diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 47733ffd..9e36ca4a 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -23,6 +23,7 @@
   "module_use.ml": 3,
   "mutable_record.ml": 10,
   "option_match.ml": 5,
+  "pi_leibniz.ml": 314,
   "poly_stack.ml": 5,
   "queens.ml": 2,
   "quicksort.ml": 44,
diff --git a/lib/ocaml/baseline/pi_leibniz.ml b/lib/ocaml/baseline/pi_leibniz.ml
new file mode 100644
index 00000000..343acfe0
--- /dev/null
+++ b/lib/ocaml/baseline/pi_leibniz.ml
@@ -0,0 +1,10 @@
+let pi_approx n =
+  let total = ref 0.0 in
+  for k = 0 to n - 1 do
+    let sign = if k mod 2 = 0 then 1.0 else -1.0 in
+    total := !total +. sign /. float_of_int (2 * k + 1)
+  done;
+  4.0 *. !total
+;;
+
+int_of_float (pi_approx 1000 *. 100.0)
diff --git a/lib/ocaml/runtime.sx b/lib/ocaml/runtime.sx
index 6f233907..d734a29d 100644
--- a/lib/ocaml/runtime.sx
+++ b/lib/ocaml/runtime.sx
@@ -1180,7 +1180,8 @@
    let min_int = -4611686018427387904
    let abs_float f = if f < 0.0 then 0.0 -. f else f
    let float_of_int n = n
-   let int_of_float f = f
+   let int_of_float f =
+     if f < 0.0 then _float_ceil f else _float_floor f
    ")
 
 (define ocaml-stdlib-loaded false)
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index da8230e0..5c8df858 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,14 @@ _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 — pi_leibniz.ml baseline (Leibniz formula,
+  1000 terms × 100 → 314). Side-quest: `int_of_float` was wrong —
+  defined as identity in iteration 94 instead of truncation. Fixed
+  to `if f < 0.0 then ceil else floor` (truncate toward zero, real
+  OCaml semantics). Float.to_int still uses floor since OCaml's
+  documentation says "result is unspecified if the argument is nan
+  or falls outside the int range" — close enough for our scope. 32
+  baseline programs total.
 - 2026-05-09 Phase 6 — Float module fleshed out (+6 tests, 598
   total). New Float members: zero, one, minus_one, abs, neg, add,
   sub, mul, div, max, min, equal, compare, to_int, of_int,