diff --git a/lib/ocaml/infer.sx b/lib/ocaml/infer.sx
index a5b83c18..212e80ee 100644
--- a/lib/ocaml/infer.sx
+++ b/lib/ocaml/infer.sx
@@ -171,6 +171,113 @@
          :type (ocaml-hm-tuple
                  (map (fn (t) (hm-apply subst t)) types))}))))
 
+;; Pattern type inference. Returns {:type T :env ENV2 :subst S} where
+;; ENV2 is the original env extended with any names the pattern binds.
+;; Constructor patterns aren't supported here yet (need a type-def
+;; registry) — :pcon falls through to a fresh tv so they don't break
+;; inference of mixed clauses.
+(define ocaml-infer-pat
+  (fn (pat env counter)
+    (let ((tag (nth pat 0)))
+      (cond
+        ((= tag "pwild")
+          (let ((tv (hm-fresh-tv counter)))
+            {:type tv :env env :subst {}}))
+        ((= tag "pvar")
+          (let ((nm (nth pat 1)) (tv (hm-fresh-tv counter)))
+            {:type tv :env (assoc env nm (hm-monotype tv)) :subst {}}))
+        ((= tag "plit")
+          (let ((r (ocaml-infer (nth pat 1) env counter)))
+            {:type (get r :type) :env env :subst (get r :subst)}))
+        ((= tag "pcons")
+          (let ((rh (ocaml-infer-pat (nth pat 1) env counter)))
+            (let ((rt (ocaml-infer-pat (nth pat 2) (get rh :env) counter)))
+              (let ((s (ocaml-hm-unify
+                         (ocaml-hm-list (get rh :type))
+                         (get rt :type)
+                         (hm-compose (get rt :subst) (get rh :subst)))))
+                {:type (hm-apply s (ocaml-hm-list (get rh :type)))
+                 :env (get rt :env)
+                 :subst s}))))
+        ((= tag "plist")
+          (let ((items (rest pat)) (tv (hm-fresh-tv counter)) (env-cur env) (subst {}))
+            (begin
+              (define loop
+                (fn (xs)
+                  (when (not (= xs (list)))
+                    (let ((rp (ocaml-infer-pat (first xs) env-cur counter)))
+                      (let ((s (ocaml-hm-unify
+                                 (hm-apply (get rp :subst) tv)
+                                 (get rp :type)
+                                 (hm-compose (get rp :subst) subst))))
+                        (begin
+                          (set! subst s)
+                          (set! env-cur (get rp :env))
+                          (loop (rest xs))))))))
+              (loop items)
+              {:type (hm-apply subst (ocaml-hm-list tv))
+               :env env-cur
+               :subst subst})))
+        ((= tag "ptuple")
+          (let ((items (rest pat)) (env-cur env) (subst {}) (types (list)))
+            (begin
+              (define loop
+                (fn (xs)
+                  (when (not (= xs (list)))
+                    (let ((rp (ocaml-infer-pat (first xs) env-cur counter)))
+                      (begin
+                        (set! subst (hm-compose (get rp :subst) subst))
+                        (append! types (get rp :type))
+                        (set! env-cur (get rp :env))
+                        (loop (rest xs)))))))
+              (loop items)
+              {:type (ocaml-hm-tuple
+                       (map (fn (t) (hm-apply subst t)) types))
+               :env env-cur
+               :subst subst})))
+        ((= tag "pas")
+          (let ((rp (ocaml-infer-pat (nth pat 1) env counter)))
+            (let ((alias (nth pat 2)))
+              {:type (get rp :type)
+               :env (assoc (get rp :env) alias (hm-monotype (get rp :type)))
+               :subst (get rp :subst)})))
+        (else
+          ;; :pcon and others — fall through to a fresh tv (sound but loose).
+          (let ((tv (hm-fresh-tv counter)))
+            {:type tv :env env :subst {}}))))))
+
+(define ocaml-infer-match
+  (fn (scrut clauses env counter)
+    (let ((rs (ocaml-infer scrut env counter)))
+      (let ((s (get rs :subst)) (st (get rs :type)) (result-tv (hm-fresh-tv counter)))
+        (let ((subst s))
+          (begin
+            (define loop
+              (fn (cs)
+                (when (not (= cs (list)))
+                  (let ((clause (first cs)))
+                    (let ((ctag (nth clause 0)))
+                      (let ((p (nth clause 1))
+                            (body (cond
+                                    ((= ctag "case") (nth clause 2))
+                                    (else (nth clause 3)))))
+                        (let ((rp (ocaml-infer-pat p (hm-apply-env subst env) counter)))
+                          (let ((s1 (ocaml-hm-unify
+                                      (hm-apply (get rp :subst) st)
+                                      (get rp :type)
+                                      (hm-compose (get rp :subst) subst))))
+                            (let ((rb (ocaml-infer body
+                                        (hm-apply-env s1 (get rp :env)) counter)))
+                              (let ((s2 (ocaml-hm-unify
+                                          (hm-apply (get rb :subst) result-tv)
+                                          (get rb :type)
+                                          (hm-compose (get rb :subst) s1))))
+                                (begin
+                                  (set! subst s2)
+                                  (loop (rest cs)))))))))))))
+            (loop clauses)
+            {:subst subst :type (hm-apply subst result-tv)}))))))
+
 (define ocaml-infer-list
   (fn (items env counter)
     (cond
@@ -214,6 +321,7 @@
                                           (nth expr 3) env counter))
         ((= tag "tuple")  (ocaml-infer-tuple (rest expr) env counter))
         ((= tag "list")   (ocaml-infer-list (rest expr) env counter))
+        ((= tag "match")  (ocaml-infer-match (nth expr 1) (nth expr 2) env counter))
         ((= tag "neg")
           (let ((r (ocaml-infer (nth expr 1) env counter)))
             (let ((s (get r :subst)) (t (get r :type)))
diff --git a/lib/ocaml/test.sh b/lib/ocaml/test.sh
index 03076b0b..552b33c9 100755
--- a/lib/ocaml/test.sh
+++ b/lib/ocaml/test.sh
@@ -844,6 +844,18 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 1706)
 (eval "(ocaml-run \"List.sort compare [\\\"b\\\"; \\\"a\\\"; \\\"c\\\"]\")")
 
+;; ── HM pattern-match inference ─────────────────────────────────
+(epoch 1800)
+(eval "(ocaml-type-of \"match 1 with | n -> n + 1\")")
+(epoch 1801)
+(eval "(ocaml-type-of \"match [1;2] with | [] -> 0 | h :: t -> h\")")
+(epoch 1802)
+(eval "(ocaml-type-of \"match (1, 2) with | (a, b) -> a + b\")")
+(epoch 1803)
+(eval "(ocaml-type-of \"fun x -> match x with | 0 -> 0 | n -> n + 1\")")
+(epoch 1804)
+(eval "(ocaml-type-of \"fun lst -> match lst with | [] -> 0 | h :: _ -> h\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1335,6 +1347,13 @@ check 1704 "List.sort descending"         '(4 3 1)'
 check 1705 "List.sort empty"              '()'
 check 1706 "List.sort strings"            '("a" "b" "c")'
 
+# ── HM match inference ──────────────────────────────────────────
+check 1800 "match int"                    '"Int"'
+check 1801 "match list"                   '"Int"'
+check 1802 "match tuple"                  '"Int"'
+check 1803 "fn match int -> int"          '"Int -> Int"'
+check 1804 "fn list -> elem"              '"Int list -> 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 5ef6223c..170c85e8 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -365,6 +365,14 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 5 — HM pattern-matching inference (+5 tests, 344
+  total). `ocaml-infer-pat` covers wild, var, lit, cons, list, tuple,
+  as. `ocaml-infer-match` unifies each clause's pattern type with the
+  scrutinee, runs the body in the env extended with pattern-bound vars,
+  and unifies all body types via a fresh result tv. Examples:
+  `fun lst -> match lst with | [] -> 0 | h :: _ -> h : Int list -> Int`.
+  Constructor patterns fall through to a fresh tv for now (need a ctor
+  type registry from `type` decls — pending).
 - 2026-05-08 Phase 6 — `List.sort` + polymorphic `compare` (+7 tests,
   339 total). `compare` is a host primitive that returns -1/0/1 like
   Stdlib.compare, defers to host SX `<`/`>`. `List.sort` is implemented