diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index d5b441f4..7bf2e72e 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -8,6 +8,7 @@
   "fizzbuzz.ml": 57,
   "list_ops.ml": 30,
   "module_use.ml": 3,
+  "mutable_record.ml": 10,
   "option_match.ml": 5,
   "quicksort.ml": 44,
   "sum_squares.ml": 385,
diff --git a/lib/ocaml/baseline/mutable_record.ml b/lib/ocaml/baseline/mutable_record.ml
new file mode 100644
index 00000000..41fb5302
--- /dev/null
+++ b/lib/ocaml/baseline/mutable_record.ml
@@ -0,0 +1,15 @@
+(* Baseline: mutable record fields via r.f <- v *)
+type counter = { mutable count : int; mutable last : int } ;;
+
+let bump c =
+  c.count <- c.count + 1 ;
+  c.last <- c.count
+;;
+
+let c = { count = 0; last = 0 } ;;
+bump c ;;
+bump c ;;
+bump c ;;
+bump c ;;
+bump c ;;
+c.count + c.last
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index d8d9ac78..41d171f2 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,10 @@ _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.
+- 2026-05-08 Phase 5.1 — mutable_record.ml baseline (14/14 pass).
+  Counter-style record with two mutable fields, bump function uses
+  `r.f <- v` to mutate. End-to-end validates type decl + record
+  literal + field access + field assignment + sequence operator.
 - 2026-05-08 Phase 2 — mutable record fields `r.f <- v` (+4 tests, 451
   total). `<-` added to op-table at level 1 (same as `:=`). Eval
   short-circuits on `<-` to mutate the lhs's field via host SX