diff --git a/apps/fs-experiment/App.tsx b/apps/fs-experiment/App.tsx
index 50c6ae2..2c972b7 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',
@@ -98,7 +99,10 @@ function App(): React.JSX.Element {
                 useSubstitution ? 'substitution-on' : 'substitution-off'
               }
               style={styles.switchRow}
-              onPress={() => setUseSubstitution(v => !v)}>
+              onPress={() => {
+                setSandboxReady(false)
+                setUseSubstitution(v => !v)
+              }}>
               <Text style={[styles.switchLabel, {color: theme.text}]}>
                 Module substitution{' '}
                 <Text style={{color: theme.textSec, fontSize: 12}}>
@@ -107,12 +111,18 @@ function App(): React.JSX.Element {
               </Text>
               <Switch
                 value={useSubstitution}
-                onValueChange={setUseSubstitution}
+                onValueChange={v => {
+                  setSandboxReady(false)
+                  setUseSubstitution(v)
+                }}
                 trackColor={{false: theme.border, true: theme.green}}
               />
             </Pressable>
           </View>
 
+          {sandboxReady && (
+            <View testID="sandbox-ready" style={styles.readyMarker} />
+          )}
           <SandboxReactNativeView
             style={styles.sandboxView}
             origin="sandbox.fs-experiment.demo"
@@ -122,7 +132,11 @@ 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 === 'string' ? JSON.parse(msg) : msg
+              if (payload?.cmd === 'ready') setSandboxReady(true)
+              console.log('Host received from sandbox:', msg)
+            }}
             onError={err =>
               console.log('Host received error from sandbox:', err)
             }
@@ -137,6 +151,10 @@ const styles = StyleSheet.create({
   root: {
     flex: 1,
   },
+  readyMarker: {
+    width: 0,
+    height: 0,
+  },
   section: {
     borderBottomWidth: StyleSheet.hairlineWidth,
   },
diff --git a/apps/fs-experiment/__tests__/sandbox-isolation.harness.tsx b/apps/fs-experiment/__tests__/sandbox-isolation.harness.tsx
index b39c19d..d2b3cc9 100644
--- a/apps/fs-experiment/__tests__/sandbox-isolation.harness.tsx
+++ b/apps/fs-experiment/__tests__/sandbox-isolation.harness.tsx
@@ -8,6 +8,7 @@ import {
   expect,
   it,
   render,
+  waitFor,
 } from 'react-native-harness'
 
 import App from '../App'
@@ -15,6 +16,7 @@ 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))
@@ -26,7 +28,16 @@ async function setSubstitution(enabled: boolean) {
 
   await userEvent.press(await screen.findByTestId('substitution-switch'))
   await screen.findByAccessibilityLabel(wantLabel)
-  await sleep(3000)
+  // Toggling substitution restarts the sandbox — wait for it to be ready again
+  // instead of a fixed sleep
+  await waitFor(
+    () => {
+      if (!screen.queryByTestId('sandbox-ready')) {
+        throw new Error('Sandbox not ready after substitution change')
+      }
+    },
+    {timeout: SANDBOX_READY_TIMEOUT}
+  )
 }
 
 async function writeAndRead(
@@ -85,8 +96,15 @@ describe('Substitution OFF', () => {
   beforeAll(async () => {
     blockCleanup = true
     await render(<App />, {timeout: RENDER_TIMEOUT})
-    await sleep(4000)
-  }, 30_000)
+    await waitFor(
+      () => {
+        if (!screen.queryByTestId('sandbox-ready')) {
+          throw new Error('Sandbox not ready')
+        }
+      },
+      {timeout: SANDBOX_READY_TIMEOUT}
+    )
+  }, SANDBOX_READY_TIMEOUT + RENDER_TIMEOUT)
 
   if (isIOS) {
     it(
diff --git a/apps/fs-experiment/rn-harness.config.mjs b/apps/fs-experiment/rn-harness.config.mjs
index 240080c..5aab348 100644
--- a/apps/fs-experiment/rn-harness.config.mjs
+++ b/apps/fs-experiment/rn-harness.config.mjs
@@ -33,6 +33,7 @@ const config = {
   forwardClientLogs: true,
   resetEnvironmentBetweenTestFiles: true,
   disableViewFlattening: true,
+  bridgeTimeout: 120_000,
 }
 
 export default config
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} />