Isomorphic SSR: server renders HTML body, client takes over with SX

Server now renders page content as HTML inside <div id="sx-root">,
visible immediately before JavaScript loads. The SX source is still
included in a <script data-mount="#sx-root"> tag for client hydration.

SSR pipeline: after aser produces the SX wire format, parse and
render-to-html it (~17ms for a 22KB page). Islands with reactive
state gracefully fall back to empty — client hydrates them.

Supporting changes:
- Load signals.sx into OCaml kernel (reactive primitives for island SSR)
- Add cek-call and context to kernel env (needed by signals/deref)
- Island-aware component accessors in sx_types.ml
- render-to-html handles Island values (renders as component with fallback)
- Fix 431 (Request Header Fields Too Large): replace SX-Components
  header (full component name list) with SX-Components-Hash (12 chars)
- CORS allow SX-Components-Hash header

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/bin/sx_server.ml

@@ -425,7 +425,7 @@ let make_server_env () =
   (* context — scope lookup.  The CEK handles this as a special form
      by walking continuation frames, but compiled VM code needs it as
      a function that reads from the scope_stacks hashtable. *)
-  bind "sx-context" (fun args ->
+  let context_impl = NativeFn ("context", fun args ->
     match args with
     | [String name] | [String name; _] ->
       let stack = try Hashtbl.find scope_stacks name with Not_found -> [] in
@@ -433,7 +433,9 @@ let make_server_env () =
        | v :: _, _ -> v
        | [], [_; default_val] -> default_val
        | [], _ -> Nil)
-    | _ -> Nil);
+    | _ -> Nil) in
+  ignore (env_bind env "sx-context" context_impl);
+  ignore (env_bind env "context" context_impl);
 
   (* qq-expand-runtime — quasiquote expansion at runtime.
      The bytecode compiler emits CALL_PRIM "qq-expand-runtime" for
@@ -464,6 +466,12 @@ let make_server_env () =
     | [fn_val; List call_args] ->
       Sx_ref.eval_expr (List (fn_val :: call_args)) (Env env)
     | _ -> raise (Eval_error "call-lambda: expected (fn args env?)"));
+  bind "cek-call" (fun args ->
+    match args with
+    | [fn_val; List call_args] -> Sx_ref.cek_call fn_val (List call_args)
+    | [fn_val; Nil] -> Sx_ref.cek_call fn_val (List [])
+    | [fn_val] -> Sx_ref.cek_call fn_val (List [])
+    | _ -> Nil);
   bind "expand-macro" (fun args ->
     match args with
     | [Macro m; List macro_args; Env e] ->
@@ -1072,6 +1080,22 @@ let rec dispatch env cmd =
       in
       let body_final = flush_batched_io body_str in
       let t2 = Unix.gettimeofday () in
+      (* Phase 1b: render the aser'd SX to HTML for isomorphic SSR.
+         The aser output is flat (all components expanded, just HTML tags),
+         so render-to-html is cheap — no component lookups needed. *)
+      let body_html =
+        try
+          let body_exprs = Sx_parser.parse_all body_final in
+          let body_expr = match body_exprs with
+            | [e] -> e | [] -> Nil | _ -> List (Symbol "<>" :: body_exprs)
+          in
+          Sx_render.render_to_html body_expr env
+        with e ->
+          Printf.eprintf "[ssr] render-to-html failed: %s\n%!" (Printexc.to_string e);
+          ""  (* fallback: client renders from SX source. Islands with
+                 reactive state may fail SSR — client hydrates them. *)
+      in
+      let t2b = Unix.gettimeofday () in
       (* Phase 2: render shell with body + all kwargs.
          Resolve symbol references (e.g. __shell-component-defs) to their
          values from the env — these were pre-injected by the bridge. *)
@@ -1082,13 +1106,15 @@ let rec dispatch env cmd =
            with _ -> try Sx_primitives.get_primitive s with _ -> v)
         | _ -> v
       ) shell_kwargs in
-      let shell_args = Keyword "page-sx" :: String body_final :: resolved_kwargs in
+      let shell_args = Keyword "page-sx" :: String body_final
+                     :: Keyword "body-html" :: String body_html
+                     :: resolved_kwargs in
       let shell_call = List (Symbol "~shared:shell/sx-page-shell" :: shell_args) in
       let html = Sx_render.render_to_html shell_call env in
       let t3 = Unix.gettimeofday () in
-      Printf.eprintf "[sx-page-full] aser=%.3fs io=%.3fs shell=%.3fs total=%.3fs body=%d html=%d\n%!"
+      Printf.eprintf "[sx-page-full] aser=%.3fs io=%.3fs ssr=%.3fs shell=%.3fs total=%.3fs body=%d ssr=%d html=%d\n%!"
-        (t1 -. t0) (t2 -. t1) (t3 -. t2) (t3 -. t0)
+        (t1 -. t0) (t2 -. t1) (t2b -. t2) (t3 -. t2b) (t3 -. t0)
-        (String.length body_final) (String.length html);
+        (String.length body_final) (String.length body_html) (String.length html);
       send_ok_string html
     with
     | Eval_error msg ->

hosts/ocaml/lib/sx_render.ml

@@ -256,6 +256,18 @@ and render_list_to_html head args env =
       let v = env_get env name in
       (match v with
        | Component _ -> render_component v args env
+       | Island i ->
+         (* Islands: render initial HTML server-side (like React SSR).
+            Log failures so we can fix them. *)
+         (try
+           let c = { c_name = i.i_name; c_params = i.i_params;
+                     c_has_children = i.i_has_children; c_body = i.i_body;
+                     c_closure = i.i_closure; c_affinity = "client";
+                     c_compiled = None } in
+           render_component (Component c) args env
+         with e ->
+           Printf.eprintf "[ssr-island] ~%s FAILED: %s\n%!" i.i_name (Printexc.to_string e);
+           "")
        | Macro m ->
          let expanded = expand_macro m args env in
          do_render_to_html expanded env

hosts/ocaml/lib/sx_types.ml

@@ -347,26 +347,32 @@ let set_lambda_name l n = match l with
 
 let component_name = function
   | Component c -> String c.c_name
+  | Island i -> String i.i_name
   | v -> raise (Eval_error ("Expected component, got " ^ type_of v))
 
 let component_params = function
   | Component c -> List (List.map (fun s -> String s) c.c_params)
+  | Island i -> List (List.map (fun s -> String s) i.i_params)
   | v -> raise (Eval_error ("Expected component, got " ^ type_of v))
 
 let component_body = function
   | Component c -> c.c_body
+  | Island i -> i.i_body
   | v -> raise (Eval_error ("Expected component, got " ^ type_of v))
 
 let component_closure = function
   | Component c -> Env c.c_closure
+  | Island i -> Env i.i_closure
   | v -> raise (Eval_error ("Expected component, got " ^ type_of v))
 
 let component_has_children = function
   | Component c -> Bool c.c_has_children
+  | Island i -> Bool i.i_has_children
   | v -> raise (Eval_error ("Expected component, got " ^ type_of v))
 
 let component_affinity = function
   | Component c -> String c.c_affinity
+  | Island _ -> String "client"
   | _ -> String "auto"
 
 let macro_params = function

shared/infrastructure/factory.py

@@ -344,7 +344,7 @@ def create_base_app(
             response.headers["Access-Control-Allow-Origin"] = origin
             response.headers["Access-Control-Allow-Credentials"] = "true"
             response.headers["Access-Control-Allow-Headers"] = (
-                "SX-Request, SX-Target, SX-Current-URL, SX-Components, SX-Css, "
+                "SX-Request, SX-Target, SX-Current-URL, SX-Components, SX-Components-Hash, SX-Css, "
                 "HX-Request, HX-Target, HX-Current-URL, HX-Trigger, "
                 "Content-Type, X-CSRFToken"
             )

shared/static/scripts/sx-browser.js

@@ -14,7 +14,7 @@
   // =========================================================================
 
   var NIL = Object.freeze({ _nil: true, toString: function() { return "nil"; } });
-  var SX_VERSION = "2026-03-23T08:59:15Z";
+  var SX_VERSION = "2026-03-23T13:21:57Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }
@@ -1582,7 +1582,13 @@ PRIMITIVES["step-sf-lambda"] = stepSfLambda;
   var val = NIL;
   var body = NIL;
   (isSxTruthy((isSxTruthy((len(restArgs) >= 2)) && isSxTruthy((typeOf(first(restArgs)) == "keyword")) && (keywordName(first(restArgs)) == "value"))) ? ((val = trampoline(evalExpr(nth(restArgs, 1), env))), (body = slice(restArgs, 2))) : (body = restArgs));
-  return (isSxTruthy(isEmpty(body)) ? makeCekValue(NIL, env, kont) : (isSxTruthy((len(body) == 1)) ? makeCekState(first(body), env, kontPush(makeScopeAccFrame(name, val, [], env), kont)) : makeCekState(first(body), env, kontPush(makeScopeAccFrame(name, val, rest(body), env), kont))));
+  scopePush(name, val);
+  return (function() {
+  var result = NIL;
+  { var _c = body; for (var _i = 0; _i < _c.length; _i++) { var expr = _c[_i]; result = trampoline(evalExpr(expr, env)); } }
+  scopePop(name);
+  return makeCekValue(result, env, kont);
+})();
 })(); };
 PRIMITIVES["step-sf-scope"] = stepSfScope;
 
@@ -1591,7 +1597,13 @@ PRIMITIVES["step-sf-scope"] = stepSfScope;
   var name = trampoline(evalExpr(first(args), env));
   var val = trampoline(evalExpr(nth(args, 1), env));
   var body = slice(args, 2);
-  return (isSxTruthy(isEmpty(body)) ? makeCekValue(NIL, env, kont) : (isSxTruthy((len(body) == 1)) ? makeCekState(first(body), env, kontPush(makeProvideFrame(name, val, [], env), kont)) : makeCekState(first(body), env, kontPush(makeProvideFrame(name, val, rest(body), env), kont))));
+  scopePush(name, val);
+  return (function() {
+  var result = NIL;
+  { var _c = body; for (var _i = 0; _i < _c.length; _i++) { var expr = _c[_i]; result = trampoline(evalExpr(expr, env)); } }
+  scopePop(name);
+  return makeCekValue(result, env, kont);
+})();
 })(); };
 PRIMITIVES["step-sf-provide"] = stepSfProvide;
 
@@ -1599,8 +1611,8 @@ PRIMITIVES["step-sf-provide"] = stepSfProvide;
   var stepSfContext = function(args, env, kont) { return (function() {
   var name = trampoline(evalExpr(first(args), env));
   var defaultVal = (isSxTruthy((len(args) >= 2)) ? trampoline(evalExpr(nth(args, 1), env)) : NIL);
-  var frame = kontFindProvide(kont, name);
+  var val = scopePeek(name);
-  return (isSxTruthy(frame) ? makeCekValue(get(frame, "value"), env, kont) : (isSxTruthy((len(args) >= 2)) ? makeCekValue(defaultVal, env, kont) : error((String("No provider for: ") + String(name)))));
+  return makeCekValue((isSxTruthy(isNil(val)) ? defaultVal : val), env, kont);
 })(); };
 PRIMITIVES["step-sf-context"] = stepSfContext;
 
@@ -1608,16 +1620,16 @@ PRIMITIVES["step-sf-context"] = stepSfContext;
   var stepSfEmit = function(args, env, kont) { return (function() {
   var name = trampoline(evalExpr(first(args), env));
   var val = trampoline(evalExpr(nth(args, 1), env));
-  var frame = kontFindScopeAcc(kont, name);
+  scopeEmit(name, val);
-  return (isSxTruthy(frame) ? (append_b(get(frame, "emitted"), val), makeCekValue(NIL, env, kont)) : error((String("No scope for emit!: ") + String(name))));
+  return makeCekValue(NIL, env, kont);
 })(); };
 PRIMITIVES["step-sf-emit"] = stepSfEmit;
 
   // step-sf-emitted
   var stepSfEmitted = function(args, env, kont) { return (function() {
   var name = trampoline(evalExpr(first(args), env));
-  var frame = kontFindScopeAcc(kont, name);
+  var val = scopePeek(name);
-  return (isSxTruthy(frame) ? makeCekValue(get(frame, "emitted"), env, kont) : error((String("No scope for emitted: ") + String(name))));
+  return makeCekValue((isSxTruthy(isNil(val)) ? [] : val), env, kont);
 })(); };
 PRIMITIVES["step-sf-emitted"] = stepSfEmitted;
 
@@ -3573,9 +3585,10 @@ PRIMITIVES["get-verb-info"] = getVerbInfo;
   var targetSel = domGetAttr(el, "sx-target");
   return (isSxTruthy(targetSel) ? dictSet(headers, "SX-Target", targetSel) : NIL);
 })();
-  if (isSxTruthy(!isSxTruthy(isEmpty(loadedComponents)))) {
+  (function() {
-  headers["SX-Components"] = join(",", loadedComponents);
+  var compHash = domGetAttr(domQuery("script[data-components][data-hash]"), "data-hash");
-}
+  return (isSxTruthy(compHash) ? dictSet(headers, "SX-Components-Hash", compHash) : NIL);
+})();
   if (isSxTruthy(cssHash)) {
   headers["SX-Css"] = cssHash;
 }

shared/sx/helpers.py

@@ -521,8 +521,18 @@ def components_for_request(source: str = "",
     elif extra_names:
         needed = extra_names
 
-    loaded_raw = request.headers.get("SX-Components", "")
+    # Check hash first (new): if client hash matches current, skip all defs.
-    loaded = set(loaded_raw.split(",")) if loaded_raw else set()
+    # Fall back to legacy name list (SX-Components) for backward compat.
+    comp_hash_header = request.headers.get("SX-Components-Hash", "")
+    if comp_hash_header:
+        from .jinja_bridge import components_for_page
+        _, current_hash = components_for_page("", service=None)
+        if comp_hash_header == current_hash:
+            return ""  # client has everything
+        loaded = set()  # hash mismatch — send all needed
+    else:
+        loaded_raw = request.headers.get("SX-Components", "")
+        loaded = set(loaded_raw.split(",")) if loaded_raw else set()
 
     parts = []
     for key, val in _COMPONENT_ENV.items():

shared/sx/ocaml_bridge.py

@@ -397,10 +397,11 @@ class OcamlBridge:
             # All directories loaded into the Python env
             all_dirs = list(set(_watched_dirs) | _dirs_from_cache)
 
-            # Web adapters (aser lives in adapter-sx.sx) — only load specific files
+            # Web adapters + signals (aser lives in adapter-sx.sx,
+            # signals.sx provides reactive primitives for island SSR)
             web_dir = os.path.join(os.path.dirname(__file__), "../../web")
             if os.path.isdir(web_dir):
-                for web_file in ["adapter-sx.sx"]:
+                for web_file in ["signals.sx", "adapter-sx.sx"]:
                     path = os.path.normpath(os.path.join(web_dir, web_file))
                     if os.path.isfile(path):
                         all_files.append(path)

shared/sx/templates/shell.sx

@@ -15,6 +15,7 @@
                                (sx-css :as string?) (sx-css-classes :as string?)
                                (component-hash :as string?) (component-defs :as string?)
                                (pages-sx :as string?) (page-sx :as string?)
+                               (body-html :as string?)
                                (asset-url :as string) (sx-js-hash :as string) (body-js-hash :as string?)
                                (head-scripts :as list?) (inline-css :as string?) (inline-head-js :as string?)
                                (init-sx :as string?) (body-scripts :as list?))
@@ -65,6 +66,8 @@ details.group{overflow:hidden}details.group>summary{list-style:none}details.grou
 .sx-error .sx-indicator{display:none}.sx-loading .sx-indicator{display:inline-flex}
 .js-wrap.open .js-pop{display:block}.js-wrap.open .js-backdrop{display:block}"))))
       (body :class "bg-stone-50 text-stone-900"
+        ;; Server-rendered HTML — visible immediately before JS loads
+        (div :id "sx-root" (raw! (or body-html "")))
         (script :type "text/sx" :data-components true :data-hash component-hash
           (raw! (or component-defs "")))
         (when init-sx
@@ -72,7 +75,7 @@ details.group{overflow:hidden}details.group>summary{list-style:none}details.grou
             (raw! init-sx)))
         (script :type "text/sx-pages"
           (raw! (or pages-sx "")))
-        (script :type "text/sx" :data-mount "body"
+        (script :type "text/sx" :data-mount "#sx-root"
           (raw! (or page-sx "")))
         (script :src (str asset-url "/scripts/sx-browser.js?v=" sx-js-hash))
         ;; Body scripts — configurable per app

web/engine.sx

@@ -125,10 +125,14 @@
         (when target-sel
           (dict-set! headers "SX-Target" target-sel)))
 
-      ;; Loaded component names
+      ;; Send component hash instead of full name list to avoid 431
-      (when (not (empty? loaded-components))
+      ;; (Request Header Fields Too Large) with many loaded components.
-        (dict-set! headers "SX-Components"
+      ;; Server uses hash to decide whether to send component definitions.
-          (join "," loaded-components)))
+      (let ((comp-hash (dom-get-attr
+                         (dom-query "script[data-components][data-hash]")
+                         "data-hash")))
+        (when comp-hash
+          (dict-set! headers "SX-Components-Hash" comp-hash)))
 
       ;; CSS class hash
       (when css-hash