diff --git a/shared/static/scripts/sx-browser.js b/shared/static/scripts/sx-browser.js
index dbc2a17..398f705 100644
--- a/shared/static/scripts/sx-browser.js
+++ b/shared/static/scripts/sx-browser.js
@@ -14,7 +14,7 @@
   // =========================================================================
 
   var NIL = Object.freeze({ _nil: true, toString: function() { return "nil"; } });
-  var SX_VERSION = "2026-03-07T00:04:07Z";
+  var SX_VERSION = "2026-03-07T00:20:00Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }
@@ -177,7 +177,8 @@
   function envExtend(env) { return merge(env); }
   function envMerge(base, overlay) { return merge(base, overlay); }
 
-  function dictSet(d, k, v) { d[k] = v; }
+  function dictSet(d, k, v) { d[k] = v; return v; }
+  PRIMITIVES["dict-set!"] = dictSet;
   function dictGet(d, k) { var v = d[k]; return v !== undefined ? v : NIL; }
 
   // Render-expression detection — lets the evaluator delegate to the active adapter.
@@ -452,6 +453,7 @@
   var range = PRIMITIVES["range"];
   function zip(a, b) { var r = []; for (var i = 0; i < Math.min(a.length, b.length); i++) r.push([a[i], b[i]]); return r; }
   function append_b(arr, x) { arr.push(x); return arr; }
+  PRIMITIVES["append!"] = append_b;
   var apply = function(f, args) { return f.apply(null, args); };
 
   // Additional primitive aliases used by adapter/engine transpiled code
diff --git a/shared/sx/primitives.py b/shared/sx/primitives.py
index cfc6d3d..cbd356d 100644
--- a/shared/sx/primitives.py
+++ b/shared/sx/primitives.py
@@ -386,6 +386,11 @@ def prim_cons(x: Any, coll: Any) -> list:
 def prim_append(coll: Any, x: Any) -> list:
     return list(coll) + [x] if coll else [x]
 
+@register_primitive("append!")
+def prim_append_mut(coll: Any, x: Any) -> list:
+    coll.append(x)
+    return coll
+
 @register_primitive("chunk-every")
 def prim_chunk_every(coll: Any, n: Any) -> list:
     n = int(n)
@@ -439,6 +444,13 @@ def prim_dissoc(d: Any, *keys_to_remove: Any) -> dict:
         result.pop(key, None)
     return result
 
+@register_primitive("dict-set!")
+def prim_dict_set_mut(d: Any, key: Any, val: Any) -> Any:
+    if isinstance(key, Keyword):
+        key = key.name
+    d[key] = val
+    return val
+
 @register_primitive("into")
 def prim_into(target: Any, coll: Any) -> Any:
     if isinstance(target, list):
diff --git a/shared/sx/ref/bootstrap_js.py b/shared/sx/ref/bootstrap_js.py
index e1704ea..739b9a3 100644
--- a/shared/sx/ref/bootstrap_js.py
+++ b/shared/sx/ref/bootstrap_js.py
@@ -1711,7 +1711,8 @@ PLATFORM_JS_PRE = '''
   function envExtend(env) { return merge(env); }
   function envMerge(base, overlay) { return merge(base, overlay); }
 
-  function dictSet(d, k, v) { d[k] = v; }
+  function dictSet(d, k, v) { d[k] = v; return v; }
+  PRIMITIVES["dict-set!"] = dictSet;
   function dictGet(d, k) { var v = d[k]; return v !== undefined ? v : NIL; }
 
   // Render-expression detection — lets the evaluator delegate to the active adapter.
@@ -1791,6 +1792,7 @@ PLATFORM_JS_POST = '''
   var range = PRIMITIVES["range"];
   function zip(a, b) { var r = []; for (var i = 0; i < Math.min(a.length, b.length); i++) r.push([a[i], b[i]]); return r; }
   function append_b(arr, x) { arr.push(x); return arr; }
+  PRIMITIVES["append!"] = append_b;
   var apply = function(f, args) { return f.apply(null, args); };
 
   // Additional primitive aliases used by adapter/engine transpiled code
diff --git a/shared/sx/ref/primitives.sx b/shared/sx/ref/primitives.sx
index 91e2904..6035a66 100644
--- a/shared/sx/ref/primitives.sx
+++ b/shared/sx/ref/primitives.sx
@@ -384,6 +384,11 @@
   :returns "list"
   :doc "Append x to end of coll (returns new list).")
 
+(define-primitive "append!"
+  :params (coll x)
+  :returns "list"
+  :doc "Mutate coll by appending x in-place. Returns coll.")
+
 (define-primitive "chunk-every"
   :params (coll n)
   :returns "list"
@@ -426,6 +431,11 @@
   :returns "dict"
   :doc "Return new dict with keys removed.")
 
+(define-primitive "dict-set!"
+  :params (d key val)
+  :returns "any"
+  :doc "Mutate dict d by setting key to val in-place. Returns val.")
+
 (define-primitive "into"
   :params (target coll)
   :returns "any"
diff --git a/shared/sx/tests/test_page_data.py b/shared/sx/tests/test_page_data.py
index d67d0ad..7cd2280 100644
--- a/shared/sx/tests/test_page_data.py
+++ b/shared/sx/tests/test_page_data.py
@@ -305,16 +305,6 @@ class TestDataCache:
         self._time = current_time_ms
         env["now-ms"] = lambda: self._time
 
-        # Mutating primitives needed by cache (available in JS, not bare Python)
-        def _dict_set(d, k, v):
-            d[k] = v
-            return v
-        def _append_b(lst, item):
-            lst.append(item)
-            return lst
-        env["dict-set!"] = _dict_set
-        env["append!"] = _append_b
-
         # Define the cache functions from orchestration.sx
         cache_src = """
         (define _page-data-cache (dict))