diff --git a/lib/ocaml/baseline/bisect.ml b/lib/ocaml/baseline/bisect.ml
new file mode 100644
index 00000000..e7e5b9e6
--- /dev/null
+++ b/lib/ocaml/baseline/bisect.ml
@@ -0,0 +1,13 @@
+let bisect f lo hi =
+  let lo = ref lo and hi = ref hi in
+  for _ = 1 to 50 do
+    let mid = (!lo +. !hi) /. 2.0 in
+    if f mid = 0.0 || f !lo *. f mid < 0.0 then hi := mid
+    else lo := mid
+  done;
+  !lo
+
+;;
+
+let r = bisect (fun x -> x *. x -. 2.0) 1.0 2.0 in
+int_of_float (r *. 100.0)
diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 3678a818..9788d392 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -5,6 +5,7 @@
   "atm.ml": 120,
   "bag.ml": 3,
   "bf_full.ml": 6,
+  "bisect.ml": 141,
   "bigint_add.ml": 28,
   "bits.ml": 21,
   "balance.ml": 3,
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index bad1dfe8..acf3e929 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 — bisect.ml baseline (root-finding via
+  bisection, sqrt(2) * 100 = 141). 50 iterations of bisection
+  searching for x^2 - 2 = 0 in [1, 2]. Tests higher-order function
+  passing (the function-to-zero is `(fun x -> x *. x -. 2.0)`),
+  multi-let `let lo = ref ... and hi = ref ...`, float arithmetic,
+  and the int_of_float truncate-toward-zero from iteration 117. 62
+  baseline programs total.
 - 2026-05-09 Phase 5.1 — base_n.ml baseline (int -> base-N string,
   length sum 2+11+3+1 = 17). 36-character digit alphabet supports up
   to base 36. Loop divides quotient by base, prepends digit. Tests: