diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 452170a2..64d27e66 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -44,6 +44,7 @@
   "luhn.ml": 2,
   "mat_mul.ml": 621,
   "max_path_tree.ml": 11,
+  "max_run.ml": 5,
   "mod_inverse.ml": 27,
   "json_pretty.ml": 24,
   "kadane.ml": 6,
diff --git a/lib/ocaml/baseline/max_run.ml b/lib/ocaml/baseline/max_run.ml
new file mode 100644
index 00000000..8e74b0d7
--- /dev/null
+++ b/lib/ocaml/baseline/max_run.ml
@@ -0,0 +1,16 @@
+let max_run xs =
+  let max_so_far = ref 0 in
+  let cur = ref 0 in
+  let last = ref None in
+  List.iter (fun x ->
+    (match !last with
+     | Some y when y = x -> cur := !cur + 1
+     | _ -> cur := 1);
+    last := Some x;
+    if !cur > !max_so_far then max_so_far := !cur
+  ) xs;
+  !max_so_far
+
+;;
+
+max_run [1;1;2;2;2;2;3;3;1;1;1] + max_run [1;2;3;4;5] + max_run []
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index efee2b01..2c5c9359 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 — max_run.ml baseline (longest consecutive
+  run, sum of three test cases = 4+1+0 = 5). Walks list with
+  `Some y when y = x` guard pattern in match for the prev-value
+  comparison; runs of equal elements increment cur, resets to 1
+  otherwise. Tests three inputs:
+    [1;1;2;2;2;2;3;3;1;1;1]   max run = 4 (the 2's)
+    [1;2;3;4;5]               max run = 1
+    []                         max run = 0
+  Sum = 5. Tests `when` guard in match arm + Option ref. 94 baseline
+  programs total.
 - 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