diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index c2530159..7da9235c 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -85,6 +85,7 @@
   "fizzbuzz.ml": 57,
   "flatten_tree.ml": 28,
   "flood_fill.ml": 7,
+  "floyd_cycle.ml": 8,
   "floyd_warshall.ml": 9,
   "lis.ml": 6,
   "list_ops.ml": 30,
diff --git a/lib/ocaml/baseline/floyd_cycle.ml b/lib/ocaml/baseline/floyd_cycle.ml
new file mode 100644
index 00000000..60ecec9f
--- /dev/null
+++ b/lib/ocaml/baseline/floyd_cycle.ml
@@ -0,0 +1,29 @@
+let f x = (x * 2 + 5) mod 17
+
+let floyd_cycle x0 =
+  let slow = ref x0 in
+  let fast = ref x0 in
+  let meet = ref false in
+  while not !meet do
+    slow := f !slow;
+    fast := f (f !fast);
+    if !slow = !fast then meet := true
+  done;
+  slow := x0;
+  let mu = ref 0 in
+  while !slow <> !fast do
+    slow := f !slow;
+    fast := f !fast;
+    mu := !mu + 1
+  done;
+  let lam = ref 1 in
+  fast := f !slow;
+  while !slow <> !fast do
+    fast := f !fast;
+    lam := !lam + 1
+  done;
+  !mu * 100 + !lam
+
+;;
+
+floyd_cycle 1
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index fe1ccc6b..3548ce41 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,17 @@ _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 — floyd_cycle.ml baseline (Floyd's tortoise
+  -and-hare cycle detection on f(x) = (2x+5) mod 17; μ=0 λ=8 →
+  encoded 8). Three phases: (1) advance slow/fast until they
+  collide inside the cycle, (2) restart slow from x0 and advance
+  both by 1 until they meet — the count is μ (tail length),
+  (3) advance fast around the cycle once until it meets slow — the
+  count is λ (cycle length). For x0=1, f(1)=7, f(7)=2, … cycles
+  through all 8 distinct values 1,7,2,9,6,0,5,15 before returning
+  to 1, so μ=0 λ=8 → 0*100+8=8. Tests three sequential while
+  loops sharing ref state, double-step `fast := f (f !fast)`,
+  meeting-condition flag pattern. 185 baseline programs total.
 - 2026-05-11 Phase 5.1 — kth_two.ml baseline (8th smallest in
   merged [1;3;5;7;9;11;13] ∪ [2;4;6;8;10;12] = 8). Two-pointer
   merge that advances the smaller-head pointer k times. Pick rule: