diff --git a/src/config/index.ts b/src/config/index.ts
index 224076a..2eec012 100644
--- a/src/config/index.ts
+++ b/src/config/index.ts
@@ -27,7 +27,7 @@ export const DEBUG_MODE = true;
 /**
  * Base URL for the backend API server
  */
-export const API_BASE_URL = 'http://192.168.56.103:8000';
+export const API_BASE_URL = 'http://localhost:8000';
 
 /**
  * API request timeout in milliseconds
@@ -64,6 +64,20 @@ export const API_ENDPOINTS = {
   },
 } as const;
 
+/**
+ * Vault storage key names (AsyncStorage keys only — not user-editable "initial values").
+ * - These are constants: the key names used to read/write/remove vault state in AsyncStorage.
+ * - The actual stored values (S0 data, '1') are set by the app; do not change these key strings
+ *   unless you are migrating storage (changing them would make existing data unfindable).
+ * - Placed in config so VaultScreen and MeScreen (and others) use the same keys in one place.
+ * - INITIALIZED: app sets to '1' after first mnemonic flow; SHARE_DEVICE: app stores serialized S0.
+ * - "Reset Vault State" = remove both keys; next vault open sees no S0 and shows mnemonic flow.
+ */
+export const VAULT_STORAGE_KEYS = {
+  INITIALIZED: 'sentinel_vault_initialized',
+  SHARE_DEVICE: 'sentinel_vault_s0',
+} as const;
+
 // =============================================================================
 // Helper Functions
 // =============================================================================
diff --git a/src/screens/MeScreen.tsx b/src/screens/MeScreen.tsx
index 5d5847b..24e360b 100644
--- a/src/screens/MeScreen.tsx
+++ b/src/screens/MeScreen.tsx
@@ -18,7 +18,7 @@ import AsyncStorage from '@react-native-async-storage/async-storage';
 import { useAuth } from '../context/AuthContext';
 import { Heir, HeirStatus, PaymentStrategy } from '../types';
 import HeritageScreen from './HeritageScreen';
-import { VAULT_STORAGE_KEYS } from './SentinelScreen';
+import { VAULT_STORAGE_KEYS } from '../config';
 
 // Mock heirs data
 const initialHeirs: Heir[] = [
@@ -248,6 +248,7 @@ export default function MeScreen() {
   });
   const [sanctumArchive, setSanctumArchive] = useState<'off' | 'standard' | 'strict'>('standard');
   const [sanctumRehearsal, setSanctumRehearsal] = useState<'monthly' | 'quarterly'>('quarterly');
+  const [resetVaultFeedback, setResetVaultFeedback] = useState<{ status: 'idle' | 'success' | 'error'; message: string }>({ status: 'idle', message: '' });
   const [triggerDisconnectDays, setTriggerDisconnectDays] = useState(30);
   const [triggerGraceDays, setTriggerGraceDays] = useState(15);
   const [triggerSource, setTriggerSource] = useState<'dual' | 'subscription' | 'activity'>('dual');
@@ -308,17 +309,29 @@ export default function MeScreen() {
   };
 
   const handleResetVault = async () => {
+    setResetVaultFeedback({ status: 'idle', message: '' });
     try {
       await AsyncStorage.multiRemove([
         VAULT_STORAGE_KEYS.INITIALIZED,
         VAULT_STORAGE_KEYS.SHARE_DEVICE,
       ]);
-      Alert.alert('Done', 'Vault state reset. Go to Sentinel → Open Shadow Vault to see first-time flow.');
+      setResetVaultFeedback({
+        status: 'success',
+        message: 'Vault state has been reset. Next time you open Shadow Vault you will see the mnemonic flow again.',
+      });
     } catch (e) {
-      Alert.alert('Error', 'Failed to reset vault state.');
+      setResetVaultFeedback({
+        status: 'error',
+        message: 'Failed to reset vault state. Please try again.',
+      });
     }
   };
 
+  const handleCloseSanctumModal = () => {
+    setResetVaultFeedback({ status: 'idle', message: '' });
+    setShowSanctumModal(false);
+  };
+
   return (
     <View style={styles.container}>
       <LinearGradient
@@ -760,7 +773,7 @@ export default function MeScreen() {
         visible={showSanctumModal}
         animationType="fade"
         transparent
-        onRequestClose={() => setShowSanctumModal(false)}
+        onRequestClose={handleCloseSanctumModal}
       >
         <View style={styles.spiritOverlay}>
           <View style={styles.spiritModal}>
@@ -908,7 +921,31 @@ export default function MeScreen() {
                     <Ionicons name="refresh" size={16} color={colors.nautical.coral} />
                     <Text style={styles.devResetText}>Reset Vault State</Text>
                   </TouchableOpacity>
-                  <Text style={styles.sanctumHint}>Clear hasVaultInitialized & Share A. Test first-open flow.</Text>
+                  <Text style={styles.sanctumHint}>Clear S0 (SHARE_DEVICE) from storage. Next vault open uses mnemonic flow.</Text>
+                  {resetVaultFeedback.status !== 'idle' && (
+                    <View
+                      style={[
+                        styles.resetVaultFeedback,
+                        resetVaultFeedback.status === 'success' ? styles.resetVaultFeedbackSuccess : styles.resetVaultFeedbackError,
+                      ]}
+                    >
+                      <Ionicons
+                        name={resetVaultFeedback.status === 'success' ? 'checkmark-circle' : 'alert-circle'}
+                        size={20}
+                        color={resetVaultFeedback.status === 'success' ? colors.sentinel?.statusNormal ?? '#6BBF8A' : colors.nautical.coral}
+                      />
+                      <Text
+                        style={[
+                          styles.resetVaultFeedbackText,
+                          resetVaultFeedback.status === 'success' ? styles.resetVaultFeedbackTextSuccess : styles.resetVaultFeedbackTextError,
+                        ]}
+                      >
+                        {resetVaultFeedback.status === 'success' ? 'Success' : 'Error'}
+                        {' — '}
+                        {resetVaultFeedback.message}
+                      </Text>
+                    </View>
+                  )}
                 </View>
               )}
             </ScrollView>
@@ -916,7 +953,7 @@ export default function MeScreen() {
               <TouchableOpacity
                 style={styles.confirmPulseButton}
                 activeOpacity={0.85}
-                onPress={() => setShowSanctumModal(false)}
+                onPress={handleCloseSanctumModal}
               >
                 <Ionicons name="checkmark-circle" size={18} color={colors.nautical.teal} />
                 <Text style={styles.confirmPulseText}>Save</Text>
@@ -924,7 +961,7 @@ export default function MeScreen() {
               <TouchableOpacity
                 style={styles.confirmPulseButton}
                 activeOpacity={0.85}
-                onPress={() => setShowSanctumModal(false)}
+                onPress={handleCloseSanctumModal}
               >
                 <Ionicons name="close-circle" size={18} color={colors.nautical.teal} />
                 <Text style={styles.confirmPulseText}>Close</Text>
@@ -1910,6 +1947,34 @@ const styles = StyleSheet.create({
     fontSize: typography.fontSize.sm,
     color: colors.nautical.coral,
   },
+  resetVaultFeedback: {
+    flexDirection: 'row',
+    alignItems: 'center',
+    gap: spacing.sm,
+    borderRadius: borderRadius.lg,
+    padding: spacing.base,
+    marginTop: spacing.md,
+  },
+  resetVaultFeedbackSuccess: {
+    backgroundColor: 'rgba(107, 191, 138, 0.2)',
+    borderWidth: 1,
+    borderColor: 'rgba(107, 191, 138, 0.5)',
+  },
+  resetVaultFeedbackError: {
+    backgroundColor: 'rgba(229, 115, 115, 0.2)',
+    borderWidth: 1,
+    borderColor: 'rgba(229, 115, 115, 0.5)',
+  },
+  resetVaultFeedbackText: {
+    flex: 1,
+    fontSize: typography.fontSize.sm,
+  },
+  resetVaultFeedbackTextSuccess: {
+    color: '#2E7D5E',
+  },
+  resetVaultFeedbackTextError: {
+    color: colors.nautical.coral,
+  },
   confirmPulseButton: {
     flexDirection: 'row',
     alignItems: 'center',
diff --git a/src/screens/SentinelScreen.tsx b/src/screens/SentinelScreen.tsx
index 8b9ffc5..3ad34b2 100644
--- a/src/screens/SentinelScreen.tsx
+++ b/src/screens/SentinelScreen.tsx
@@ -65,6 +65,8 @@ const initialLogs: KillSwitchLog[] = [
   { id: '4', action: 'HEARTBEAT_CONFIRMED', timestamp: new Date('2024-01-15T11:20:00') },
 ];
 
+export { VAULT_STORAGE_KEYS } from '../config';
+
 export default function SentinelScreen() {
   const [status, setStatus] = useState<SystemStatus>('normal');
   const [lastSubscriptionCheck] = useState(new Date('2024-01-18T00:00:00'));
diff --git a/src/screens/VaultScreen.tsx b/src/screens/VaultScreen.tsx
index 0f629f1..d7d9576 100644
--- a/src/screens/VaultScreen.tsx
+++ b/src/screens/VaultScreen.tsx
@@ -25,6 +25,8 @@ import { VaultAsset, VaultAssetType, Heir } from '../types';
 import BiometricModal from '../components/common/BiometricModal';
 import { useAuth } from '../context/AuthContext';
 import { useVaultAssets } from '../hooks/useVaultAssets';
+import { VAULT_STORAGE_KEYS } from '../config';
+import { mnemonicToEntropy, splitSecret, serializeShare } from '../utils/sss';
 
 // Asset type configuration with nautical theme
 const assetTypeConfig: Record<VaultAssetType, { icon: string; iconType: 'ionicons' | 'feather' | 'material' | 'fontawesome5'; label: string }> = {
@@ -168,6 +170,7 @@ export default function VaultScreen() {
   const [accountProvider, setAccountProvider] = useState<'bank' | 'steam' | 'facebook' | 'custom'>('bank');
   const [showMnemonic, setShowMnemonic] = useState(false);
   const [showLegacyAssignCta, setShowLegacyAssignCta] = useState(false);
+  const [hasS0, setHasS0] = useState<boolean | null>(null);
   const [mnemonicWords, setMnemonicWords] = useState<string[]>([]);
   const [mnemonicParts, setMnemonicParts] = useState<string[][]>([]);
   const [mnemonicStep, setMnemonicStep] = useState<1 | 2 | 3 | 4 | 5>(1);
@@ -185,15 +188,31 @@ export default function VaultScreen() {
   const mnemonicRef = useRef<View>(null);
   const progressTimerRef = useRef<ReturnType<typeof setInterval> | null>(null);
 
+  // Detect S0 (TEE/SE): if present, later open shows biometric only; if not, mnemonic flow
   useEffect(() => {
-    if (!isUnlocked) {
-      const timer = setTimeout(() => {
-        setShowBiometric(true);
-      }, 500);
-      return () => clearTimeout(timer);
-    }
+    let cancelled = false;
+    AsyncStorage.getItem(VAULT_STORAGE_KEYS.SHARE_DEVICE)
+      .then((v) => {
+        if (!cancelled) setHasS0(!!v);
+      })
+      .catch(() => {
+        if (!cancelled) setHasS0(false);
+      });
+    return () => { cancelled = true; };
   }, []);
 
+  // Only when S0 exists and vault not unlocked: show biometric after short delay.
+  // When hasS0 is false or null, never show biometric — go straight to mnemonic flow.
+  useEffect(() => {
+    if (hasS0 !== true) {
+      setShowBiometric(false);
+      return;
+    }
+    if (isUnlocked) return;
+    const timer = setTimeout(() => setShowBiometric(true), 500);
+    return () => clearTimeout(timer);
+  }, [isUnlocked, hasS0]);
+
   useEffect(() => {
     if (isUnlocked) {
       Animated.timing(fadeAnim, {
@@ -335,28 +354,49 @@ export default function VaultScreen() {
 
   const handleHeirDecision = (share: boolean) => {
     // Placeholder for future heir flow
-    setShowMnemonic(false);
-    setIsUnlocked(true);
+    finishMnemonicThenShowBiometric();
   };
 
   const handleSubmitAssignment = () => {
     // Placeholder for submitting assignment to API
-    setShowMnemonic(false);
-    setIsUnlocked(true);
+    finishMnemonicThenShowBiometric();
   };
 
   const handleHeirYes = () => {
-    setShowMnemonic(false);
-    setIsUnlocked(true);
+    finishMnemonicThenShowBiometric();
     setShowLegacyAssignCta(true);
   };
 
   const handleHeirNo = () => {
-    setShowMnemonic(false);
-    setIsUnlocked(true);
+    finishMnemonicThenShowBiometric();
     setShowLegacyAssignCta(true);
   };
 
+  /** After mnemonic flow: persist S0 (simulated TEE/SE via AsyncStorage), then show biometric; unlock on success */
+  const finishMnemonicThenShowBiometric = async () => {
+    try {
+      const wordList = bip39.wordlists.english;
+      const entropy = mnemonicToEntropy(mnemonicWords, wordList);
+      const shares = splitSecret(entropy);
+      const s0 = shares[0]; // device share (S0)
+      // S0 is stored in AsyncStorage under SHARE_DEVICE — app-level storage, not hardware TEE/SE
+      await AsyncStorage.setItem(VAULT_STORAGE_KEYS.SHARE_DEVICE, serializeShare(s0));
+      await AsyncStorage.setItem(VAULT_STORAGE_KEYS.INITIALIZED, '1');
+      setHasS0(true);
+      setShowMnemonic(false);
+      setShowBiometric(true);
+    } catch (e) {
+      if (__DEV__) console.warn('finishMnemonicThenShowBiometric', e);
+      setShowMnemonic(false);
+      setShowBiometric(true);
+    }
+  };
+
+  const handleBiometricSuccess = () => {
+    setShowBiometric(false);
+    setIsUnlocked(true);
+  };
+
   const handleOpenLegacyAssign = () => {
     setSelectedHeir(null);
     setSelectedHeirAsset(null);
@@ -888,7 +928,7 @@ export default function VaultScreen() {
               
               <TouchableOpacity
                 style={styles.unlockButton}
-                onPress={() => setShowBiometric(true)}
+                onPress={hasS0 ? () => setShowBiometric(true) : handleUnlock}
                 activeOpacity={0.8}
               >
                 <LinearGradient
@@ -897,8 +937,10 @@ export default function VaultScreen() {
                   start={{ x: 0, y: 0 }}
                   end={{ x: 1, y: 0 }}
                 >
-                  <Ionicons name="finger-print" size={20} color={colors.vault.background} />
-                  <Text style={styles.unlockButtonText}>Captain's Verification</Text>
+                  <Ionicons name={hasS0 ? 'finger-print' : 'key'} size={20} color={colors.vault.background} />
+                  <Text style={styles.unlockButtonText}>
+                    {hasS0 === true ? "Captain's Verification" : hasS0 === false ? 'Enter Vault' : 'Loading…'}
+                  </Text>
                 </LinearGradient>
               </TouchableOpacity>
             </View>
@@ -906,8 +948,8 @@ export default function VaultScreen() {
         </LinearGradient>
 
         <BiometricModal
-          visible={showBiometric}
-          onSuccess={handleUnlock}
+          visible={hasS0 === true && showBiometric}
+          onSuccess={handleBiometricSuccess}
           onCancel={() => setShowBiometric(false)}
           title="Enter the Vault"
           message="Verify your identity to access your treasures"