diff --git a/apps/fs-experiment/App.tsx b/apps/fs-experiment/App.tsx
index 50c6ae2..c28ef7e 100644
--- a/apps/fs-experiment/App.tsx
+++ b/apps/fs-experiment/App.tsx
@@ -40,6 +40,7 @@ const SANDBOXED_SUBSTITUTIONS: Record<string, string> = {
 function App(): React.JSX.Element {
   const isDarkMode = useColorScheme() === 'dark'
   const [useSubstitution, setUseSubstitution] = useState(false)
+  const [sandboxReady, setSandboxReady] = useState(false)
 
   const theme = {
     bg: isDarkMode ? '#000' : '#fff',
@@ -114,6 +115,7 @@ function App(): React.JSX.Element {
           </View>
 
           <SandboxReactNativeView
+            testID={sandboxReady ? 'sandbox-ready' : undefined}
             style={styles.sandboxView}
             origin="sandbox.fs-experiment.demo"
             componentName="SandboxApp"
@@ -122,7 +124,12 @@ function App(): React.JSX.Element {
             turboModuleSubstitutions={
               useSubstitution ? SANDBOXED_SUBSTITUTIONS : undefined
             }
-            onMessage={msg => console.log('Host received from sandbox:', msg)}
+            onMessage={msg => {
+              const payload =
+                typeof msg.data === 'string' ? JSON.parse(msg.data) : msg.data
+              if (payload?.cmd === 'ready') setSandboxReady(true)
+              console.log('Host received from sandbox:', msg)
+            }}
             onError={err =>
               console.log('Host received error from sandbox:', err)
             }
diff --git a/apps/fs-experiment/__tests__/sandbox-isolation.harness.tsx b/apps/fs-experiment/__tests__/sandbox-isolation.harness.tsx
index b39c19d..3812644 100644
--- a/apps/fs-experiment/__tests__/sandbox-isolation.harness.tsx
+++ b/apps/fs-experiment/__tests__/sandbox-isolation.harness.tsx
@@ -15,6 +15,7 @@ import App from '../App'
 const isIOS = Platform.OS === 'ios'
 const TEST_TIMEOUT = 60_000
 const RENDER_TIMEOUT = 10_000
+const SANDBOX_READY_TIMEOUT = 90_000
 
 async function sleep(ms: number) {
   return new Promise(r => setTimeout(r, ms))
@@ -85,8 +86,12 @@ describe('Substitution OFF', () => {
   beforeAll(async () => {
     blockCleanup = true
     await render(<App />, {timeout: RENDER_TIMEOUT})
-    await sleep(4000)
-  }, 30_000)
+    await screen.findByTestId(
+      'sandbox-ready',
+      {},
+      {timeout: SANDBOX_READY_TIMEOUT}
+    )
+  }, SANDBOX_READY_TIMEOUT + RENDER_TIMEOUT)
 
   if (isIOS) {
     it(
diff --git a/apps/fs-experiment/sandbox.js b/apps/fs-experiment/sandbox.js
index e0bc3a3..b116f39 100644
--- a/apps/fs-experiment/sandbox.js
+++ b/apps/fs-experiment/sandbox.js
@@ -58,6 +58,9 @@ function installTestCommandHandler() {
 function SandboxApp(props) {
   useEffect(() => {
     installTestCommandHandler()
+    if (typeof globalThis.postMessage === 'function') {
+      globalThis.postMessage({cmd: 'ready'})
+    }
   }, [])
 
   return <FileOpsUI testIDPrefix="sandbox" {...props} />