diff --git a/lib/ocaml/runtime.sx b/lib/ocaml/runtime.sx
index 5afd020f..97d1835a 100644
--- a/lib/ocaml/runtime.sx
+++ b/lib/ocaml/runtime.sx
@@ -435,6 +435,32 @@
          match m with
          | [] -> 0
          | _ :: t -> 1 + cardinal t
+
+       let rec iter f m =
+         match m with
+         | [] -> ()
+         | (k, v) :: t -> f k v; iter f t
+
+       let rec fold f m acc =
+         match m with
+         | [] -> acc
+         | (k, v) :: t -> fold f t (f k v acc)
+
+       let rec map f m =
+         match m with
+         | [] -> []
+         | (k, v) :: t -> (k, f v) :: map f t
+
+       let rec filter p m =
+         match m with
+         | [] -> []
+         | (k, v) :: t ->
+           if p k v then (k, v) :: filter p t else filter p t
+
+       let rec is_empty m =
+         match m with
+         | [] -> true
+         | _ -> false
      end
    end ;;
 
@@ -472,6 +498,36 @@
          match s with
          | [] -> 0
          | _ :: t -> 1 + cardinal t
+
+       let rec iter f s =
+         match s with
+         | [] -> ()
+         | h :: t -> f h; iter f t
+
+       let rec fold f s acc =
+         match s with
+         | [] -> acc
+         | h :: t -> fold f t (f h acc)
+
+       let rec filter p s =
+         match s with
+         | [] -> []
+         | h :: t -> if p h then h :: filter p t else filter p t
+
+       let rec is_empty s =
+         match s with
+         | [] -> true
+         | _ -> false
+
+       let rec union a b =
+         match b with
+         | [] -> a
+         | h :: t -> union (add h a) t
+
+       let rec inter a b =
+         match a with
+         | [] -> []
+         | h :: t -> if mem h b then h :: inter t b else inter t b
      end
    end")
 
diff --git a/lib/ocaml/test.sh b/lib/ocaml/test.sh
index 0c2fdd5d..0a0e75ba 100755
--- a/lib/ocaml/test.sh
+++ b/lib/ocaml/test.sh
@@ -1034,6 +1034,16 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 3303)
 (eval "(ocaml-run-program \"module IntOrd = struct let compare a b = compare a b end ;; module IntSet = Set.Make(IntOrd) ;; IntSet.mem 2 (IntSet.add 3 (IntSet.add 1 (IntSet.add 2 IntSet.empty)))\")")
 
+;; ── Map/Set fold/iter/filter/union/inter ──────────────────────
+(epoch 3400)
+(eval "(ocaml-run-program \"module IntOrd = struct let compare a b = compare a b end ;; module IntMap = Map.Make(IntOrd) ;; let m = IntMap.add 1 10 (IntMap.add 2 20 IntMap.empty) ;; IntMap.fold (fun k v acc -> acc + v) m 0\")")
+(epoch 3401)
+(eval "(ocaml-run-program \"module IntOrd = struct let compare a b = compare a b end ;; module IntMap = Map.Make(IntOrd) ;; let m = IntMap.add 1 10 IntMap.empty ;; IntMap.is_empty m\")")
+(epoch 3402)
+(eval "(ocaml-run-program \"module IntOrd = struct let compare a b = compare a b end ;; module IntSet = Set.Make(IntOrd) ;; let a = IntSet.add 1 (IntSet.add 2 IntSet.empty) ;; let b = IntSet.add 2 (IntSet.add 3 IntSet.empty) ;; IntSet.elements (IntSet.union a b)\")")
+(epoch 3403)
+(eval "(ocaml-run-program \"module IntOrd = struct let compare a b = compare a b end ;; module IntSet = Set.Make(IntOrd) ;; let a = IntSet.add 1 (IntSet.add 2 (IntSet.add 3 IntSet.empty)) ;; let b = IntSet.add 2 (IntSet.add 3 (IntSet.add 4 IntSet.empty)) ;; IntSet.elements (IntSet.inter a b)\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1636,6 +1646,12 @@ check 3301 "Map.cardinal"                 '2'
 check 3302 "Set.elements sorted"          '(1 2 3)'
 check 3303 "Set.mem"                      'true'
 
+# ── Map/Set fold/iter/filter/union/inter ───────────────────────
+check 3400 "Map.fold sum"                 '30'
+check 3401 "Map.is_empty false"           'false'
+check 3402 "Set.union"                    '(1 2 3)'
+check 3403 "Set.inter"                    '(2 3)'
+
 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 d009b9d7..bc10a8fb 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -386,6 +386,9 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 6 — Map/Set extensions: iter/fold/map/filter/
+  is_empty + Set.union/inter (+4 tests, 422 total). Functor
+  bodies grow naturally — all in OCaml syntax.
 - 2026-05-08 Phase 6 — `Map.Make` / `Set.Make` functors written in
   OCaml (+4 tests, 418 total). Sorted association list / sorted list
   backed (linear ops, but correct). Both take an `Ord` module supplying