diff --git a/lib/hyperscript/runtime.sx b/lib/hyperscript/runtime.sx
index 8b71b93e..06b5b3a4 100644
--- a/lib/hyperscript/runtime.sx
+++ b/lib/hyperscript/runtime.sx
@@ -172,7 +172,7 @@
     (n thunk)
     (define
       do-repeat
-      (fn (i) (when (< i n) (thunk) (do-repeat (+ i 1)))))
+      (fn (i) (when (< i n) (do (thunk) (do-repeat (+ i 1))))))
     (do-repeat 0)))
 
 ;; Repeat forever (until break — relies on exception/continuation).
diff --git a/shared/static/wasm/sx/hs-runtime.sx b/shared/static/wasm/sx/hs-runtime.sx
new file mode 100644
index 00000000..c277c351
--- /dev/null
+++ b/shared/static/wasm/sx/hs-runtime.sx
@@ -0,0 +1,316 @@
+;; _hyperscript runtime shims
+;;
+;; Thin wrappers over web/lib/dom.sx and web/lib/browser.sx primitives
+;; that implement hyperscript-specific semantics (async transparency,
+;; class toggling, event waiting, iteration, type coercion).
+;;
+;; These are the functions that hs-to-sx (compiler.sx) emits calls to.
+;; Each is a pure define — no platform dependency beyond the DOM/browser
+;; primitives already available in the SX web platform.
+
+;; ── Event handling ──────────────────────────────────────────────
+
+;; Register an event listener. Returns unlisten function.
+;; (hs-on target event-name handler) → unlisten-fn
+(define
+  hs-on
+  (fn (target event-name handler) (dom-listen target event-name handler)))
+
+;; Register for every occurrence (no queuing — each fires independently).
+;; Stock hyperscript queues by default; "every" disables queuing.
+(define
+  hs-on-every
+  (fn (target event-name handler) (dom-listen target event-name handler)))
+
+;; Run an initializer function immediately.
+;; (hs-init thunk) — called at element boot time
+(define hs-init (fn (thunk) (thunk)))
+
+;; ── Async / timing ──────────────────────────────────────────────
+
+;; Wait for a duration in milliseconds.
+;; In hyperscript, wait is async-transparent — execution pauses.
+;; Here we use perform/IO suspension for true pause semantics.
+(define hs-wait (fn (ms) (perform (list (quote io-sleep) ms))))
+
+;; Wait for a DOM event on a target.
+;; (hs-wait-for target event-name) — suspends until event fires
+(define
+  hs-wait-for
+  (fn
+    (target event-name)
+    (perform (list (quote io-wait-event) target event-name))))
+
+;; Wait for CSS transitions/animations to settle on an element.
+(define hs-settle (fn (target) (perform (list (quote io-settle) target))))
+
+;; ── Class manipulation ──────────────────────────────────────────
+
+;; Toggle a single class on an element.
+(define
+  hs-toggle-class!
+  (fn
+    (target cls)
+    (if
+      (dom-has-class? target cls)
+      (dom-remove-class target cls)
+      (dom-add-class target cls))))
+
+;; Toggle between two classes — exactly one is active at a time.
+(define
+  hs-toggle-between!
+  (fn
+    (target cls1 cls2)
+    (if
+      (dom-has-class? target cls1)
+      (do (dom-remove-class target cls1) (dom-add-class target cls2))
+      (do (dom-remove-class target cls2) (dom-add-class target cls1)))))
+
+;; Take a class from siblings — add to target, remove from others.
+;; (hs-take! target cls) — like radio button class behavior
+(define
+  hs-take!
+  (fn
+    (target cls)
+    (let
+      ((parent (dom-parent target)))
+      (when
+        parent
+        (for-each
+          (fn (child) (dom-remove-class child cls))
+          (dom-child-list parent)))
+      (dom-add-class target cls))))
+
+;; ── DOM insertion ───────────────────────────────────────────────
+
+;; Put content at a position relative to a target.
+;; pos: "into" | "before" | "after"
+(define
+  hs-put!
+  (fn
+    (value pos target)
+    (cond
+      ((= pos "into") (dom-set-inner-html target value))
+      ((= pos "before")
+        (dom-insert-adjacent-html target "beforebegin" value))
+      ((= pos "after") (dom-insert-adjacent-html target "afterend" value)))))
+
+;; ── Navigation / traversal ──────────────────────────────────────
+
+;; Navigate to a URL.
+(define hs-navigate! (fn (url) (perform (list (quote io-navigate) url))))
+
+;; Find next sibling matching a selector (or any sibling).
+(define
+  hs-next
+  (fn
+    (target sel)
+    (if
+      (= sel "*")
+      (dom-next-sibling target)
+      (let
+        ((sibling (dom-next-sibling target)))
+        (define
+          find-next
+          (fn
+            (el)
+            (cond
+              ((nil? el) nil)
+              ((dom-matches? el sel) el)
+              (true (find-next (dom-next-sibling el))))))
+        (find-next sibling)))))
+
+;; Find previous sibling matching a selector.
+(define
+  hs-previous
+  (fn
+    (target sel)
+    (if
+      (= sel "*")
+      (dom-get-prop target "previousElementSibling")
+      (let
+        ((sibling (dom-get-prop target "previousElementSibling")))
+        (define
+          find-prev
+          (fn
+            (el)
+            (cond
+              ((nil? el) nil)
+              ((dom-matches? el sel) el)
+              (true (find-prev (dom-get-prop el "previousElementSibling"))))))
+        (find-prev sibling)))))
+
+;; First element matching selector within a scope.
+(define hs-query-first (fn (sel) (dom-query sel)))
+
+;; Last element matching selector.
+(define
+  hs-query-last
+  (fn
+    (sel)
+    (let
+      ((all (dom-query-all (dom-body) sel)))
+      (if (> (len all) 0) (nth all (- (len all) 1)) nil))))
+
+;; First/last within a specific scope.
+(define hs-first (fn (scope sel) (dom-query-all scope sel)))
+
+(define
+  hs-last
+  (fn
+    (scope sel)
+    (let
+      ((all (dom-query-all scope sel)))
+      (if (> (len all) 0) (nth all (- (len all) 1)) nil))))
+
+;; ── Iteration ───────────────────────────────────────────────────
+
+;; Repeat a thunk N times.
+(define
+  hs-repeat-times
+  (fn
+    (n thunk)
+    (define
+      do-repeat
+      (fn (i) (when (< i n) (do (thunk) (do-repeat (+ i 1))))))
+    (do-repeat 0)))
+
+;; Repeat forever (until break — relies on exception/continuation).
+(define
+  hs-repeat-forever
+  (fn
+    (thunk)
+    (define do-forever (fn () (thunk) (do-forever)))
+    (do-forever)))
+
+;; ── Fetch ───────────────────────────────────────────────────────
+
+;; Fetch a URL, parse response according to format.
+;; (hs-fetch url format) — format is "json" | "text" | "html"
+(define
+  hs-fetch
+  (fn
+    (url format)
+    (let
+      ((response (perform (list (quote io-fetch) url))))
+      (cond
+        ((= format "json") (perform (list (quote io-parse-json) response)))
+        ((= format "text") (perform (list (quote io-parse-text) response)))
+        ((= format "html") (perform (list (quote io-parse-html) response)))
+        (true response)))))
+
+;; ── Type coercion ───────────────────────────────────────────────
+
+;; Coerce a value to a type by name.
+;; (hs-coerce value type-name) — type-name is "Int", "Float", "String", etc.
+(define
+  hs-coerce
+  (fn
+    (value type-name)
+    (cond
+      ((= type-name "Int") (floor (+ value 0)))
+      ((= type-name "Integer") (floor (+ value 0)))
+      ((= type-name "Float") (+ value 0))
+      ((= type-name "Number") (+ value 0))
+      ((= type-name "String") (str value))
+      ((= type-name "Boolean") (if value true false))
+      ((= type-name "Array") (if (list? value) value (list value)))
+      (true value))))
+
+;; ── Object creation ─────────────────────────────────────────────
+
+;; Make a new object of a given type.
+;; (hs-make type-name) — creates empty object/collection
+(define
+  hs-add
+  (fn (a b) (if (or (string? a) (string? b)) (str a b) (+ a b))))
+
+;; ── Behavior installation ───────────────────────────────────────
+
+;; Install a behavior on an element.
+;; A behavior is a function that takes (me ...params) and sets up features.
+;; (hs-install behavior-fn me ...args)
+(define
+  hs-make
+  (fn
+    (type-name)
+    (cond
+      ((= type-name "Object") (dict))
+      ((= type-name "Array") (list))
+      ((= type-name "Set") (list))
+      ((= type-name "Map") (dict))
+      (true (dict)))))
+
+;; ── Measurement ─────────────────────────────────────────────────
+
+;; Measure an element's bounding rect, store as local variables.
+;; Returns a dict with x, y, width, height, top, left, right, bottom.
+(define hs-install (fn (behavior-fn) (behavior-fn me)))
+
+;; ── Transition ──────────────────────────────────────────────────
+
+;; Transition a CSS property to a value, optionally with duration.
+;; (hs-transition target prop value duration)
+(define
+  hs-measure
+  (fn (target) (perform (list (quote io-measure) target))))
+
+(define
+  hs-transition
+  (fn
+    (target prop value duration)
+    (when
+      duration
+      (dom-set-style
+        target
+        "transition"
+        (str prop " " (/ duration 1000) "s")))
+    (dom-set-style target prop value)
+    (when duration (hs-settle target))))
+
+(define
+  hs-type-check
+  (fn
+    (value type-name)
+    (if
+      (nil? value)
+      true
+      (cond
+        ((= type-name "Number") (number? value))
+        ((= type-name "String") (string? value))
+        ((= type-name "Boolean") (or (= value true) (= value false)))
+        ((= type-name "Array") (list? value))
+        ((= type-name "Object") (dict? value))
+        (true true)))))
+
+(define
+  hs-type-check!
+  (fn
+    (value type-name)
+    (if (nil? value) false (hs-type-check value type-name))))
+
+(define
+  hs-strict-eq
+  (fn (a b) (and (= (type-of a) (type-of b)) (= a b))))
+
+(define
+  hs-falsy?
+  (fn (v) (or (nil? v) (= v false) (and (string? v) (= v "")))))
+
+(define
+  hs-matches?
+  (fn
+    (target pattern)
+    (if
+      (string? target)
+      (if (= pattern ".*") true (string-contains? target pattern))
+      false)))
+
+(define
+  hs-contains?
+  (fn
+    (collection item)
+    (cond
+      ((list? collection) (some (fn (x) (= x item)) collection))
+      ((string? collection) (string-contains? collection item))
+      (true false))))
\ No newline at end of file
diff --git a/tests/playwright/sx-inspect.js b/tests/playwright/sx-inspect.js
index 93c7dab9..c0fe7aa7 100644
--- a/tests/playwright/sx-inspect.js
+++ b/tests/playwright/sx-inspect.js
@@ -1296,6 +1296,339 @@ async function modeEvalAt(browser, url, phase, expr) {
   return { url, phase, expr, result: evalResult, bootLog: bootLogs };
 }
 
+// ---------------------------------------------------------------------------
+// Mode: sandbox — offline WASM kernel in a blank page, no server needed
+//
+// General-purpose browser test environment. Injects the real WASM kernel
+// into about:blank with full FFI, IO suspension, and DOM.
+//
+// Predefined stacks (via stack param):
+//   "core"  — kernel + FFI only (default)
+//   "web"   — full web stack (render, signals, DOM, adapters, boot)
+//   "hs"    — web + hyperscript modules
+//   "test"  — web + test framework
+//
+// Custom files loaded after the stack. Setup expression runs before eval.
+// All IO suspensions are traced. Results formatted via SX inspect.
+// ---------------------------------------------------------------------------
+
+const SANDBOX_STACKS = {
+  // Paths relative to shared/static/wasm/sx/
+  web: [
+    'render', 'core-signals', 'signals', 'deps', 'router',
+    'page-helpers', 'freeze', 'dom', 'browser',
+    'adapter-html', 'adapter-sx', 'adapter-dom',
+    'boot-helpers', 'hypersx', 'engine', 'orchestration', 'boot',
+  ],
+  hs: [
+    // web stack first, then hyperscript from lib/
+    '_web_',
+    'hs-tokenizer', 'hs-parser', 'hs-compiler', 'hs-runtime', 'hs-integration',
+  ],
+  test: [
+    '_web_',
+    'harness', 'harness-reactive', 'harness-web',
+  ],
+};
+
+async function modeSandbox(page, expr, files, setup, stack) {
+  const fs = require('fs');
+  const path = require('path');
+  const PROJECT_ROOT = path.resolve(__dirname, '../..');
+  const WASM_DIR = path.join(PROJECT_ROOT, 'shared/static/wasm');
+  const SX_DIR = path.join(WASM_DIR, 'sx');
+
+  const consoleLogs = [];
+  page.on('console', msg => {
+    consoleLogs.push({ type: msg.type(), text: msg.text().slice(0, 500) });
+  });
+
+  // 1. Navigate to blank page
+  await page.goto('about:blank');
+
+  // 2. Inject WASM kernel
+  const kernelSrc = fs.readFileSync(path.join(WASM_DIR, 'sx_browser.bc.js'), 'utf8');
+  await page.addScriptTag({ content: kernelSrc });
+  await page.waitForFunction('!!window.SxKernel', { timeout: 10000 });
+
+  // 3. Register FFI primitives + IO suspension driver
+  await page.evaluate(() => {
+    const K = window.SxKernel;
+
+    K.registerNative('host-global', args => {
+      const n = args[0];
+      return (n in globalThis) ? globalThis[n] : null;
+    });
+    K.registerNative('host-get', args => {
+      if (args[0] == null) return null;
+      const v = args[0][args[1]];
+      return v === undefined ? null : v;
+    });
+    K.registerNative('host-set!', args => {
+      if (args[0] != null) args[0][args[1]] = args[2];
+      return args[2];
+    });
+    K.registerNative('host-call', args => {
+      const [obj, method, ...rest] = args;
+      if (obj == null) {
+        const fn = globalThis[method];
+        if (typeof fn === 'function') return fn.apply(null, rest);
+        return null;
+      }
+      if (typeof obj[method] !== 'function') return null;
+      try { const r = obj[method].apply(obj, rest); return r === undefined ? null : r; }
+      catch(e) { console.error('[sandbox] host-call error:', method, e.message); return null; }
+    });
+    K.registerNative('host-new', args => {
+      const name = args[0];
+      const cArgs = args.slice(1);
+      const Ctor = typeof name === 'string' ? globalThis[name] : name;
+      if (typeof Ctor !== 'function') return null;
+      return new Ctor(...cArgs);
+    });
+    K.registerNative('host-callback', args => {
+      const fn = args[0];
+      if (typeof fn === 'function' && fn.__sx_handle === undefined) return fn;
+      if (fn && fn.__sx_handle !== undefined) {
+        return function() {
+          const r = K.callFn(fn, Array.from(arguments));
+          window._driveAsync(r);
+          return r;
+        };
+      }
+      return function() {};
+    });
+    K.registerNative('host-typeof', args => {
+      const obj = args[0];
+      if (obj == null) return 'nil';
+      if (obj instanceof Element) return 'element';
+      if (obj instanceof Text) return 'text';
+      if (obj instanceof DocumentFragment) return 'fragment';
+      if (obj instanceof Document) return 'document';
+      if (obj instanceof Event) return 'event';
+      if (obj instanceof Promise) return 'promise';
+      return typeof obj;
+    });
+    K.registerNative('host-await', args => {
+      const [promise, callback] = args;
+      if (promise && typeof promise.then === 'function') {
+        const cb = (callback && callback.__sx_handle !== undefined)
+          ? v => K.callFn(callback, [v])
+          : () => {};
+        promise.then(cb);
+      }
+    });
+    K.registerNative('load-library!', args => false);
+
+    // IO suspension driver — traces every suspension/resume
+    // _asyncPending tracks in-flight IO chains so the sandbox can wait for completion
+    window._ioTrace = [];
+    window._asyncPending = 0;
+    window._driveAsync = function driveAsync(result) {
+      if (!result || !result.suspended) { return; }
+      window._asyncPending++;
+      const req = result.request;
+      const items = req && (req.items || req);
+      const op = items && items[0];
+      const opName = typeof op === 'string' ? op : (op && op.name) || String(op);
+      const arg = items && items[1];
+      window._ioTrace.push({ op: opName, arg: arg });
+
+      function doResume(val, delay) {
+        setTimeout(() => {
+          try {
+            const resumed = result.resume(val);
+            window._asyncPending--;
+            driveAsync(resumed);
+          } catch(e) {
+            window._asyncPending--;
+            console.error('[sandbox] resume error:', e.message);
+            window._ioTrace.push({ op: 'ERROR', arg: e.message });
+          }
+        }, delay);
+      }
+
+      if (opName === 'io-sleep' || opName === 'wait') {
+        doResume(null, Math.min(typeof arg === 'number' ? arg : 0, 10));
+      } else if (opName === 'io-navigate') {
+        window._asyncPending--;
+      } else if (opName === 'io-fetch') {
+        doResume({ ok: true, text: '' }, 1);
+      } else {
+        window._asyncPending--;
+        console.warn('[sandbox] unhandled IO:', opName);
+        window._ioTrace.push({ op: 'unhandled', arg: opName });
+      }
+    };
+
+    K.eval('(define SX_VERSION "sandbox-1.0")');
+    K.eval('(define SX_ENGINE "ocaml-vm-sandbox")');
+    K.eval('(define parse sx-parse)');
+    K.eval('(define serialize sx-serialize)');
+  });
+
+  // 4. Load stack modules
+  const loadErrors = [];
+  const loadedModules = [];
+
+  function resolveStack(name) {
+    const mods = SANDBOX_STACKS[name] || [];
+    const result = [];
+    for (const m of mods) {
+      if (m === '_web_') result.push(...resolveStack('web'));
+      else result.push(m);
+    }
+    return result;
+  }
+
+  if (stack && stack !== 'core') {
+    const modules = resolveStack(stack);
+    if (modules.length > 0) {
+      // beginModuleLoad if available
+      await page.evaluate(() => {
+        if (window.SxKernel.beginModuleLoad) window.SxKernel.beginModuleLoad();
+      });
+      for (const mod of modules) {
+        // Try .sx file in wasm/sx/ dir
+        const sxPath = path.join(SX_DIR, mod + '.sx');
+        // Also try lib/hyperscript/ for hs- prefixed modules
+        const libPath = path.join(PROJECT_ROOT, 'lib/hyperscript', mod.replace(/^hs-/, '') + '.sx');
+        let src;
+        try {
+          src = fs.existsSync(sxPath)
+            ? fs.readFileSync(sxPath, 'utf8')
+            : fs.readFileSync(libPath, 'utf8');
+        } catch(e) {
+          loadErrors.push(mod + ': file not found');
+          continue;
+        }
+        const err = await page.evaluate(src => {
+          try { window.SxKernel.load(src); return null; }
+          catch(e) { return e.message; }
+        }, src);
+        if (err) loadErrors.push(mod + ': ' + err);
+        else loadedModules.push(mod);
+      }
+      await page.evaluate(() => {
+        if (window.SxKernel.endModuleLoad) window.SxKernel.endModuleLoad();
+      });
+    }
+  }
+
+  // 5. Load custom files
+  for (const f of files) {
+    const abs = path.isAbsolute(f) ? f : path.join(PROJECT_ROOT, f);
+    try {
+      const src = fs.readFileSync(abs, 'utf8');
+      const err = await page.evaluate(src => {
+        try { window.SxKernel.load(src); return null; }
+        catch(e) { return e.message; }
+      }, src);
+      if (err) loadErrors.push(f + ': ' + err);
+      else loadedModules.push(path.basename(f));
+    } catch(e) {
+      loadErrors.push(f + ': ' + e.message);
+    }
+  }
+
+  // 6. Run setup expression
+  if (setup) {
+    const setupErr = await page.evaluate(s => {
+      try { window.SxKernel.eval(s); return null; }
+      catch(e) { return e.message; }
+    }, setup);
+    if (setupErr) loadErrors.push('setup: ' + setupErr);
+  }
+
+  // 7. Evaluate — wraps in thunk + callFn for IO suspension support.
+  //    After synchronous eval, waits for all _driveAsync chains to settle.
+  const evalResult = await page.evaluate(expr => {
+    return new Promise(resolve => {
+      const K = window.SxKernel;
+      window._ioTrace = [];
+      window._asyncPending = 0;
+      let syncResult;
+      try {
+        K.eval('(define _sandbox_thunk (fn () ' + expr + '))');
+        syncResult = K.callFn(K.eval('_sandbox_thunk'), []);
+      } catch(e) {
+        resolve({ result: 'Error: ' + e.message, ioTrace: window._ioTrace });
+        return;
+      }
+
+      // If the thunk itself suspended (direct perform), drive it inline
+      if (syncResult && syncResult.suspended) {
+        function tracingDrive(r) {
+          if (!r || !r.suspended) {
+            waitAndResolve(r);
+            return;
+          }
+          const req = r.request;
+          const items = req && (req.items || req);
+          const op = items && items[0];
+          const opName = typeof op === 'string' ? op : (op && op.name) || String(op);
+          const arg = items && items[1];
+          window._ioTrace.push({ op: opName, arg: arg });
+          if (opName === 'io-sleep' || opName === 'wait') {
+            setTimeout(() => {
+              try { tracingDrive(r.resume(null)); }
+              catch(e) { resolve({ result: 'Error: ' + e.message, ioTrace: window._ioTrace }); }
+            }, Math.min(typeof arg === 'number' ? arg : 0, 10));
+          } else if (opName === 'io-fetch') {
+            setTimeout(() => {
+              try { tracingDrive(r.resume({ ok: true, text: '' })); }
+              catch(e) { resolve({ result: 'Error: ' + e.message, ioTrace: window._ioTrace }); }
+            }, 1);
+          } else {
+            resolve({ result: 'unhandled IO: ' + opName, ioTrace: window._ioTrace });
+          }
+        }
+        tracingDrive(syncResult);
+        return;
+      }
+
+      // Synchronous result — but there may be pending _driveAsync chains
+      // (e.g. from host-callback event handlers triggered during eval)
+      waitAndResolve(syncResult);
+
+      function waitAndResolve(r) {
+        const display = (r === null || r === undefined) ? 'nil'
+          : typeof r === 'string' ? r
+          : typeof r === 'object' ? JSON.stringify(r) : String(r);
+        // Poll for _asyncPending to reach 0 (max 5s)
+        let waited = 0;
+        function check() {
+          if (window._asyncPending <= 0 || waited > 5000) {
+            resolve({ result: display, ioTrace: window._ioTrace });
+          } else {
+            waited += 20;
+            setTimeout(check, 20);
+          }
+        }
+        // Give first tick a chance to start
+        setTimeout(check, 20);
+      }
+    });
+  }, expr);
+
+  // 8. Collect logs
+  const ioLogs = consoleLogs.filter(l =>
+    l.text.includes('[sandbox]') || l.text.includes('[sx]') ||
+    l.type === 'error' || l.type === 'warning'
+  );
+
+  return {
+    mode: 'sandbox',
+    stack: stack || 'core',
+    modules: loadedModules.length > 0 ? loadedModules : undefined,
+    files: files.length > 0 ? files.map(f => path.basename(f)) : undefined,
+    result: evalResult.result,
+    ioTrace: evalResult.ioTrace.length > 0 ? evalResult.ioTrace : undefined,
+    loadErrors: loadErrors.length > 0 ? loadErrors : undefined,
+    log: ioLogs.length > 0 ? ioLogs : undefined,
+  };
+}
+
 async function main() {
   const argsJson = process.argv[2] || '{}';
   let args;
@@ -1354,6 +1687,9 @@ async function main() {
       case 'eval-at':
         result = await modeEvalAt(browser, url, args.phase || 'before-hydrate', args.expr || '(type-of ~cssx/tw)');
         break;
+      case 'sandbox':
+        result = await modeSandbox(page, args.expr || '"hello"', args.files || [], args.setup || '', args.stack || '');
+        break;
       default:
         result = { error: `Unknown mode: ${mode}` };
     }