diff --git a/lib/ocaml/baseline/atm.ml b/lib/ocaml/baseline/atm.ml
new file mode 100644
index 00000000..cf8b66e0
--- /dev/null
+++ b/lib/ocaml/baseline/atm.ml
@@ -0,0 +1,17 @@
+type account = { mutable balance : int }
+
+exception Insufficient
+
+let withdraw acct amt =
+  if amt > acct.balance then raise Insufficient
+  else acct.balance <- acct.balance - amt
+
+let deposit acct amt = acct.balance <- acct.balance + amt
+
+;;
+
+let a = { balance = 100 } in
+deposit a 50;
+withdraw a 30;
+try (withdraw a 200; -1)
+with Insufficient -> a.balance
diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index ccdfe1bb..1db31844 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -2,6 +2,7 @@
   "ackermann.ml": 125,
   "anagram_check.ml": 2,
   "anagrams.ml": 3,
+  "atm.ml": 120,
   "bag.ml": 3,
   "bf_full.ml": 6,
   "bigint_add.ml": 28,
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index b6486d70..accd5f67 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,15 @@ _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 — atm.ml baseline (mutable record + custom
+  exception + try/with, balance 120 after rollback). Models a bank
+  account: deposit/withdraw mutate the balance field; an over-draw
+  raises `Insufficient` which the caller catches and falls back to
+  the unchanged balance. Starting at 100, +50 = 150, -30 = 120,
+  attempted -200 throws and the handler returns 120. Combines
+  mutable record fields, user exception declaration, and
+  try-with-bare-pattern in a realistic micro-pattern. 59 baseline
+  programs total.
 - 2026-05-09 Phase 5.1 — bf_full.ml baseline (Brainfuck interpreter
   with `[`/`]` loops, `+++[.-]` → 3+2+1 = 6). Extends the iter-92
   brainfuck.ml subset with bracket matching: `[` jumps past matching