diff --git a/hosts/ocaml/bootstrap.py b/hosts/ocaml/bootstrap.py
index 9f04f7ae..e8df8b4e 100644
--- a/hosts/ocaml/bootstrap.py
+++ b/hosts/ocaml/bootstrap.py
@@ -82,7 +82,10 @@ let cek_run_iterative state =
       s := cek_step !s
     done;
     (match cek_suspended_p !s with
-     | Bool true -> raise (Eval_error "IO suspension in non-IO context")
+     | Bool true ->
+       (match !_cek_io_suspend_hook with
+        | Some hook -> hook !s
+        | None -> raise (Eval_error "IO suspension in non-IO context"))
      | _ -> cek_value !s)
   with Eval_error msg ->
     _last_error_kont_ref := cek_kont !s;
@@ -308,6 +311,23 @@ def compile_spec_to_ml(spec_dir: str | None = None) -> str:
         output
     )
 
+    # Patch transpiled cek_run to invoke _cek_io_suspend_hook on suspension
+    # instead of unconditionally raising Eval_error. This is the fix for the
+    # tree-walk eval_expr path: sf_letrec init exprs / non-last body exprs,
+    # macro bodies, qq_expand, dynamic-wind / scope / provide bodies all use
+    # `trampoline (eval_expr ...)` and were swallowing CEK suspensions as
+    # "IO suspension in non-IO context" errors. With the hook, the suspension
+    # propagates as VmSuspended to the outer driver (browser callFn / server
+    # eval_expr_io). When the hook is unset (pure-CEK harness), the legacy
+    # error is preserved as the fallback.
+    output = re.sub(
+        r'\(raise \(Eval_error \(value_to_str \(String "IO suspension in non-IO context"\)\)\)\)',
+        '(match !_cek_io_suspend_hook with Some hook -> hook final | None -> '
+        '(raise (Eval_error (value_to_str (String "IO suspension in non-IO context")))))',
+        output,
+        count=1,
+    )
+
     return output
 
 
diff --git a/hosts/ocaml/browser/sx_browser.ml b/hosts/ocaml/browser/sx_browser.ml
index 2abbbf17..60511546 100644
--- a/hosts/ocaml/browser/sx_browser.ml
+++ b/hosts/ocaml/browser/sx_browser.ml
@@ -344,6 +344,12 @@ let api_eval src_js =
     sync_env_to_vm ();
     return_via_side_channel (value_to_js result)
   with
+  | Sx_vm.VmSuspended _ ->
+    (* Top-level eval encountered an IO suspension propagated via the
+       cek_run hook (perform inside letrec init / non-last body / macro /
+       qq tree-walked path). K.eval doesn't drive resumption — surface as
+       a clear error so the caller knows to use callFn instead. *)
+    Js.Unsafe.inject (Js.string "Error: IO suspension in non-IO context (use callFn for IO-aware paths)")
   | Eval_error msg -> Js.Unsafe.inject (Js.string ("Error: " ^ msg))
   | Parse_error msg -> Js.Unsafe.inject (Js.string ("Parse error: " ^ msg))
 
@@ -371,6 +377,8 @@ let api_eval_vm src_js =
     ) _vm_globals;
     return_via_side_channel (value_to_js result)
   with
+  | Sx_vm.VmSuspended _ ->
+    Js.Unsafe.inject (Js.string "Error: IO suspension in non-IO context (use callFn for IO-aware paths)")
   | Eval_error msg -> Js.Unsafe.inject (Js.string ("Error: " ^ msg))
   | Parse_error msg -> Js.Unsafe.inject (Js.string ("Parse error: " ^ msg))
   | Not_found -> Js.Unsafe.inject (Js.string "Error: compile-module not loaded")
@@ -381,7 +389,10 @@ let api_eval_expr expr_js _env_js =
     let result = Sx_ref.eval_expr expr (Env global_env) in
     sync_env_to_vm ();
     return_via_side_channel (value_to_js result)
-  with Eval_error msg ->
+  with
+  | Sx_vm.VmSuspended _ ->
+    Js.Unsafe.inject (Js.string "Error: IO suspension in non-IO context (use callFn for IO-aware paths)")
+  | Eval_error msg ->
     Js.Unsafe.inject (Js.string ("Error: " ^ msg))
 
 let api_load src_js =
diff --git a/hosts/ocaml/browser/test_letrec_resume.js b/hosts/ocaml/browser/test_letrec_resume.js
new file mode 100644
index 00000000..550355cf
--- /dev/null
+++ b/hosts/ocaml/browser/test_letrec_resume.js
@@ -0,0 +1,172 @@
+#!/usr/bin/env node
+// Repro: letrec sibling bindings nil after perform/resume in browser kernel
+//
+// Bug: After a CEK IO suspension (perform / hs-wait) resumes in the
+// WASM browser kernel, calling a sibling letrec binding could return
+// nil, with the error surfaced as `[sx] resume: Not callable: nil`.
+//
+// Root cause: cek-run / cek_run_iterative raised
+// `"IO suspension in non-IO context"` when a tree-walked eval_expr
+// (sf_letrec init exprs / non-last body, macro body, qq unquote, scope
+// body, provide body, dynamic-wind) hit a perform. The CEK suspension
+// was created correctly but never propagated through the OCaml-side
+// _cek_io_suspend_hook, so the outer driver never saw VmSuspended.
+//
+// Fix: cek_run / cek_run_iterative now invoke _cek_io_suspend_hook on
+// suspension (raising VmSuspended for the outer driver). When the hook
+// is unset (pure-CEK harness), they fall back to the legacy error.
+//
+// This test exercises the WASM kernel through K.callFn — the path that
+// browser event handlers use. Suspension surfaces as a JS object with
+// {suspended, request, resume(result)} that the test drives synchronously.
+//
+// Companion: spec/tests/test-letrec-resume-treewalk.sx tests the
+// CEK-only path through the OCaml test runner.
+
+const path = require('path');
+const fs = require('fs');
+
+const KERNEL = path.join(__dirname, '..', '_build', 'default', 'browser', 'sx_browser.bc.js');
+if (!fs.existsSync(KERNEL)) {
+  console.error('FATAL: missing ' + KERNEL + ' — run `dune build` from hosts/ocaml first');
+  process.exit(2);
+}
+require(KERNEL);
+const K = globalThis.SxKernel;
+
+let passed = 0, failed = 0;
+const failures = [];
+
+function test(name, fn) {
+  try {
+    const r = fn();
+    if (r === true) {
+      passed++;
+      console.log('  PASS: ' + name);
+    } else {
+      failed++;
+      failures.push({ name, error: 'got ' + JSON.stringify(r) });
+      console.log('  FAIL: ' + name + ' — got ' + JSON.stringify(r));
+    }
+  } catch (e) {
+    failed++;
+    failures.push({ name, error: e.message || String(e) });
+    console.log('  FAIL: ' + name + ' — ' + (e.message || e));
+  }
+}
+
+function driveSync(result) {
+  while (result && typeof result === 'object' && result.suspended) {
+    result = result.resume(null);
+  }
+  return result;
+}
+
+function callExpr(src) {
+  K.eval('(define _t-fn (fn () ' + src + '))');
+  const fn = K.eval('_t-fn');
+  return driveSync(K.callFn(fn, []));
+}
+
+console.log('\n=== letrec + perform/resume regression tests ===\n');
+
+test('basic letrec without perform', () =>
+  callExpr('(letrec ((f (fn () "ok"))) (f))') === 'ok');
+
+test('callFn perform suspends and resumes with nil', () => {
+  K.eval('(define _t-perform (fn () (perform {:op "io"})))');
+  let r = K.callFn(K.eval('_t-perform'), []);
+  if (!r || !r.suspended) return 'no suspension: ' + JSON.stringify(r);
+  return r.resume(null) === null;
+});
+
+test('letrec, single binding, perform/resume', () =>
+  callExpr('(letrec ((f (fn () (perform {:op "io"})))) (f))') === null);
+
+test('letrec, 2 bindings, body calls sibling after suspended call', () =>
+  callExpr(`
+    (letrec
+      ((wait-then (fn () (do (perform {:op "io"}) "wait-done")))
+       (other-fn (fn () "other-result")))
+      (do (wait-then) (other-fn)))`) === 'other-result');
+
+test('letrec, suspending fn calls sibling after own perform', () =>
+  callExpr(`
+    (letrec
+      ((wait-and-call (fn () (do (perform {:op "io"}) (other-fn))))
+       (other-fn (fn () "from-sibling")))
+      (wait-and-call))`) === 'from-sibling');
+
+test('letrec, fn references sibling value after perform/resume', () =>
+  callExpr(`
+    (letrec
+      ((shared "shared-state")
+       (do-fn (fn () (do (perform {:op "io"}) shared))))
+      (do-fn))`) === 'shared-state');
+
+test('letrec, recursive self-call after perform (wait-boot pattern)', () => {
+  K.eval('(define _wb-c 0)');
+  K.eval('(set! _wb-c 0)');
+  return callExpr(`
+    (letrec ((wait-boot (fn ()
+                (do (perform {:op "io"})
+                    (if (>= _wb-c 1)
+                        "done"
+                        (do (set! _wb-c (+ 1 _wb-c))
+                            (wait-boot)))))))
+      (wait-boot))`) === 'done';
+});
+
+test('top-level define + perform + sibling call after resume', () => {
+  K.eval('(define do-suspend-x (fn () (do (perform {:op "io"}) (do-other-x))))');
+  K.eval('(define do-other-x (fn () "ok-from-other"))');
+  return callExpr('(do-suspend-x)') === 'ok-from-other';
+});
+
+test('letrec, two performs (sequential) then sibling call', () =>
+  callExpr(`
+    (letrec
+      ((wait-twice (fn () (do (perform {:op "io1"}) (perform {:op "io2"}) (other))))
+       (other (fn () "after-double")))
+      (wait-twice))`) === 'after-double');
+
+// === Tree-walk paths that previously raised "IO suspension in non-IO context" ===
+
+test('letrec init expr with perform — suspension propagates (no error)', () => {
+  let r;
+  try { r = callExpr('(letrec ((x (perform {:op "io"}))) "ok")'); }
+  catch (e) { return 'threw: ' + e.message; }
+  return r === null || r === 'ok';
+});
+
+test('letrec non-last body with perform — suspension propagates (no error)', () => {
+  let r;
+  try { r = callExpr('(letrec ((x 1)) (perform {:op "io"}) "after")'); }
+  catch (e) { return 'threw: ' + e.message; }
+  return r === null || r === 'after';
+});
+
+test('macro body with perform — suspension propagates', () => {
+  K.eval('(defmacro _m1 (form) (do (perform {:op "io"}) form))');
+  let r;
+  try { r = callExpr('(_m1 "macro-ok")'); }
+  catch (e) { return 'threw: ' + e.message; }
+  return r === 'macro-ok' || r === null;
+});
+
+test('quasiquote unquote with perform — suspension propagates', () => {
+  let r;
+  try { r = callExpr('(let ((y "yyy")) `(a ,(do (perform {:op "io"}) y) c))'); }
+  catch (e) { return 'threw: ' + e.message; }
+  return r !== undefined;
+});
+
+console.log('\n--- Results ---');
+console.log('passed: ' + passed);
+console.log('failed: ' + failed);
+if (failed > 0) {
+  console.log('\nFailures:');
+  failures.forEach(f => console.log('  - ' + f.name + ': ' + f.error));
+  process.exit(1);
+}
+process.exit(0);
diff --git a/hosts/ocaml/lib/sx_ref.ml b/hosts/ocaml/lib/sx_ref.ml
index e71a889e..2b12cc22 100644
--- a/hosts/ocaml/lib/sx_ref.ml
+++ b/hosts/ocaml/lib/sx_ref.ml
@@ -614,7 +614,7 @@ and cek_step_loop state =
 
 (* cek-run *)
 and cek_run state =
-  (let final = (cek_step_loop (state)) in (if sx_truthy ((cek_suspended_p (final))) then (raise (Eval_error (value_to_str (String "IO suspension in non-IO context")))) else (cek_value (final))))
+  (let final = (cek_step_loop (state)) in (if sx_truthy ((cek_suspended_p (final))) then (match !_cek_io_suspend_hook with Some hook -> hook final | None -> (raise (Eval_error (value_to_str (String "IO suspension in non-IO context"))))) else (cek_value (final))))
 
 (* cek-resume *)
 and cek_resume suspended_state result' =
@@ -1052,7 +1052,14 @@ let cek_run_iterative state =
       s := cek_step !s
     done;
     (match cek_suspended_p !s with
-     | Bool true -> raise (Eval_error "IO suspension in non-IO context")
+     | Bool true ->
+       (* Propagate suspension via the OCaml-side hook so it converts to
+          VmSuspended and flows to the outer driver (value_to_js / resume
+          callback). Without the hook (pure CEK harness), keep the legacy
+          error so test runners surface the misuse. *)
+       (match !_cek_io_suspend_hook with
+        | Some hook -> hook !s
+        | None -> raise (Eval_error "IO suspension in non-IO context"))
      | _ -> cek_value !s)
   with Eval_error msg ->
     _last_error_kont_ref := cek_kont !s;
diff --git a/spec/tests/test-letrec-resume-treewalk.sx b/spec/tests/test-letrec-resume-treewalk.sx
new file mode 100644
index 00000000..816856d5
--- /dev/null
+++ b/spec/tests/test-letrec-resume-treewalk.sx
@@ -0,0 +1,61 @@
+;; Letrec + perform/resume — tree-walk paths regression
+;;
+;; Before the fix: cek-run raised "IO suspension in non-IO context" when
+;; a perform fired inside any tree-walked eval_expr path:
+;;   - sf_letrec init exprs / non-last body exprs (the original bug)
+;;   - sf_dynamic_wind, sf_scope, sf_provide bodies
+;;   - expand_macro body, qq_expand unquote
+;;
+;; After the fix: cek-run invokes _cek_io_suspend_hook which (in this test
+;; runner) resolves the IO and returns the result, allowing evaluation to
+;; complete successfully. End-to-end: the expression returns a value
+;; instead of throwing the swallowed-suspension error.
+;;
+;; The test runner's hook resolves IO requests via resolve_io (mock).
+;; In production browser, the hook raises VmSuspended → JS resume callback.
+
+(defsuite
+  "letrec-resume-treewalk-init"
+  (deftest
+    "perform in letrec init expr — evaluates without error"
+    (let
+      ((result (eval-expr-cek (quote (letrec ((x (perform {:op "request-arg" :args (list "k")}))) (or x "default"))) (make-env))))
+      (assert (or (string? result) (nil? result)))))
+  (deftest
+    "perform in letrec non-last body — evaluates without error"
+    (let
+      ((result (eval-expr-cek (quote (letrec ((x 42)) (perform {:op "request-arg" :args (list "k")}) (+ x 1))) (make-env))))
+      (assert= result 43)))
+  (deftest
+    "letrec init perform result reachable — no nil sibling"
+    (let
+      ((result (eval-expr-cek (quote (letrec ((x 99) (y (fn () x))) (perform {:op "request-arg" :args (list "k")}) (y))) (make-env))))
+      (assert= result 99))))
+
+
+(defsuite
+  "letrec-resume-treewalk-bodies"
+  (deftest
+    "perform inside scope body — evaluates without error"
+    (let
+      ((result (eval-expr-cek (quote (scope "test-scope" :value 1 (perform {:op "request-arg" :args (list "k")}) "scope-done")) (make-env))))
+      (assert= result "scope-done")))
+  (deftest
+    "perform inside provide body — evaluates without error"
+    (let
+      ((result (eval-expr-cek (quote (provide "test-key" 7 (perform {:op "request-arg" :args (list "k")}) "provide-done")) (make-env))))
+      (assert= result "provide-done"))))
+
+
+(defsuite
+  "letrec-resume-treewalk-recursive"
+  (deftest
+    "letrec sibling fn called after perform/resume — sibling not nil"
+    (let
+      ((result (eval-expr-cek (quote (letrec ((sib "from-sibling") (suspending (fn () (do (perform {:op "request-arg" :args (list "k")}) sib)))) (suspending))) (make-env))))
+      (assert= result "from-sibling")))
+  (deftest
+    "letrec — suspending fn calls sibling fn after own perform"
+    (let
+      ((result (eval-expr-cek (quote (letrec ((wait-and-call (fn () (do (perform {:op "request-arg" :args (list "k")}) (other)))) (other (fn () "called-after-resume"))) (wait-and-call))) (make-env))))
+      (assert= result "called-after-resume"))))