diff --git a/lib/ocaml/infer.sx b/lib/ocaml/infer.sx
index 211c880f..c07bf1ef 100644
--- a/lib/ocaml/infer.sx
+++ b/lib/ocaml/infer.sx
@@ -228,8 +228,8 @@
 (define ocaml-infer-pcon
   (fn (name arg-pats env counter)
     (cond
-      ((has-key? ocaml-hm-ctors name)
-        (let ((ctor-type (hm-instantiate (get ocaml-hm-ctors name) counter))
+      ((ocaml-hm-ctor-has? name)
+        (let ((ctor-type (hm-instantiate (ocaml-hm-ctor-lookup name) counter))
               (env-cur env) (subst {}))
           (let ((cur-type (list nil)))
             (begin
@@ -388,21 +388,60 @@
             {:subst subst
              :type (ocaml-hm-list (hm-apply subst elem-tv))}))))))
 
-(define ocaml-hm-ctors (ocaml-hm-ctor-env))
+;; Mutable cell so user `type` declarations can extend the registry.
+(define ocaml-hm-ctors (list (ocaml-hm-ctor-env)))
+
+(define ocaml-hm-ctor-lookup
+  (fn (name) (get (nth ocaml-hm-ctors 0) name)))
+
+(define ocaml-hm-ctor-has?
+  (fn (name) (has-key? (nth ocaml-hm-ctors 0) name)))
+
+(define ocaml-hm-ctor-register!
+  (fn (name scheme)
+    (set-nth! ocaml-hm-ctors 0
+      (merge (nth ocaml-hm-ctors 0) (dict name scheme)))))
+
+;; Process a :type-def AST. For each ctor, build its scheme:
+;;   nullary `A` → scheme [] (con NAME [param-tvs...])
+;;   ctor `B of int` → scheme [] (int -> (con NAME [...]))
+;; Argument types are ignored for now (they're raw source strings) — we
+;; assume `int`. A future iteration parses arg types properly.
+(define ocaml-hm-register-type-def!
+  (fn (type-def)
+    (let ((name (nth type-def 1))
+          (params (nth type-def 2))
+          (ctors (nth type-def 3)))
+      (let ((param-tvs (map hm-tv params)))
+        (let ((self-type (hm-con name param-tvs)))
+          (begin
+            (define register-ctor
+              (fn (ctor)
+                (let ((cname (first ctor))
+                      (arg-srcs (rest ctor)))
+                  (cond
+                    ((= (len arg-srcs) 0)
+                      (ocaml-hm-ctor-register! cname
+                        (hm-scheme params self-type)))
+                    (else
+                      ;; Single-arg ctor with arg type `int` (placeholder).
+                      ;; Multi-arg or other-typed ctors fall back to int.
+                      (ocaml-hm-ctor-register! cname
+                        (hm-scheme params
+                          (hm-arrow (hm-int) self-type))))))))
+            (for-each register-ctor ctors)))))))
 
 (set! ocaml-infer
   (fn (expr env counter)
     (let ((tag (nth expr 0)))
       (cond
         ((= tag "con")
-          ;; (:con NAME) — look up constructor type, instantiate fresh.
           (let ((name (nth expr 1)))
             (cond
-              ((has-key? ocaml-hm-ctors name)
+              ((ocaml-hm-ctor-has? name)
                 {:subst {}
-                 :type (hm-instantiate (get ocaml-hm-ctors name) counter)})
+                 :type (hm-instantiate (ocaml-hm-ctor-lookup name) counter)})
               (else
-                ;; Unknown ctor — treat as a fresh polymorphic type.
                 {:subst {} :type (hm-fresh-tv counter)}))))
         ((= tag "int")    {:subst {} :type (hm-int)})
         ((= tag "float")  {:subst {} :type (hm-int)})  ;; treat float as int for now
@@ -448,6 +487,61 @@
       (let ((r (ocaml-infer expr env counter)))
         (ocaml-hm-format-type (hm-apply (get r :subst) (get r :type)))))))
 
+;; Program-level type inference: process decls in order, registering
+;; type-defs with the ctor registry, threading let-bindings into the
+;; env, and returning the type of the last expression-level form.
+(define ocaml-type-of-program
+  (fn (src)
+    (let ((prog (ocaml-parse-program src))
+          (env (ocaml-hm-builtin-env))
+          (counter (ocaml-hm-counter))
+          (last-type (hm-tv "?")))
+      (begin
+        (define run-decl
+          (fn (decl)
+            (let ((tag (nth decl 0)))
+              (cond
+                ((= tag "type-def") (ocaml-hm-register-type-def! decl))
+                ((= tag "exception-def") nil)
+                ((= tag "def")
+                  (let ((nm (nth decl 1)) (ps (nth decl 2)) (rh (nth decl 3)))
+                    (let ((rhs-expr (cond
+                                      ((= (len ps) 0) rh)
+                                      (else (list :fun ps rh)))))
+                      (let ((r (ocaml-infer rhs-expr env counter)))
+                        (let ((s (get r :subst)) (t (get r :type)))
+                          (let ((env2 (hm-apply-env s env)))
+                            (let ((scheme (hm-generalize t env2)))
+                              (begin
+                                (set! env (assoc env2 nm scheme))
+                                (set! last-type t)))))))))
+                ((= tag "def-rec")
+                  (let ((nm (nth decl 1)) (ps (nth decl 2)) (rh (nth decl 3)))
+                    (let ((rec-tv (hm-fresh-tv counter)))
+                      (let ((env-rec (assoc env nm (hm-monotype rec-tv)))
+                            (rhs-expr (cond
+                                        ((= (len ps) 0) rh)
+                                        (else (list :fun ps rh)))))
+                        (let ((r (ocaml-infer rhs-expr env-rec counter)))
+                          (let ((s (get r :subst)) (t (get r :type)))
+                            (let ((s2 (ocaml-hm-unify (hm-apply s rec-tv) t s)))
+                              (let ((env2 (hm-apply-env s2 env)))
+                                (let ((scheme (hm-generalize (hm-apply s2 t) env2)))
+                                  (begin
+                                    (set! env (assoc env2 nm scheme))
+                                    (set! last-type t)))))))))))
+                ((= tag "expr")
+                  (let ((r (ocaml-infer (nth decl 1) env counter)))
+                    (set! last-type
+                      (hm-apply (get r :subst) (get r :type)))))
+                (else nil)))))
+        (define loop
+          (fn (xs)
+            (when (not (= xs (list)))
+              (begin (run-decl (first xs)) (loop (rest xs))))))
+        (loop (rest prog))
+        (ocaml-hm-format-type last-type)))))
+
 ;; Pretty-print a type as an OCaml-style string for testing. Only handles
 ;; the constructors we use: Int / Bool / String / Unit / -> / type-vars.
 (define ocaml-hm-format-type
diff --git a/lib/ocaml/test.sh b/lib/ocaml/test.sh
index 7ebdec78..4a6ad7b0 100755
--- a/lib/ocaml/test.sh
+++ b/lib/ocaml/test.sh
@@ -886,6 +886,20 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 2005)
 (eval "(ocaml-type-of \"let rec sum lst = match lst with | [] -> 0 | h :: t -> h + sum t in sum [1; 2; 3]\")")
 
+;; ── HM with user type declarations ─────────────────────────────
+(epoch 2100)
+(eval "(ocaml-type-of-program \"type color = Red | Green | Blue;; Red\")")
+(epoch 2101)
+(eval "(ocaml-type-of-program \"type shape = Circle of int | Square of int;; Circle 5\")")
+(epoch 2102)
+(eval "(ocaml-type-of-program \"type color = Red | Green;; let c = Red;; c\")")
+(epoch 2103)
+(eval "(ocaml-type-of-program \"type shape = Circle of int | Square of int;; let area s = match s with | Circle r -> r * r | Square s -> s * s;; area\")")
+(epoch 2104)
+(eval "(ocaml-type-of-program \"let x = 1;; let y = 2;; x + y\")")
+(epoch 2105)
+(eval "(ocaml-type-of-program \"let rec fact n = if n = 0 then 1 else n * fact (n - 1);; fact 5\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1401,6 +1415,14 @@ check 2003 "type 1::list"                 '"Int list"'
 check 2004 "type [1] @ [2;3]"             '"Int list"'
 check 2005 "type sum"                     '"Int"'
 
+# ── HM with user type-defs ──────────────────────────────────────
+check 2100 "user type Red : color"        '"color"'
+check 2101 "user type Circle 5 : shape"   '"shape"'
+check 2102 "let c = Red; c"               '"color"'
+check 2103 "shape -> Int"                 '"shape -> Int"'
+check 2104 "program x+y"                  '"Int"'
+check 2105 "program fact 5"               '"Int"'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index ec4b930b..328791eb 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -227,10 +227,12 @@ SX CEK evaluator (both JS and OCaml hosts)
       pattern matching, let-rec, modules.)_
 - [x] Type variables: `'a`, `'b`; unification with occur-check (kit).
 - [x] Let-polymorphism: generalise at let-bindings (kit `hm-generalize`).
-- [~] ADT types: `option`/`result` ctors hardcoded in
-      `ocaml-hm-ctor-env`; `:con NAME` and `:pcon NAME …` look up the
-      scheme and instantiate. User type-defs would extend the registry.
-      Format: `Int option`, `(Int, 'b) result`.
+- [x] ADT types: `option`/`result` ctors seeded;
+      `ocaml-hm-register-type-def!` registers user types from `:type-def`.
+      `ocaml-type-of-program` threads decls through the env, registering
+      types and binding `let` schemes. `:con NAME` / `:pcon NAME …`
+      instantiate from the registry. _(Caveat: ctor arg types currently
+      default to `int` — proper type parsing pending.)_
 - [~] Function types `T1 -> T2` work; tuples (`'a * 'b`) and lists
       (`'a list`) supported. Records pending.
 - [ ] Type signatures: `val f : int -> int` — verify against inferred type.
@@ -368,6 +370,18 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 5 — HM with user `type` declarations (+6 tests, 363
+  total). `ocaml-hm-ctors` is now a mutable list cell; user type-defs
+  register their constructors via `ocaml-hm-register-type-def!`. New
+  `ocaml-type-of-program` processes top-level decls in order: type-def
+  registers ctors, def/def-rec generalize, exception-def is a no-op,
+  expr returns its inferred type. Examples:
+    `type color = Red | Green | Blue;; Red : color`
+    `type shape = Circle of int | Square of int;; let area s = match s
+     with | Circle r -> r * r | Square s -> s * s;; area : shape -> Int`
+  Caveat: ctor arg types parsed as raw source strings; the registry
+  defaults to `int` for any single-arg ctor. Proper type-source parsing
+  is pending.
 - 2026-05-08 Phase 5 — HM let-rec inference + `::`/`@` operator types
   (+6 tests, 357 total). `ocaml-infer-let-rec` pre-binds the function
   name to a fresh tv, infers rhs (which can recursively call name),