diff --git a/lib/ocaml/baseline/bs_rotated.ml b/lib/ocaml/baseline/bs_rotated.ml
new file mode 100644
index 00000000..87978aeb
--- /dev/null
+++ b/lib/ocaml/baseline/bs_rotated.ml
@@ -0,0 +1,25 @@
+let bs_rotated arr target =
+  let lo = ref 0 in
+  let hi = ref (Array.length arr - 1) in
+  let result = ref (-1) in
+  while !lo <= !hi && !result = -1 do
+    let mid = (!lo + !hi) / 2 in
+    if arr.(mid) = target then result := mid
+    else if arr.(!lo) <= arr.(mid) then begin
+      if target >= arr.(!lo) && target < arr.(mid) then
+        hi := mid - 1
+      else
+        lo := mid + 1
+    end else begin
+      if target > arr.(mid) && target <= arr.(!hi) then
+        lo := mid + 1
+      else
+        hi := mid - 1
+    end
+  done;
+  !result
+
+;;
+
+let a = [| 4; 5; 6; 7; 0; 1; 2 |] in
+bs_rotated a 0 + bs_rotated a 7 * 10 + bs_rotated a 3 * 100
diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 85242cde..a0d601ac 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -23,6 +23,7 @@
   "btree.ml": 39,
   "brainfuck.ml": 75,
   "bs_bounds.ml": 3211,
+  "bs_rotated.ml": -66,
   "bsearch.ml": 7,
   "caesar.ml": 215,
   "calc.ml": 13,
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 5135c066..ae9dfdc7 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.
+- 2026-05-11 Phase 5.1 — bs_rotated.ml baseline (binary search in
+  rotated sorted array; encoded result -66). For [4;5;6;7;0;1;2]:
+  - search 0 → index 4
+  - search 7 → index 3
+  - search 3 → −1 (not present)
+  Encoded: 4 + 3*10 + (-1)*100 = -66. Each step decides which half
+  is sorted by comparing arr[lo] vs arr[mid], then checks whether
+  the target falls in that sorted half. First baseline with a
+  negative top-level result; test runner uses literal `grep -qF`
+  so the leading minus is fine. 196 baseline programs total.
 - 2026-05-11 Phase 5.1 — task_scheduler.ml baseline (task cooldown
   formula, "AAABBC" with n=2 → 7 intervals). Counts each letter,
   finds max frequency `m` and the number of letters that hit that