /dev/null
+++ b/lib/ocaml/baseline/subseq_check.ml
@@ -0,0 +1,18 @@
+let is_subseq s t =
+  let i = ref 0 in
+  let n = String.length s in
+  let m = String.length t in
+  let j = ref 0 in
+  while !i < n && !j < m do
+    if s.[!i] = t.[!j] then i := !i + 1;
+    j := !j + 1
+  done;
+  !i = n
+
+;;
+
+(if is_subseq "abc" "ahbgdc" then 1 else 0) +
+(if is_subseq "axc" "ahbgdc" then 1 else 0) +
+(if is_subseq "" "anything" then 1 else 0) +
+(if is_subseq "abc" "abc" then 1 else 0) +
+(if is_subseq "abcd" "abc" then 1 else 0)
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 3a66d964..efee2b01 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 — subseq_check.ml baseline (string is
+  subsequence?, 3/5 yes). Two-pointer walk: advance `i` only on
+  match, always advance `j`. Match if `i` reaches `n` (consumed
+  all of s). Five test cases:
+    abc in ahbgdc      yes
+    axc in ahbgdc      no  (no x in t)
+    "" in anything     yes (empty trivially)
+    abc in abc         yes
+    abcd in abc        no  (s longer)
+  Sum = 3. 93 baseline programs total.
 - 2026-05-09 Phase 5.1 — merge_intervals.ml baseline (LeetCode #56,
   total length 9 + 3 = 12). Sort by start, then sweep maintaining a
   current `(cs, ce)` window — extend `ce` if next start ≤ ce, else