diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 35b76e82..26b0e10a 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -15,6 +15,7 @@
   "csv.ml": 10,
   "exception_handle.ml": 4,
   "expr_eval.ml": 16,
+  "expr_simp.ml": 22,
   "factorial.ml": 3628800,
   "fraction.ml": 7,
   "frequency.ml": 5,
diff --git a/lib/ocaml/baseline/expr_simp.ml b/lib/ocaml/baseline/expr_simp.ml
new file mode 100644
index 00000000..a68c65b6
--- /dev/null
+++ b/lib/ocaml/baseline/expr_simp.ml
@@ -0,0 +1,33 @@
+type expr =
+  | Num of int
+  | Add of expr * expr
+  | Mul of expr * expr
+
+let rec simp e =
+  match e with
+  | Num n -> Num n
+  | Add (a, b) ->
+    (match (simp a, simp b) with
+     | (Num 0, x) -> x
+     | (x, Num 0) -> x
+     | (Num n, Num m) -> Num (n + m)
+     | (a', b') -> Add (a', b'))
+  | Mul (a, b) ->
+    (match (simp a, simp b) with
+     | (Num 0, _) -> Num 0
+     | (_, Num 0) -> Num 0
+     | (Num 1, x) -> x
+     | (x, Num 1) -> x
+     | (Num n, Num m) -> Num (n * m)
+     | (a', b') -> Mul (a', b'))
+
+let rec eval e =
+  match e with
+  | Num n -> n
+  | Add (a, b) -> eval a + eval b
+  | Mul (a, b) -> eval a * eval b
+
+;;
+
+let e = Add (Mul (Num 3, Num 5), Add (Num 0, Mul (Num 1, Num 7))) in
+eval (simp e)
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 68e5a959..8c117cba 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 — expr_simp.ml baseline (symbolic expression
+  simplifier, eval (simp e) = 22). Recursive ADT with three
+  constructors (Num/Add/Mul). simp does bottom-up rewrite using
+  algebraic identities: x+0 → x, 0+x → x, x*0 → 0, 0*x → 0, x*1 → x,
+  1*x → x, constant folding both. Uses tuple pattern in nested match
+  (`match (simp a, simp b) with`). For `Add (Mul (Num 3, Num 5),
+  Add (Num 0, Mul (Num 1, Num 7)))` → simp → `Add (Num 15, Num 7)`
+  → eval → 22. 51 baseline programs total.
 - 2026-05-09 Phase 5.1 — mat_mul.ml baseline (3x3 row-major matrix
   multiply, sum of result = 621). Triple-nested for loop over
   i / j / k with row-major indexing `c.(i * n + j) <- c.(i * n + j) +