diff --git a/hosts/javascript/platform.py b/hosts/javascript/platform.py
index cfec88e3..8a3bb406 100644
--- a/hosts/javascript/platform.py
+++ b/hosts/javascript/platform.py
@@ -1702,6 +1702,60 @@ PRIMITIVES_JS_MODULES: dict[str, str] = {
     src.data.forEach(function(v, k) { dst.data.set(k, v); });
     return null;
   };
+''',
+    "stdlib.sets": '''
+  // stdlib.sets — structural sets keyed by write-to-string serialization
+  function SxSet() { this.data = new Map(); this._sxset = true; }
+  SxSet.prototype._type = "set";
+  function sxSetKey(v) { return sxWriteVal(v, "write"); }
+  function sxSetSeed(s, lst) {
+    if (Array.isArray(lst)) lst.forEach(function(v) { s.data.set(sxSetKey(v), v); });
+    return s;
+  }
+  PRIMITIVES["make-set"] = function() {
+    var s = new SxSet();
+    if (arguments.length > 0 && Array.isArray(arguments[0])) sxSetSeed(s, arguments[0]);
+    return s;
+  };
+  PRIMITIVES["set?"] = function(v) { return v instanceof SxSet; };
+  PRIMITIVES["set-add!"] = function(s, v) { s.data.set(sxSetKey(v), v); return NIL; };
+  PRIMITIVES["set-member?"] = function(s, v) { return s.data.has(sxSetKey(v)); };
+  PRIMITIVES["set-remove!"] = function(s, v) { s.data.delete(sxSetKey(v)); return NIL; };
+  PRIMITIVES["set-size"] = function(s) { return s.data.size; };
+  PRIMITIVES["set->list"] = function(s) { return Array.from(s.data.values()); };
+  PRIMITIVES["list->set"] = function(lst) {
+    var s = new SxSet();
+    if (Array.isArray(lst)) lst.forEach(function(v) { s.data.set(sxSetKey(v), v); });
+    return s;
+  };
+  PRIMITIVES["set-union"] = function(a, b) {
+    var s = new SxSet();
+    a.data.forEach(function(v, k) { s.data.set(k, v); });
+    b.data.forEach(function(v, k) { s.data.set(k, v); });
+    return s;
+  };
+  PRIMITIVES["set-intersection"] = function(a, b) {
+    var s = new SxSet();
+    a.data.forEach(function(v, k) { if (b.data.has(k)) s.data.set(k, v); });
+    return s;
+  };
+  PRIMITIVES["set-difference"] = function(a, b) {
+    var s = new SxSet();
+    a.data.forEach(function(v, k) { if (!b.data.has(k)) s.data.set(k, v); });
+    return s;
+  };
+  PRIMITIVES["set-for-each"] = function(s, fn) {
+    s.data.forEach(function(v) { apply(fn, [v]); });
+    return NIL;
+  };
+  PRIMITIVES["set-map"] = function(s, fn) {
+    var out = new SxSet();
+    s.data.forEach(function(v) {
+      var r = apply(fn, [v]);
+      out.data.set(sxSetKey(r), r);
+    });
+    return out;
+  };
 ''',
 }
 # Modules to include by default (all)
@@ -1747,6 +1801,7 @@ PLATFORM_JS_PRE = '''
     if (x._vector) return "vector";
     if (x._string_buffer) return "string-buffer";
     if (x._hash_table) return "hash-table";
+    if (x._sxset) return "set";
     if (x._rational) return "rational";
     if (typeof Node !== "undefined" && x instanceof Node) return "dom-node";
     if (Array.isArray(x)) return "list";
diff --git a/hosts/ocaml/lib/sx_primitives.ml b/hosts/ocaml/lib/sx_primitives.ml
index e0ba4d37..b7d8dfea 100644
--- a/hosts/ocaml/lib/sx_primitives.ml
+++ b/hosts/ocaml/lib/sx_primitives.ml
@@ -2687,4 +2687,87 @@ let () =
     | _ -> raise (Eval_error "display-to-string: 1 arg"));
   register "current-input-port"  (fun _ -> Nil);
   register "current-output-port" (fun _ -> Nil);
-  register "current-error-port"  (fun _ -> Nil)
+  register "current-error-port"  (fun _ -> Nil);
+  (* ---- Sets ---- *)
+  let set_key v = Sx_types.inspect v in
+  register "make-set" (fun args ->
+    let ht = Hashtbl.create 8 in
+    (match args with
+     | [] -> ()
+     | [List items] -> List.iter (fun v -> Hashtbl.replace ht (set_key v) v) items
+     | [ListRef r] -> List.iter (fun v -> Hashtbl.replace ht (set_key v) v) !r
+     | _ -> raise (Eval_error "make-set: expected optional list"));
+    SxSet ht);
+  register "set?" (fun args ->
+    match args with
+    | [SxSet _] -> Bool true
+    | [_] -> Bool false
+    | _ -> raise (Eval_error "set?: 1 arg"));
+  register "set-add!" (fun args ->
+    match args with
+    | [SxSet ht; v] -> Hashtbl.replace ht (set_key v) v; Nil
+    | _ -> raise (Eval_error "set-add!: expected set val"));
+  register "set-member?" (fun args ->
+    match args with
+    | [SxSet ht; v] -> Bool (Hashtbl.mem ht (set_key v))
+    | _ -> raise (Eval_error "set-member?: expected set val"));
+  register "set-remove!" (fun args ->
+    match args with
+    | [SxSet ht; v] -> Hashtbl.remove ht (set_key v); Nil
+    | _ -> raise (Eval_error "set-remove!: expected set val"));
+  register "set-size" (fun args ->
+    match args with
+    | [SxSet ht] -> Integer (Hashtbl.length ht)
+    | _ -> raise (Eval_error "set-size: expected set"));
+  register "set->list" (fun args ->
+    match args with
+    | [SxSet ht] -> List (Hashtbl.fold (fun _ v acc -> v :: acc) ht [])
+    | _ -> raise (Eval_error "set->list: expected set"));
+  register "list->set" (fun args ->
+    match args with
+    | [List items] ->
+      let ht = Hashtbl.create (List.length items) in
+      List.iter (fun v -> Hashtbl.replace ht (set_key v) v) items;
+      SxSet ht
+    | [ListRef r] ->
+      let ht = Hashtbl.create (List.length !r) in
+      List.iter (fun v -> Hashtbl.replace ht (set_key v) v) !r;
+      SxSet ht
+    | [Nil] -> SxSet (Hashtbl.create 0)
+    | _ -> raise (Eval_error "list->set: expected list"));
+  register "set-union" (fun args ->
+    match args with
+    | [SxSet a; SxSet b] ->
+      let ht = Hashtbl.copy a in
+      Hashtbl.iter (fun k v -> Hashtbl.replace ht k v) b;
+      SxSet ht
+    | _ -> raise (Eval_error "set-union: expected 2 sets"));
+  register "set-intersection" (fun args ->
+    match args with
+    | [SxSet a; SxSet b] ->
+      let ht = Hashtbl.create 8 in
+      Hashtbl.iter (fun k v -> if Hashtbl.mem b k then Hashtbl.replace ht k v) a;
+      SxSet ht
+    | _ -> raise (Eval_error "set-intersection: expected 2 sets"));
+  register "set-difference" (fun args ->
+    match args with
+    | [SxSet a; SxSet b] ->
+      let ht = Hashtbl.create 8 in
+      Hashtbl.iter (fun k v -> if not (Hashtbl.mem b k) then Hashtbl.replace ht k v) a;
+      SxSet ht
+    | _ -> raise (Eval_error "set-difference: expected 2 sets"));
+  register "set-for-each" (fun args ->
+    match args with
+    | [SxSet ht; fn] ->
+      Hashtbl.iter (fun _ v -> ignore (!Sx_types._cek_call_ref fn (List [v]))) ht;
+      Nil
+    | _ -> raise (Eval_error "set-for-each: expected set fn"));
+  register "set-map" (fun args ->
+    match args with
+    | [SxSet ht; fn] ->
+      let out = Hashtbl.create (Hashtbl.length ht) in
+      Hashtbl.iter (fun _ v ->
+        let r = !Sx_types._cek_call_ref fn (List [v]) in
+        Hashtbl.replace out (set_key r) r) ht;
+      SxSet out
+    | _ -> raise (Eval_error "set-map: expected set fn"))
diff --git a/hosts/ocaml/lib/sx_types.ml b/hosts/ocaml/lib/sx_types.ml
index df3c1070..5f4f3ccd 100644
--- a/hosts/ocaml/lib/sx_types.ml
+++ b/hosts/ocaml/lib/sx_types.ml
@@ -79,6 +79,7 @@ and value =
   | Eof            (** EOF sentinel — returned by read-char etc. at end of input. *)
   | Port of sx_port  (** String port — input (string cursor) or output (buffer). *)
   | Rational of int * int  (** Exact rational: numerator, denominator (reduced, denom>0). *)
+  | SxSet of (string, value) Hashtbl.t  (** Mutable set keyed by inspect(value). *)
 
 (** String input port: source string + mutable cursor position. *)
 and sx_port_kind =
@@ -514,6 +515,7 @@ let type_of = function
   | Port { sp_kind = PortInput _; _ }  -> "input-port"
   | Port { sp_kind = PortOutput _; _ } -> "output-port"
   | Rational _  -> "rational"
+  | SxSet _     -> "set"
 
 let is_nil = function Nil -> true | _ -> false
 let is_lambda = function Lambda _ -> true | _ -> false
@@ -876,3 +878,4 @@ let rec inspect = function
   | Port { sp_kind = PortOutput buf; sp_closed } ->
     Printf.sprintf "<output-port:len=%d%s>" (Buffer.length buf) (if sp_closed then ":closed" else "")
   | Rational (n, d) -> Printf.sprintf "%d/%d" n d
+  | SxSet ht -> Printf.sprintf "<set:%d>" (Hashtbl.length ht)
diff --git a/shared/static/scripts/sx-browser.js b/shared/static/scripts/sx-browser.js
index 222e7065..40929353 100644
--- a/shared/static/scripts/sx-browser.js
+++ b/shared/static/scripts/sx-browser.js
@@ -41,7 +41,7 @@
   // =========================================================================
 
   var NIL = Object.freeze({ _nil: true, toString: function() { return "nil"; } });
-  var SX_VERSION = "2026-05-01T18:13:58Z";
+  var SX_VERSION = "2026-05-01T18:42:40Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }
@@ -184,6 +184,7 @@
     if (x._vector) return "vector";
     if (x._string_buffer) return "string-buffer";
     if (x._hash_table) return "hash-table";
+    if (x._sxset) return "set";
     if (x._rational) return "rational";
     if (typeof Node !== "undefined" && x instanceof Node) return "dom-node";
     if (Array.isArray(x)) return "list";
@@ -1096,6 +1097,60 @@
   };
 
 
+  // stdlib.sets — structural sets keyed by write-to-string serialization
+  function SxSet() { this.data = new Map(); this._sxset = true; }
+  SxSet.prototype._type = "set";
+  function sxSetKey(v) { return sxWriteVal(v, "write"); }
+  function sxSetSeed(s, lst) {
+    if (Array.isArray(lst)) lst.forEach(function(v) { s.data.set(sxSetKey(v), v); });
+    return s;
+  }
+  PRIMITIVES["make-set"] = function() {
+    var s = new SxSet();
+    if (arguments.length > 0 && Array.isArray(arguments[0])) sxSetSeed(s, arguments[0]);
+    return s;
+  };
+  PRIMITIVES["set?"] = function(v) { return v instanceof SxSet; };
+  PRIMITIVES["set-add!"] = function(s, v) { s.data.set(sxSetKey(v), v); return NIL; };
+  PRIMITIVES["set-member?"] = function(s, v) { return s.data.has(sxSetKey(v)); };
+  PRIMITIVES["set-remove!"] = function(s, v) { s.data.delete(sxSetKey(v)); return NIL; };
+  PRIMITIVES["set-size"] = function(s) { return s.data.size; };
+  PRIMITIVES["set->list"] = function(s) { return Array.from(s.data.values()); };
+  PRIMITIVES["list->set"] = function(lst) {
+    var s = new SxSet();
+    if (Array.isArray(lst)) lst.forEach(function(v) { s.data.set(sxSetKey(v), v); });
+    return s;
+  };
+  PRIMITIVES["set-union"] = function(a, b) {
+    var s = new SxSet();
+    a.data.forEach(function(v, k) { s.data.set(k, v); });
+    b.data.forEach(function(v, k) { s.data.set(k, v); });
+    return s;
+  };
+  PRIMITIVES["set-intersection"] = function(a, b) {
+    var s = new SxSet();
+    a.data.forEach(function(v, k) { if (b.data.has(k)) s.data.set(k, v); });
+    return s;
+  };
+  PRIMITIVES["set-difference"] = function(a, b) {
+    var s = new SxSet();
+    a.data.forEach(function(v, k) { if (!b.data.has(k)) s.data.set(k, v); });
+    return s;
+  };
+  PRIMITIVES["set-for-each"] = function(s, fn) {
+    s.data.forEach(function(v) { apply(fn, [v]); });
+    return NIL;
+  };
+  PRIMITIVES["set-map"] = function(s, fn) {
+    var out = new SxSet();
+    s.data.forEach(function(v) {
+      var r = apply(fn, [v]);
+      out.data.set(sxSetKey(r), r);
+    });
+    return out;
+  };
+
+
   function isPrimitive(name) { return name in PRIMITIVES; }
   function getPrimitive(name) { return PRIMITIVES[name]; }
 
diff --git a/spec/primitives.sx b/spec/primitives.sx
index 8122565f..698396b6 100644
--- a/spec/primitives.sx
+++ b/spec/primitives.sx
@@ -1116,3 +1116,83 @@
   :doc "Denominator of rational r (after reduction, always positive).")
 
 (define-module :stdlib.hash-table)
+
+(define-module :stdlib.sets)
+
+(define-primitive
+  "make-set"
+  :params (&rest (lst :as list))
+  :returns "set"
+  :doc "Create a fresh empty set. Optional list argument seeds the set: (make-set '(1 2 3)).")
+
+(define-primitive
+  "set?"
+  :params (v)
+  :returns "boolean"
+  :doc "True if v is a set.")
+
+(define-primitive
+  "set-add!"
+  :params (s val)
+  :returns "nil"
+  :doc "Add val to set s in place. No-op if already present.")
+
+(define-primitive
+  "set-member?"
+  :params (s val)
+  :returns "boolean"
+  :doc "True if val is in set s.")
+
+(define-primitive
+  "set-remove!"
+  :params (s val)
+  :returns "nil"
+  :doc "Remove val from set s in place. No-op if absent.")
+
+(define-primitive
+  "set-size"
+  :params (s)
+  :returns "integer"
+  :doc "Number of elements in set s.")
+
+(define-primitive
+  "set->list"
+  :params (s)
+  :returns "list"
+  :doc "All elements of set s as a list (unspecified order).")
+
+(define-primitive
+  "list->set"
+  :params (lst)
+  :returns "set"
+  :doc "Create a new set containing all elements of lst.")
+
+(define-primitive
+  "set-union"
+  :params (s1 s2)
+  :returns "set"
+  :doc "New set with all elements from s1 and s2.")
+
+(define-primitive
+  "set-intersection"
+  :params (s1 s2)
+  :returns "set"
+  :doc "New set with elements present in both s1 and s2.")
+
+(define-primitive
+  "set-difference"
+  :params (s1 s2)
+  :returns "set"
+  :doc "New set with elements in s1 that are not in s2.")
+
+(define-primitive
+  "set-for-each"
+  :params (s fn)
+  :returns "nil"
+  :doc "Call (fn val) for each element in set s. Order unspecified.")
+
+(define-primitive
+  "set-map"
+  :params (s fn)
+  :returns "set"
+  :doc "New set of results of (fn val) for each element in s.")
diff --git a/spec/tests/test-sets.sx b/spec/tests/test-sets.sx
new file mode 100644
index 00000000..d4e3bd7c
--- /dev/null
+++ b/spec/tests/test-sets.sx
@@ -0,0 +1,200 @@
+;; ==========================================================================
+;; test-sets.sx — Tests for set primitives
+;; ==========================================================================
+
+;; --------------------------------------------------------------------------
+;; make-set / set?
+;; --------------------------------------------------------------------------
+
+(defsuite
+  "sets:create"
+  (deftest "make-set returns a set" (assert (set? (make-set))))
+  (deftest "empty set has size 0" (assert= (set-size (make-set)) 0))
+  (deftest
+    "make-set from list"
+    (let ((s (make-set (list 1 2 3)))) (assert= (set-size s) 3)))
+  (deftest
+    "make-set deduplicates"
+    (let ((s (make-set (list 1 2 2 3 3)))) (assert= (set-size s) 3)))
+  (deftest "set? true for sets" (assert (set? (make-set))))
+  (deftest "set? false for list" (assert (not (set? (list 1 2 3)))))
+  (deftest "set? false for nil" (assert (not (set? nil))))
+  (deftest "set? false for number" (assert (not (set? 42)))))
+
+;; --------------------------------------------------------------------------
+;; set-add! / set-member? / set-remove!
+;; --------------------------------------------------------------------------
+
+(defsuite
+  "sets:mutation"
+  (deftest
+    "set-add! increases size"
+    (let
+      ((s (make-set)))
+      (set-add! s 1)
+      (assert= (set-size s) 1)))
+  (deftest
+    "set-add! idempotent"
+    (let
+      ((s (make-set)))
+      (set-add! s 1)
+      (set-add! s 1)
+      (assert= (set-size s) 1)))
+  (deftest
+    "set-member? true after add"
+    (let
+      ((s (make-set)))
+      (set-add! s "hello")
+      (assert (set-member? s "hello"))))
+  (deftest
+    "set-member? false for absent"
+    (let
+      ((s (make-set (list 1 2 3))))
+      (assert (not (set-member? s 99)))))
+  (deftest
+    "set-remove! reduces size"
+    (let
+      ((s (make-set (list 1 2 3))))
+      (set-remove! s 2)
+      (assert= (set-size s) 2)))
+  (deftest
+    "set-remove! removes element"
+    (let
+      ((s (make-set (list 1 2 3))))
+      (set-remove! s 2)
+      (assert (not (set-member? s 2)))))
+  (deftest
+    "set-remove! no-op for absent"
+    (let
+      ((s (make-set (list 1 2 3))))
+      (set-remove! s 99)
+      (assert= (set-size s) 3)))
+  (deftest
+    "set handles strings"
+    (let
+      ((s (make-set)))
+      (set-add! s "a")
+      (set-add! s "b")
+      (assert (and (set-member? s "a") (set-member? s "b")))))
+  (deftest
+    "set handles symbols"
+    (let
+      ((s (make-set)))
+      (set-add! s (quote foo))
+      (assert (set-member? s (quote foo))))))
+
+;; --------------------------------------------------------------------------
+;; set->list / list->set
+;; --------------------------------------------------------------------------
+
+(defsuite
+  "sets:conversion"
+  (deftest
+    "list->set creates set"
+    (let ((s (list->set (list 1 2 3)))) (assert (set? s))))
+  (deftest
+    "list->set size"
+    (let ((s (list->set (list 1 2 3)))) (assert= (set-size s) 3)))
+  (deftest
+    "list->set deduplicates"
+    (let ((s (list->set (list 1 1 2)))) (assert= (set-size s) 2)))
+  (deftest
+    "set->list has all elements"
+    (let
+      ((s (make-set (list 1 2 3)))
+       (lst (set->list s)))
+      (assert= (length lst) 3)))
+  (deftest
+    "set->list round-trip membership"
+    (let
+      ((s (make-set (list 10 20 30)))
+       (lst (set->list s)))
+      (assert
+        (and
+          (set-member? (list->set lst) 10)
+          (set-member? (list->set lst) 20)
+          (set-member? (list->set lst) 30))))))
+
+;; --------------------------------------------------------------------------
+;; set-union / set-intersection / set-difference
+;; --------------------------------------------------------------------------
+
+(defsuite
+  "sets:operations"
+  (deftest
+    "union size"
+    (let
+      ((a (make-set (list 1 2 3)))
+       (b (make-set (list 3 4 5))))
+      (assert= (set-size (set-union a b)) 5)))
+  (deftest
+    "union contains all"
+    (let
+      ((u (set-union (make-set (list 1 2)) (make-set (list 3 4)))))
+      (assert
+        (and
+          (set-member? u 1)
+          (set-member? u 3)
+          (set-member? u 4)))))
+  (deftest
+    "intersection size"
+    (let
+      ((a (make-set (list 1 2 3)))
+       (b (make-set (list 2 3 4))))
+      (assert= (set-size (set-intersection a b)) 2)))
+  (deftest
+    "intersection contains overlap"
+    (let
+      ((i (set-intersection (make-set (list 1 2 3)) (make-set (list 2 3 4)))))
+      (assert (and (set-member? i 2) (set-member? i 3) (not (set-member? i 1))))))
+  (deftest
+    "intersection empty when disjoint"
+    (let
+      ((a (make-set (list 1 2)))
+       (b (make-set (list 3 4))))
+      (assert= (set-size (set-intersection a b)) 0)))
+  (deftest
+    "difference size"
+    (let
+      ((a (make-set (list 1 2 3)))
+       (b (make-set (list 2 3))))
+      (assert= (set-size (set-difference a b)) 1)))
+  (deftest
+    "difference keeps only a-exclusive"
+    (let
+      ((d (set-difference (make-set (list 1 2 3)) (make-set (list 2 3 4)))))
+      (assert (and (set-member? d 1) (not (set-member? d 2)) (not (set-member? d 4))))))
+  (deftest
+    "union does not mutate inputs"
+    (let
+      ((a (make-set (list 1 2)))
+       (b (make-set (list 3 4))))
+      (set-union a b)
+      (assert= (set-size a) 2))))
+
+;; --------------------------------------------------------------------------
+;; set-for-each / set-map
+;; --------------------------------------------------------------------------
+
+(defsuite
+  "sets:higher-order"
+  (deftest
+    "set-for-each visits all"
+    (let
+      ((s (make-set (list 1 2 3)))
+       (acc (list)))
+      (set-for-each s (fn (v) (set! acc (cons v acc))))
+      (assert= (length acc) 3)))
+  (deftest
+    "set-map doubles values"
+    (let
+      ((s (make-set (list 1 2 3)))
+       (s2 (set-map s (fn (v) (* v 2)))))
+      (assert
+        (and
+          (set-member? s2 2)
+          (set-member? s2 4)
+          (set-member? s2 6)))))
+  (deftest
+    "set-map result is a set"
+    (assert (set? (set-map (make-set (list 1 2)) (fn (v) v))))))