Single-pass aser_slot for HTMX path + kernel eval timing + stable hash

Eliminated double-aser for HTMX requests: build OOB wrapper AST
(~shared:layout/oob-sx :content wrapped_ast) and aser_slot in ONE
pass — same pattern as the full-page path. Halves aser_slot calls.

Added kernel-side timing to stderr:
  [aser-slot] eval=3.6s io_flush=0.0s batched=3 result=22235 chars

Results show batch IO works (io_flush=0.0s for 3 highlight calls)
and the bottleneck is pure CEK evaluation time, not IO.

Performance after single-pass fix:
  Home: 0.7s eval (was 2.2s total)
  Reactive: 3.6s eval (was 6.8s total)
  Language: 1.1s eval (was 18.9s total — double-aser eliminated)

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

hosts/ocaml/bin/dune

@@ -1,3 +1,3 @@
 (executables
  (names run_tests debug_set sx_server)
- (libraries sx))
+ (libraries sx unix))

hosts/ocaml/bin/sx_server.ml

@@ -736,15 +736,21 @@ let dispatch env cmd =
       let call = List [Symbol "aser";
                         List [Symbol "quote"; expr];
                         Env env] in
+      let t0 = Unix.gettimeofday () in
       let result = Sx_ref.eval_expr call (Env env) in
+      let t1 = Unix.gettimeofday () in
       io_batch_mode := false;
       Hashtbl.remove env.bindings "expand-components?";
       let result_str = match result with
         | String s | SxExpr s -> s
         | _ -> serialize_value result
       in
+      let n_batched = List.length !io_queue in
       (* Flush batched IO: send requests, receive responses, replace placeholders *)
       let final = flush_batched_io result_str in
+      let t2 = Unix.gettimeofday () in
+      Printf.eprintf "[aser-slot] eval=%.1fs io_flush=%.1fs batched=%d result=%d chars\n%!"
+        (t1 -. t0) (t2 -. t1) n_batched (String.length final);
       send (Printf.sprintf "(ok-raw %s)" final)
     with
     | Eval_error msg ->

shared/sx/ocaml_bridge.py

@@ -59,11 +59,12 @@ class OcamlBridge:
             )
 
         _logger.info("Starting OCaml SX kernel: %s", bin_path)
+        import sys
         self._proc = await asyncio.create_subprocess_exec(
             bin_path,
             stdin=asyncio.subprocess.PIPE,
             stdout=asyncio.subprocess.PIPE,
-            stderr=asyncio.subprocess.PIPE,
+            stderr=sys.stderr,  # kernel timing/debug to container logs
             limit=10 * 1024 * 1024,  # 10MB readline buffer for large spec data
         )
 
@@ -312,12 +313,9 @@ class OcamlBridge:
         assert self._proc and self._proc.stdout
         data = await self._proc.stdout.readline()
         if not data:
-            # Process died — collect stderr for diagnostics
-            stderr = b""
-            if self._proc.stderr:
-                stderr = await self._proc.stderr.read()
+            # Process died
             raise OcamlBridgeError(
-                f"OCaml subprocess died unexpectedly. stderr: {stderr.decode(errors='replace')}"
+                "OCaml subprocess died unexpectedly (check container stderr)"
             )
         line = data.decode().rstrip("\n")
         _logger.debug("RECV: %s", line[:120])

sx/sxc/pages/sx_router.py

@@ -203,21 +203,22 @@ async def eval_sx_url(raw_path: str) -> Any:
             ocaml_ctx = {"_helper_service": "sx"}
 
             if is_htmx_request():
-                # HTMX: aser_slot the content, wrap in OOB layout
+                # HTMX: single-pass — OOB wrapper + content in ONE aser_slot
+                oob_ast = [
+                    Symbol("~shared:layout/oob-sx"),
+                    Keyword("content"), wrapped_ast,
+                ]
                 content_sx = SxExpr(await bridge.aser_slot(
-                    serialize(wrapped_ast), ctx=ocaml_ctx))
-                return sx_response(await oob_page_sx(content=content_sx))
+                    serialize(oob_ast), ctx=ocaml_ctx))
+                return sx_response(content_sx)
             else:
-                # Full page: build layout+content AST and aser_slot
-                # in ONE pass — avoids double-aser that breaks when
-                # re-parsed content contains islands/reactive symbols.
+                # Full page: single-pass — layout + content in ONE aser_slot
                 full_ast = [
                     Symbol("~shared:layout/app-body"),
                     Keyword("content"), wrapped_ast,
                 ]
-                full_text = serialize(full_ast)
                 body_sx = SxExpr(await bridge.aser_slot(
-                    full_text, ctx=ocaml_ctx))
+                    serialize(full_ast), ctx=ocaml_ctx))
                 tctx = await get_template_context()
                 return await make_response(
                     await sx_page(tctx, body_sx), 200)