diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 167d68c1..4d328b0d 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -1,6 +1,7 @@
 {
   "closures.ml": 315,
   "exception_handle.ml": 4,
+  "expr_eval.ml": 16,
   "factorial.ml": 3628800,
   "list_ops.ml": 30,
   "module_use.ml": 3,
diff --git a/lib/ocaml/baseline/expr_eval.ml b/lib/ocaml/baseline/expr_eval.ml
new file mode 100644
index 00000000..59ee0e43
--- /dev/null
+++ b/lib/ocaml/baseline/expr_eval.ml
@@ -0,0 +1,19 @@
+(* Baseline: a tiny expression evaluator using ADTs + match *)
+type expr =
+  | Lit of int
+  | Add of expr * expr
+  | Mul of expr * expr
+  | Neg of expr
+;;
+
+let rec eval e =
+  match e with
+  | Lit n -> n
+  | Add (a, b) -> eval a + eval b
+  | Mul (a, b) -> eval a * eval b
+  | Neg x -> 0 - eval x
+;;
+
+(* (1 + 2) * (3 + 4) - 5 = 21 - 5 = 16 *)
+eval
+  (Add (Mul (Add (Lit 1, Lit 2), Add (Lit 3, Lit 4)), Neg (Lit 5)))
diff --git a/lib/ocaml/baseline/run.sh b/lib/ocaml/baseline/run.sh
index 6e62e688..8d5e75e7 100755
--- a/lib/ocaml/baseline/run.sh
+++ b/lib/ocaml/baseline/run.sh
@@ -36,7 +36,7 @@ for f in lib/ocaml/baseline/*.ml; do
 (eval "(ocaml-run-program (file-read \"$f\"))")
 EOF
 
-  output=$(timeout 60 "$SX_SERVER" < "$TMP" 2>/dev/null)
+  output=$(timeout 120 "$SX_SERVER" < "$TMP" 2>/dev/null)
   rm -f "$TMP"
 
   result=$(echo "$output" | awk '
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 44e30220..9e5ba3b5 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -377,6 +377,11 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 5.1 — expr_eval.ml baseline (9/9 pass). A tiny
+  arithmetic-expression evaluator using ADT (`type expr = Lit | Add |
+  Mul | Neg`) + recursive eval + pattern match — exercises the full
+  type-decl + ctor + match pipeline end-to-end. Per-program timeout
+  bumped to 120s in run.sh.
 - 2026-05-08 Phase 3 — polymorphic variants `` `Tag `` (+4 tests, 382
   total). Tokenizer recognises backtick followed by an upper ident,
   tokenizing identically to nominal ctors. Parser and evaluator treat