diff --git a/hosts/ocaml/bin/run_tests.ml b/hosts/ocaml/bin/run_tests.ml
index efa7ffde..90ca8d6b 100644
--- a/hosts/ocaml/bin/run_tests.ml
+++ b/hosts/ocaml/bin/run_tests.ml
@@ -1289,7 +1289,11 @@ let run_spec_tests env test_files =
       done;
       if is_suspended !s then begin
         let request = Sx_runtime.get_val !s (String "request") in
-        let op = match Sx_runtime.get_val request (String "op") with String s -> s | _ -> "" in
+        let req_list = match request with List l -> l | ListRef { contents = l } -> l | _ -> [] in
+        let op = match req_list with
+          | String o :: _ -> o
+          | Symbol o :: _ -> o
+          | _ -> (match Sx_runtime.get_val request (String "op") with String s -> s | _ -> "") in
         let response = match op with
           | "import" ->
             let lib_spec = Sx_runtime.get_val request (String "library") in
@@ -1321,7 +1325,22 @@ let run_spec_tests env test_files =
             Hashtbl.replace d "ascent" (Number (size *. 0.8));
             Hashtbl.replace d "descent" (Number (size *. 0.2));
             Dict d
-          | _ -> Nil  (* Other IO ops return nil in test context *)
+          | "io-sleep" | "io-wait" | "io-settle" | "io-wait-for" -> Nil
+          | "io-fetch" ->
+            let args = match req_list with _ :: rest -> rest | _ -> [] in
+            let format = match args with _ :: String f :: _ -> f | _ -> "text" in
+            (match format with
+             | "json" ->
+               let j = Hashtbl.create 2 in
+               Hashtbl.replace j "foo" (Number 1.0); Dict j
+             | "response" ->
+               let resp = Hashtbl.create 4 in
+               Hashtbl.replace resp "ok" (Bool true);
+               Hashtbl.replace resp "status" (Number 200.0);
+               Hashtbl.replace resp "text" (String "yay");
+               Dict resp
+             | _ -> String "yay")
+          | _ -> Nil
         in
         s := Sx_ref.cek_resume !s response;
         loop ()
@@ -2130,28 +2149,35 @@ let run_spec_tests env test_files =
     Hashtbl.replace mock_body "textContent" (String "");
     Nil);
 
-  (* IO suspension resolver — handles perform/wait/fetch in event handlers *)
+  (* IO suspension resolver — handles perform/wait/fetch in event handlers.
+     Requests come as Lists: ("op-name" arg1 arg2 ...) or Dicts with "op" key *)
   Sx_types._cek_io_resolver := Some (fun request _suspended ->
-    match request with
-    | Dict d ->
-      let op = match Hashtbl.find_opt d "op" with Some (String s) -> s | _ -> "" in
-      (match op with
-       | "io-sleep" | "io-wait" -> Nil  (* instant resume in tests *)
-       | "io-settle" -> Nil
-       | "io-fetch" ->
-         let url = match Hashtbl.find_opt d "args" with
-           | Some (List (String u :: _)) -> u | _ -> "" in
-         (* Return mock fetch response *)
+    let op, args = match request with
+      | List (String op :: rest) -> op, rest
+      | Dict d ->
+        let op = match Hashtbl.find_opt d "op" with Some (String s) -> s | _ -> "" in
+        let a = match Hashtbl.find_opt d "args" with Some (List l) -> l | _ -> [] in
+        op, a
+      | _ -> "", []
+    in
+    match op with
+    | "io-sleep" | "io-wait" | "io-settle" | "io-wait-for" -> Nil
+    | "io-fetch" ->
+      let url = match args with String u :: _ -> u | _ -> "" in
+      let format = match args with _ :: String f :: _ -> f | _ -> "text" in
+      let body = "yay" in
+      (match format with
+       | "json" ->
+         let j = Hashtbl.create 2 in
+         Hashtbl.replace j "foo" (Number 1.0); Dict j
+       | "response" ->
          let resp = Hashtbl.create 4 in
          Hashtbl.replace resp "ok" (Bool true);
          Hashtbl.replace resp "status" (Number 200.0);
          Hashtbl.replace resp "url" (String url);
-         Hashtbl.replace resp "text" (String (Printf.sprintf "{\"foo\":1}"));
-         Hashtbl.replace resp "json" (let j = Hashtbl.create 2 in Hashtbl.replace j "foo" (Number 1.0); Dict j);
-         Hashtbl.replace resp "headers" (Dict (Hashtbl.create 0));
+         Hashtbl.replace resp "text" (String body);
          Dict resp
-       | "io-wait-for" -> Nil  (* event wait — instant *)
-       | _ -> Nil)
+       | _ -> String body)
     | _ -> Nil);
 
   (* Load modules needed by tests *)
@@ -2391,6 +2417,33 @@ let run_spec_tests env test_files =
      code-from-value, and other late-bound functions must be visible. *)
   !_jit_refresh_globals ();
 
+  (* Rebind try-call to use eval_with_io for IO-aware test execution *)
+  ignore (Sx_types.env_bind env "try-call" (NativeFn ("try-call", fun args ->
+    match args with
+    | [thunk] ->
+      Sx_ref.step_limit := 0;
+      Sx_ref.step_count := 0;
+      (try
+        ignore (eval_with_io (List [thunk]) (Env env));
+        Sx_ref.step_limit := 0;
+        let d = Hashtbl.create 2 in
+        Hashtbl.replace d "ok" (Bool true);
+        Dict d
+      with
+      | Eval_error msg ->
+        Sx_ref.step_limit := 0;
+        let d = Hashtbl.create 2 in
+        Hashtbl.replace d "ok" (Bool false);
+        Hashtbl.replace d "error" (String msg);
+        Dict d
+      | exn ->
+        Sx_ref.step_limit := 0;
+        let d = Hashtbl.create 2 in
+        Hashtbl.replace d "ok" (Bool false);
+        Hashtbl.replace d "error" (String (Printexc.to_string exn));
+        Dict d)
+    | _ -> raise (Eval_error "try-call: expected 1 arg"))));
+
   List.iter (fun path ->
     if Sys.file_exists path then begin
       let name = Filename.basename path in