diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index c4ea6f4e..27ecf27b 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -7,6 +7,7 @@
   "factorial.ml": 3628800,
   "fizzbuzz.ml": 57,
   "list_ops.ml": 30,
+  "lambda_calc.ml": 7,
   "memo_fib.ml": 75025,
   "module_use.ml": 3,
   "mutable_record.ml": 10,
diff --git a/lib/ocaml/baseline/lambda_calc.ml b/lib/ocaml/baseline/lambda_calc.ml
new file mode 100644
index 00000000..7944be4e
--- /dev/null
+++ b/lib/ocaml/baseline/lambda_calc.ml
@@ -0,0 +1,41 @@
+(* Baseline: untyped lambda calculus with closures over a Hashtbl env *)
+type term =
+  | Var of string
+  | Abs of string * term
+  | App of term * term
+  | Num of int
+;;
+
+type value =
+  | VNum of int
+  | VClos of string * term * (string * value) list
+;;
+
+let rec lookup name env =
+  match env with
+  | [] -> failwith "unbound"
+  | (n, v) :: t -> if n = name then v else lookup name t
+;;
+
+let rec eval env t =
+  match t with
+  | Num n -> VNum n
+  | Var x -> lookup x env
+  | Abs (x, body) -> VClos (x, body, env)
+  | App (f, a) ->
+    let fv = eval env f in
+    let av = eval env a in
+    (match fv with
+     | VClos (param, body, captured) ->
+       eval ((param, av) :: captured) body
+     | _ -> failwith "not a function")
+;;
+
+let unwrap v = match v with VNum n -> n | _ -> failwith "not a number" ;;
+
+(* (\x. \y. x) 7 99 = 7 *)
+let term =
+  App (App (Abs ("x", Abs ("y", Var "x")), Num 7), Num 99)
+;;
+
+unwrap (eval [] term)
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index b404aad6..879a0226 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,12 @@ _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 — lambda_calc.ml baseline (17/17 pass). Untyped
+  lambda calculus interpreter using two ADTs (`type term = Var | Abs |
+  App | Num`, `type value = VNum | VClos`), an env as `(string * value)
+  list`, and recursive eval. `(\x.\y.x) 7 99 = 7` end-to-end. Demonstrates
+  the substrate handles a non-trivial AST + closure-based evaluator
+  written in OCaml-on-SX.
 - 2026-05-08 Phase 6 — Char predicates: is_digit/is_alpha/is_alnum/
   is_whitespace/is_upper/is_lower (+7 tests, 461 total). All written
   in OCaml in runtime.sx using Char.code + ASCII range checks.