diff --git a/shared/static/scripts/sx-browser.js b/shared/static/scripts/sx-browser.js
index 7f18587..511b10a 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 = "2026-03-19T11:12:01Z";
+  var SX_VERSION = "2026-03-19T12:40:33Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }
@@ -2679,7 +2679,7 @@ PRIMITIVES["aser"] = aser;
   return (isSxTruthy((name == "<>")) ? aserFragment(args, env) : (isSxTruthy((name == "raw!")) ? aserCall("raw!", args, env) : (isSxTruthy(startsWith(name, "~")) ? (function() {
   var comp = (isSxTruthy(envHas(env, name)) ? envGet(env, name) : NIL);
   var expandAll = (isSxTruthy(envHas(env, "expand-components?")) ? expandComponents_p() : false);
-  return (isSxTruthy((isSxTruthy(comp) && isMacro(comp))) ? aser(expandMacro(comp, args, env), env) : (isSxTruthy((isSxTruthy(comp) && isSxTruthy(isComponent(comp)) && isSxTruthy(sxOr(expandAll, (componentAffinity(comp) == "server"))) && !isSxTruthy((componentAffinity(comp) == "client")))) ? aserExpandComponent(comp, args, env) : aserCall(name, args, env)));
+  return (isSxTruthy((isSxTruthy(comp) && isMacro(comp))) ? aser(expandMacro(comp, args, env), env) : (isSxTruthy((isSxTruthy(comp) && isSxTruthy(isComponent(comp)) && isSxTruthy(!isSxTruthy(isIsland(comp))) && isSxTruthy(sxOr(expandAll, (componentAffinity(comp) == "server"))) && !isSxTruthy((componentAffinity(comp) == "client")))) ? aserExpandComponent(comp, args, env) : aserCall(name, args, env)));
 })() : (isSxTruthy((name == "lake")) ? aserCall(name, args, env) : (isSxTruthy((name == "marsh")) ? aserCall(name, args, env) : (isSxTruthy(contains(HTML_TAGS, name)) ? aserCall(name, args, env) : (isSxTruthy(sxOr(isSpecialForm(name), isHoForm(name))) ? aserSpecial(name, expr, env) : (isSxTruthy((isSxTruthy(envHas(env, name)) && isMacro(envGet(env, name)))) ? aser(expandMacro(envGet(env, name), args, env), env) : (function() {
   var f = trampoline(evalExpr(head, env));
   var evaledArgs = map(function(a) { return trampoline(evalExpr(a, env)); }, args);
diff --git a/shared/sx/tests/test_ocaml_helpers.py b/shared/sx/tests/test_ocaml_helpers.py
new file mode 100644
index 0000000..987f37f
--- /dev/null
+++ b/shared/sx/tests/test_ocaml_helpers.py
@@ -0,0 +1,355 @@
+"""Tests for OCaml kernel ↔ Python page helper IO bridge.
+
+Verifies that:
+1. Helper injection registers functions in the OCaml kernel
+2. The kernel can call helpers via (io-request "helper" ...)
+3. aser_slot expands components that use helpers
+4. Caching eliminates redundant IO round-trips
+
+Usage:
+    pytest shared/sx/tests/test_ocaml_helpers.py -v
+"""
+
+import asyncio
+import os
+import sys
+import time
+import unittest
+
+_project_root = os.path.abspath(os.path.join(os.path.dirname(__file__), "../../.."))
+if _project_root not in sys.path:
+    sys.path.insert(0, _project_root)
+
+from shared.sx.ocaml_bridge import OcamlBridge, OcamlBridgeError, _DEFAULT_BIN, _escape
+
+
+def _skip_if_no_binary():
+    bin_path = os.path.abspath(_DEFAULT_BIN)
+    if not os.path.isfile(bin_path):
+        raise unittest.SkipTest(
+            f"OCaml binary not found at {bin_path}. "
+            f"Build with: cd hosts/ocaml && eval $(opam env) && dune build"
+        )
+
+
+class TestHelperInjection(unittest.IsolatedAsyncioTestCase):
+    """Test that page helpers can be injected into the OCaml kernel."""
+
+    @classmethod
+    def setUpClass(cls):
+        _skip_if_no_binary()
+
+    async def asyncSetUp(self):
+        self.bridge = OcamlBridge()
+        await self.bridge.start()
+        # Load spec + adapter (needed for aser)
+        spec_dir = os.path.join(_project_root, "spec")
+        web_dir = os.path.join(_project_root, "web")
+        for f in ["parser.sx", "render.sx"]:
+            path = os.path.join(spec_dir, f)
+            if os.path.isfile(path):
+                async with self.bridge._lock:
+                    self.bridge._send(f'(load "{_escape(path)}")')
+                    await self.bridge._read_until_ok(ctx=None)
+        adapter = os.path.join(web_dir, "adapter-sx.sx")
+        if os.path.isfile(adapter):
+            async with self.bridge._lock:
+                self.bridge._send(f'(load "{_escape(adapter)}")')
+                await self.bridge._read_until_ok(ctx=None)
+
+    async def asyncTearDown(self):
+        await self.bridge.stop()
+
+    async def _inject_test_helper(self, name: str, nargs: int):
+        """Inject a single helper proxy into the kernel."""
+        param_names = " ".join(chr(97 + i) for i in range(nargs))
+        arg_list = " ".join(chr(97 + i) for i in range(nargs))
+        sx_def = f'(define {name} (fn ({param_names}) (helper "{name}" {arg_list})))'
+        async with self.bridge._lock:
+            self.bridge._send(f'(load-source "{_escape(sx_def)}")')
+            await self.bridge._read_until_ok(ctx=None)
+
+    async def test_helper_call_returns_value(self):
+        """Injected helper can be called and returns IO result."""
+        # The "helper" native binding is already in the kernel.
+        # Define a test helper that calls (helper "json-encode" value)
+        await self._inject_test_helper("json-encode", 1)
+
+        # Call it via eval — should yield io-request, Python dispatches
+        result = await self.bridge.eval(
+            '(json-encode "hello")',
+            ctx={"_helper_service": "sx"}
+        )
+        self.assertIn("hello", result)
+
+    async def test_helper_with_two_args(self):
+        """Helper with 2 args works (e.g. highlight pattern)."""
+        # Define a 2-arg test helper via the generic helper binding
+        sx_def = '(define test-two-args (fn (a b) (helper "json-encode" (str a ":" b))))'
+        async with self.bridge._lock:
+            self.bridge._send(f'(load-source "{_escape(sx_def)}")')
+            await self.bridge._read_until_ok(ctx=None)
+
+        result = await self.bridge.eval(
+            '(test-two-args "hello" "world")',
+            ctx={"_helper_service": "sx"}
+        )
+        self.assertIn("hello:world", result)
+
+    async def test_undefined_helper_errors(self):
+        """Calling an uninjected helper raises an error."""
+        with self.assertRaises(OcamlBridgeError) as cm:
+            await self.bridge.eval('(nonexistent-helper "arg")')
+        self.assertIn("Undefined symbol", str(cm.exception))
+
+    async def test_helper_in_aser(self):
+        """Helper works inside aser — result inlined in SX output."""
+        await self._inject_test_helper("json-encode", 1)
+
+        # aser a component-like expression that calls the helper
+        result = await self.bridge.aser(
+            '(div :class "test" (json-encode "hello"))',
+            ctx={"_helper_service": "sx"}
+        )
+        # The aser should evaluate json-encode and inline the result
+        self.assertIn("div", result)
+        self.assertIn("hello", result)
+
+    async def test_helper_in_aser_slot_component(self):
+        """aser_slot expands component containing helper call."""
+        await self._inject_test_helper("json-encode", 1)
+
+        # Define a component that calls the helper
+        async with self.bridge._lock:
+            self.bridge._send(
+                '(load-source "(defcomp ~test/code-display (&key code) '
+                '(pre (code code)))")'
+            )
+            await self.bridge._read_until_ok(ctx=None)
+
+        # aser_slot should expand the component, evaluating the body
+        result = await self.bridge.aser_slot(
+            '(~test/code-display :code (json-encode "test-value"))',
+            ctx={"_helper_service": "sx"}
+        )
+        # Should contain expanded HTML tags, not component call
+        self.assertIn("pre", result)
+        self.assertIn("test-value", result)
+        # Should NOT contain the component call
+        self.assertNotIn("~test/code-display", result)
+
+
+class TestHelperIOPerformance(unittest.IsolatedAsyncioTestCase):
+    """Test that helper IO round-trips are fast enough for production."""
+
+    @classmethod
+    def setUpClass(cls):
+        _skip_if_no_binary()
+
+    async def asyncSetUp(self):
+        self.bridge = OcamlBridge()
+        await self.bridge.start()
+        spec_dir = os.path.join(_project_root, "spec")
+        web_dir = os.path.join(_project_root, "web")
+        for f in ["parser.sx", "render.sx"]:
+            path = os.path.join(spec_dir, f)
+            if os.path.isfile(path):
+                async with self.bridge._lock:
+                    self.bridge._send(f'(load "{_escape(path)}")')
+                    await self.bridge._read_until_ok(ctx=None)
+        adapter = os.path.join(web_dir, "adapter-sx.sx")
+        if os.path.isfile(adapter):
+            async with self.bridge._lock:
+                self.bridge._send(f'(load "{_escape(adapter)}")')
+                await self.bridge._read_until_ok(ctx=None)
+
+    async def asyncTearDown(self):
+        await self.bridge.stop()
+
+    async def test_sequential_helper_calls_timing(self):
+        """Measure round-trip time for sequential helper calls."""
+        # Inject json-encode as a fast helper
+        param_names = "a"
+        sx_def = '(define json-encode (fn (a) (helper "json-encode" a)))'
+        async with self.bridge._lock:
+            self.bridge._send(f'(load-source "{_escape(sx_def)}")')
+            await self.bridge._read_until_ok(ctx=None)
+
+        # Time 20 sequential calls (simulating a page with 20 highlight calls)
+        n_calls = 20
+        calls = " ".join(f'(json-encode "{i}")' for i in range(n_calls))
+        expr = f'(list {calls})'
+
+        start = time.monotonic()
+        result = await self.bridge.eval(expr, ctx={"_helper_service": "sx"})
+        elapsed = time.monotonic() - start
+
+        # Should complete in under 5 seconds (generous for 20 IO round-trips)
+        self.assertLess(elapsed, 5.0,
+            f"20 helper IO round-trips took {elapsed:.1f}s (target: <5s)")
+
+    async def test_aser_slot_with_many_helper_calls(self):
+        """aser_slot with multiple helper calls completes in reasonable time."""
+        sx_def = '(define json-encode (fn (a) (helper "json-encode" a)))'
+        async with self.bridge._lock:
+            self.bridge._send(f'(load-source "{_escape(sx_def)}")')
+            await self.bridge._read_until_ok(ctx=None)
+
+        # Define a component with multiple helper calls
+        comp_def = (
+            '(defcomp ~test/multi-helper (&key)'
+            '  (div'
+            '    (p (json-encode "a"))'
+            '    (p (json-encode "b"))'
+            '    (p (json-encode "c"))'
+            '    (p (json-encode "d"))'
+            '    (p (json-encode "e"))))'
+        )
+        async with self.bridge._lock:
+            self.bridge._send(f'(load-source "{_escape(comp_def)}")')
+            await self.bridge._read_until_ok(ctx=None)
+
+        start = time.monotonic()
+        result = await self.bridge.aser_slot(
+            '(~test/multi-helper)',
+            ctx={"_helper_service": "sx"}
+        )
+        elapsed = time.monotonic() - start
+
+        self.assertIn("div", result)
+        self.assertLess(elapsed, 3.0,
+            f"aser_slot with 5 helpers took {elapsed:.1f}s (target: <3s)")
+
+
+class TestAserSlotClientAffinity(unittest.IsolatedAsyncioTestCase):
+    """Test that :affinity :client components are NOT expanded by aser_slot."""
+
+    @classmethod
+    def setUpClass(cls):
+        _skip_if_no_binary()
+
+    async def asyncSetUp(self):
+        self.bridge = OcamlBridge()
+        await self.bridge.start()
+        spec_dir = os.path.join(_project_root, "spec")
+        web_dir = os.path.join(_project_root, "web")
+        for f in ["parser.sx", "render.sx"]:
+            path = os.path.join(spec_dir, f)
+            if os.path.isfile(path):
+                async with self.bridge._lock:
+                    self.bridge._send(f'(load "{_escape(path)}")')
+                    await self.bridge._read_until_ok(ctx=None)
+        adapter = os.path.join(web_dir, "adapter-sx.sx")
+        if os.path.isfile(adapter):
+            async with self.bridge._lock:
+                self.bridge._send(f'(load "{_escape(adapter)}")')
+                await self.bridge._read_until_ok(ctx=None)
+
+    async def asyncTearDown(self):
+        await self.bridge.stop()
+
+    async def test_client_affinity_not_expanded(self):
+        """Components with :affinity :client stay as calls in aser_slot."""
+        comp_def = (
+            '(defcomp ~test/client-only () :affinity :client'
+            '  (div "browser-only-content"))'
+        )
+        async with self.bridge._lock:
+            self.bridge._send(f'(load-source "{_escape(comp_def)}")')
+            await self.bridge._read_until_ok(ctx=None)
+
+        result = await self.bridge.aser_slot('(~test/client-only)')
+        # Should remain as a component call, NOT expanded
+        self.assertIn("~test/client-only", result)
+        self.assertNotIn("browser-only-content", result)
+
+    async def test_server_affinity_expanded(self):
+        """Components with :affinity :server are expanded by regular aser."""
+        comp_def = (
+            '(defcomp ~test/server-only (&key label) :affinity :server'
+            '  (div :class "server" label))'
+        )
+        async with self.bridge._lock:
+            self.bridge._send(f'(load-source "{_escape(comp_def)}")')
+            await self.bridge._read_until_ok(ctx=None)
+
+        result = await self.bridge.aser(
+            '(~test/server-only :label "hello")')
+        # Should be expanded — div with class, not component call
+        self.assertIn("div", result)
+        self.assertIn("server", result)
+        self.assertNotIn("~test/server-only", result)
+
+    async def test_auto_affinity_not_expanded_by_aser(self):
+        """Default affinity components are NOT expanded by regular aser."""
+        comp_def = (
+            '(defcomp ~test/auto-comp (&key label)'
+            '  (div "auto-content" label))'
+        )
+        async with self.bridge._lock:
+            self.bridge._send(f'(load-source "{_escape(comp_def)}")')
+            await self.bridge._read_until_ok(ctx=None)
+
+        result = await self.bridge.aser(
+            '(~test/auto-comp :label "hi")')
+        # Should remain as component call
+        self.assertIn("~test/auto-comp", result)
+
+    async def test_island_not_expanded_by_aser_slot(self):
+        """Islands are NEVER expanded server-side, even with expand-all."""
+        island_def = (
+            '(defisland ~test/reactive-isle (&key label)'
+            '  (div (deref (signal 0)) label))'
+        )
+        async with self.bridge._lock:
+            self.bridge._send(f'(load-source "{_escape(island_def)}")')
+            await self.bridge._read_until_ok(ctx=None)
+
+        result = await self.bridge.aser_slot(
+            '(~test/reactive-isle :label "hello")')
+        # Island should be serialized as a call, NOT expanded
+        self.assertIn("~test/reactive-isle", result)
+        # Body content (deref, signal) should NOT appear
+        self.assertNotIn("deref", result)
+        self.assertNotIn("signal", result)
+
+    async def test_island_preserved_inside_expanded_component(self):
+        """Island calls survive inside aser_slot-expanded components."""
+        src = (
+            '(defisland ~test/inner-isle (&key v) (span (deref (signal v))))'
+            '(defcomp ~test/outer-comp (&key title)'
+            '  (div (h1 title) (~test/inner-isle :v 42)))'
+        )
+        async with self.bridge._lock:
+            self.bridge._send(f'(load-source "{_escape(src)}")')
+            await self.bridge._read_until_ok(ctx=None)
+
+        result = await self.bridge.aser_slot(
+            '(~test/outer-comp :title "Test")')
+        # Outer component expanded
+        self.assertNotIn("~test/outer-comp", result)
+        self.assertIn("div", result)
+        self.assertIn("Test", result)
+        # Inner island preserved as call
+        self.assertIn("~test/inner-isle", result)
+
+    async def test_auto_affinity_expanded_by_aser_slot(self):
+        """Default affinity components ARE expanded by aser_slot."""
+        comp_def = (
+            '(defcomp ~test/auto-comp2 (&key label)'
+            '  (div "expanded" label))'
+        )
+        async with self.bridge._lock:
+            self.bridge._send(f'(load-source "{_escape(comp_def)}")')
+            await self.bridge._read_until_ok(ctx=None)
+
+        result = await self.bridge.aser_slot(
+            '(~test/auto-comp2 :label "hi")')
+        # Should be expanded
+        self.assertIn("div", result)
+        self.assertIn("expanded", result)
+        self.assertNotIn("~test/auto-comp2", result)
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/web/adapter-sx.sx b/web/adapter-sx.sx
index e8278ad..cb01454 100644
--- a/web/adapter-sx.sx
+++ b/web/adapter-sx.sx
@@ -87,6 +87,7 @@
                   (and comp (macro? comp))
                     (aser (expand-macro comp args env) env)
                   (and comp (component? comp)
+                       (not (island? comp))
                        (or expand-all
                            (= (component-affinity comp) "server"))
                        ;; :affinity :client components are never expanded