diff --git a/hosts/ocaml/bin/run_tests.ml b/hosts/ocaml/bin/run_tests.ml
index d3798742..eb804bb2 100644
--- a/hosts/ocaml/bin/run_tests.ml
+++ b/hosts/ocaml/bin/run_tests.ml
@@ -1309,16 +1309,39 @@ let run_spec_tests env test_files =
   (* Rebind vm-execute-module and code-from-value to native OCaml implementations.
      The SX versions from vm.sx run bytecode step-by-step in the interpreter —
      far too slow for the test suite. Native versions use the compiled OCaml VM. *)
-  (* Rebind vm-execute-module to use the native OCaml VM directly.
-     The SX version from vm.sx runs bytecode step-by-step in the interpreter.
-     code-from-value stays as the SX version — it produces dicts that
-     vm-execute-module converts to native vm_code internally. *)
+  (* Wrap SX vm-execute-module to seed empty globals with primitives + env.
+     The SX VM resolves CALL_PRIM/GLOBAL_GET from globals — without seeding,
+     even (+ 1 2) fails. We keep the SX version (not native Sx_vm) so
+     suspension tests work (SX VM suspends via dict, native VM via exception). *)
+  let sx_vm_execute = try Some (Sx_types.env_get env "vm-execute-module") with _ -> None in
   ignore (Sx_types.env_bind env "vm-execute-module" (NativeFn ("vm-execute-module", fun args ->
     match args with
     | [code; Dict globals] ->
+      if Hashtbl.length globals = 0 then begin
+        Hashtbl.iter (fun name fn ->
+          Hashtbl.replace globals name (NativeFn (name, fn))
+        ) Sx_primitives.primitives;
+        let rec add_env e =
+          Hashtbl.iter (fun id v ->
+            let name = Sx_types.unintern id in
+            if not (Hashtbl.mem globals name) then
+              Hashtbl.replace globals name v) e.Sx_types.bindings;
+          match e.Sx_types.parent with Some p -> add_env p | None -> ()
+        in add_env env
+      end;
+      (* Use native VM for speed — much faster than SX step-by-step *)
       let c = Sx_vm.code_from_value code in
-      Sx_vm.execute_module c globals
-    | _ -> Nil)));
+      (try Sx_vm.execute_module c globals
+       with Sx_vm.VmSuspended (_request, _saved_vm) ->
+         (* Fall back to SX version for suspension handling *)
+         Hashtbl.remove globals "__io_request";
+         match sx_vm_execute with
+         | Some fn -> Sx_ref.cek_call fn (List [code; Dict globals])
+         | None -> Nil)
+    | _ ->
+      match sx_vm_execute with
+      | Some fn -> Sx_ref.cek_call fn (List args)
+      | None -> Nil)));
   load_module "signals.sx" spec_dir;  (* core reactive primitives *)
   load_module "signals.sx" web_dir;   (* web extensions *)
   load_module "freeze.sx" lib_dir;
diff --git a/hosts/ocaml/lib/sx_vm.ml b/hosts/ocaml/lib/sx_vm.ml
index bbb992ce..a79ab860 100644
--- a/hosts/ocaml/lib/sx_vm.ml
+++ b/hosts/ocaml/lib/sx_vm.ml
@@ -193,7 +193,30 @@ let code_from_value v =
     let rest_arity = match find2 "rest-arity" "vc-rest-arity" with
       | Some (Number n) -> int_of_float n | _ -> -1
     in
-    { vc_arity = arity; vc_rest_arity = rest_arity; vc_locals = arity + 16;
+    (* Compute locals from bytecode: scan for highest LOCAL_GET/LOCAL_SET slot.
+       The compiler's arity may undercount when nested lets add many locals. *)
+    let max_local = ref (arity - 1) in
+    let len = Array.length bc_list in
+    let i = ref 0 in
+    while !i < len do
+      let op = bc_list.(!i) in
+      if (op = 16 (* LOCAL_GET *) || op = 17 (* LOCAL_SET *)) && !i + 1 < len then
+        (let slot = bc_list.(!i + 1) in
+         if slot > !max_local then max_local := slot;
+         i := !i + 2)
+      else if op = 18 (* UPVALUE_GET *) || op = 19 (* UPVALUE_SET *)
+           || op = 8 (* JUMP_IF_FALSE *) || op = 33 (* JUMP_IF_FALSE_u16 *)
+           || op = 34 (* JUMP_IF_TRUE *) then
+        i := !i + 2
+      else if op = 1 (* CONST *) || op = 20 (* GLOBAL_GET *) || op = 21 (* GLOBAL_SET *)
+           || op = 32 (* JUMP *) || op = 51 (* CLOSURE *) || op = 52 (* CALL_PRIM *)
+           || op = 64 (* MAKE_LIST *) || op = 65 (* MAKE_DICT *) then
+        i := !i + 3  (* u16 operand *)
+      else
+        i := !i + 1
+    done;
+    let locals = !max_local + 1 + 16 in  (* +16 headroom for temporaries *)
+    { vc_arity = arity; vc_rest_arity = rest_arity; vc_locals = locals;
       vc_bytecode = bc_list; vc_constants = constants;
       vc_bytecode_list = None; vc_constants_list = None }
   | _ -> { vc_arity = 0; vc_rest_arity = -1; vc_locals = 16; vc_bytecode = [||]; vc_constants = [||];
@@ -833,28 +856,34 @@ let jit_compile_lambda (l : lambda) globals =
   if !_jit_compiling then (
     (* Already compiling — prevent cascade. The CEK will handle this call. *)
     None
+  ) else if List.mem "&key" l.l_params || List.mem ":as" l.l_params then (
+    (* &key/:as require complex runtime argument processing that the compiler
+       doesn't emit. These functions must run via CEK. *)
+    None
   ) else
   try
     _jit_compiling := true;
     let compile_fn = try Hashtbl.find globals "compile"
       with Not_found -> (_jit_compiling := false; raise (Eval_error "JIT: compiler not loaded")) in
-    (* Reconstruct the (fn (params) body) form so the compiler produces
-       a proper closure. l.l_body is the inner body; we need the full
-       function form with params so the compiled code binds them. *)
     let param_syms = List (List.map (fun s -> Symbol s) l.l_params) in
     let fn_expr = List [Symbol "fn"; param_syms; l.l_body] in
     let quoted = List [Symbol "quote"; fn_expr] in
-    (* Use Symbol "compile" so the CEK resolves it from the env, not
-       an embedded VmClosure value — the CEK dispatches VmClosure calls
-       differently when the value is resolved from env vs embedded in AST. *)
-    ignore compile_fn;
-    let compile_env = Sx_types.env_extend (Sx_types.make_env ()) in
-    Hashtbl.iter (fun k v -> Hashtbl.replace compile_env.bindings (Sx_types.intern k) v) globals;
-    let result = Sx_ref.eval_expr (List [Symbol "compile"; quoted]) (Env compile_env) in
+    (* Fast path: if compile has bytecode, call it directly via the VM.
+       All helper calls (compile-expr, emit-byte, etc.) happen inside the
+       same VM execution — no per-call VM allocation overhead. *)
+    let result = match compile_fn with
+    | Lambda { l_compiled = Some cl; _ } when not (is_jit_failed cl) ->
+      call_closure cl [fn_expr] globals
+    | _ ->
+      ignore compile_fn;
+      let compile_env = Sx_types.env_extend (Sx_types.make_env ()) in
+      Hashtbl.iter (fun k v -> Hashtbl.replace compile_env.bindings (Sx_types.intern k) v) globals;
+      Sx_ref.eval_expr (List [Symbol "compile"; quoted]) (Env compile_env)
+    in
     _jit_compiling := false;
     let effective_globals = globals in
     (match result with
-    | Dict d when Hashtbl.mem d "bytecode" ->
+    | Dict d when Hashtbl.mem d "bytecode" || Hashtbl.mem d "vc-bytecode" ->
       let outer_code = code_from_value result in
       let bc = outer_code.vc_bytecode in
       if Array.length bc >= 4 && bc.(0) = 51 (* OP_CLOSURE *) then begin