diff --git a/src/cliproxy/account-safety.ts b/src/cliproxy/account-safety.ts
new file mode 100644
index 00000000..522c248e
--- /dev/null
+++ b/src/cliproxy/account-safety.ts
@@ -0,0 +1,376 @@
+/**
+ * Account Safety Guards
+ *
+ * Prevents Google account bans by:
+ * 1. Cross-provider isolation (auto-pause conflicting accounts at launch, restore on exit)
+ * 2. Ban/disable detection (auto-pauses affected accounts on error response)
+ * 3. Crash recovery (restores stale auto-pauses from dead sessions)
+ *
+ * Ref: https://github.com/kaitranntt/ccs/issues/509
+ */
+
+import * as fs from 'fs';
+import * as path from 'path';
+import { warn, info } from '../utils/ui';
+import { CLIProxyProvider } from './types';
+import { loadAccountsRegistry, pauseAccount, resumeAccount } from './accounts/registry';
+import { getCcsDir } from '../utils/config-manager';
+
+/** Providers that use Google OAuth (ban risk when overlapping) */
+const GOOGLE_OAUTH_PROVIDERS: CLIProxyProvider[] = ['gemini', 'agy', 'codex'];
+
+// --- Auto-pause persistence (crash recovery) ---
+
+interface AutoPausedSession {
+  initiator: CLIProxyProvider;
+  pid: number;
+  pausedAt: string;
+  accounts: Array<{ provider: CLIProxyProvider; accountId: string }>;
+}
+
+interface AutoPausedFile {
+  sessions: AutoPausedSession[];
+}
+
+function getAutoPausedPath(): string {
+  return path.join(getCcsDir(), 'cliproxy', 'auto-paused.json');
+}
+
+function loadAutoPaused(): AutoPausedFile {
+  try {
+    const filePath = getAutoPausedPath();
+    if (fs.existsSync(filePath)) {
+      const data = JSON.parse(fs.readFileSync(filePath, 'utf-8'));
+      if (Array.isArray(data.sessions)) return { sessions: data.sessions };
+    }
+  } catch {
+    // Corrupted or malformed file — start fresh
+  }
+  return { sessions: [] };
+}
+
+function saveAutoPaused(data: AutoPausedFile): void {
+  const filePath = getAutoPausedPath();
+  if (data.sessions.length === 0) {
+    try {
+      fs.unlinkSync(filePath);
+    } catch {
+      /* already gone */
+    }
+    return;
+  }
+  const dir = path.dirname(filePath);
+  if (!fs.existsSync(dir)) fs.mkdirSync(dir, { recursive: true, mode: 0o700 });
+  fs.writeFileSync(filePath, JSON.stringify(data, null, 2) + '\n', { mode: 0o600 });
+}
+
+/**
+ * Check if a process is alive. NOTE: PIDs can be recycled by the OS.
+ * If a stale PID is reused by an unrelated process, cleanup is deferred until that process exits.
+ * This is acceptable — next CCS launch will self-heal via cleanupStaleAutoPauses().
+ */
+function isPidAlive(pid: number): boolean {
+  try {
+    process.kill(pid, 0);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+/**
+ * Detect same email registered under multiple Google OAuth providers.
+ * This is the primary cause of account bans — Google sees concurrent
+ * OAuth usage from different client IDs as suspicious activity.
+ *
+ * Returns map of email -> providers it appears in (only duplicates).
+ */
+export function detectCrossProviderDuplicates(): Map<string, CLIProxyProvider[]> {
+  const registry = loadAccountsRegistry();
+
+  // Build email -> providers mapping (only Google OAuth providers)
+  const emailProviders = new Map<string, CLIProxyProvider[]>();
+
+  for (const provider of GOOGLE_OAUTH_PROVIDERS) {
+    const providerAccounts = registry.providers[provider];
+    if (!providerAccounts) continue;
+
+    for (const [, account] of Object.entries(providerAccounts.accounts)) {
+      const email = account.email;
+      if (!email || account.paused) continue;
+
+      const normalized = email.toLowerCase();
+      const existing = emailProviders.get(normalized) ?? [];
+      existing.push(provider);
+      emailProviders.set(normalized, existing);
+    }
+  }
+
+  // Filter to only duplicates (email in 2+ providers)
+  const duplicates = new Map<string, CLIProxyProvider[]>();
+  for (const [email, providers] of emailProviders) {
+    if (providers.length > 1) {
+      duplicates.set(email, providers);
+    }
+  }
+
+  return duplicates;
+}
+
+/**
+ * Check if a newly registered account creates a cross-provider conflict.
+ * Returns the conflicting providers, or null if no conflict.
+ */
+export function checkNewAccountConflict(
+  provider: CLIProxyProvider,
+  email: string | undefined
+): CLIProxyProvider[] | null {
+  if (!email || !GOOGLE_OAUTH_PROVIDERS.includes(provider)) return null;
+
+  const registry = loadAccountsRegistry();
+  const normalized = email.toLowerCase();
+  const conflicts: CLIProxyProvider[] = [];
+
+  for (const other of GOOGLE_OAUTH_PROVIDERS) {
+    if (other === provider) continue;
+
+    const providerAccounts = registry.providers[other];
+    if (!providerAccounts) continue;
+
+    for (const [, account] of Object.entries(providerAccounts.accounts)) {
+      if (account.email?.toLowerCase() === normalized && !account.paused) {
+        conflicts.push(other);
+        break;
+      }
+    }
+  }
+
+  return conflicts.length > 0 ? conflicts : null;
+}
+
+/**
+ * Display cross-provider duplicate warning at session launch.
+ * Returns true if warning was shown.
+ */
+export function warnCrossProviderDuplicates(provider: CLIProxyProvider): boolean {
+  if (!GOOGLE_OAUTH_PROVIDERS.includes(provider)) return false;
+
+  const duplicates = detectCrossProviderDuplicates();
+  if (duplicates.size === 0) return false;
+
+  console.error('');
+  console.error(warn('Account safety: cross-provider duplicate detected'));
+  console.error('    Same Google account across providers risks account bans (ref: #509).');
+  console.error('');
+
+  for (const [email, providers] of duplicates) {
+    console.error(`    ${maskEmail(email)} -> ${providers.join(', ')}`);
+  }
+
+  console.error('');
+  console.error('    Fix: pause duplicate with "ccs <provider> --pause <account>"');
+  console.error('    or use separate Google accounts per provider.');
+  console.error('');
+
+  return true;
+}
+
+/**
+ * Warn about a specific new account conflict during OAuth registration.
+ */
+export function warnNewAccountConflict(
+  email: string,
+  conflictingProviders: CLIProxyProvider[]
+): void {
+  console.error('');
+  console.error(warn('Account safety: this email is used by another provider'));
+  console.error(
+    `    ${maskEmail(email)} is also registered under: ${conflictingProviders.join(', ')}`
+  );
+  console.error('    Concurrent usage may cause Google to ban your account.');
+  console.error('    Consider pausing the duplicate or using a different account.');
+  console.error('');
+}
+
+// --- Enforcement: auto-pause/restore ---
+
+/**
+ * Restore auto-paused accounts from crashed sessions (dead PIDs).
+ * Call at launch BEFORE enforceProviderIsolation().
+ */
+export function cleanupStaleAutoPauses(): void {
+  const data = loadAutoPaused();
+  if (data.sessions.length === 0) return;
+
+  const alive: AutoPausedSession[] = [];
+
+  for (const session of data.sessions) {
+    if (isPidAlive(session.pid)) {
+      alive.push(session);
+      continue;
+    }
+    // Dead PID — restore accounts
+    for (const { provider, accountId } of session.accounts) {
+      resumeAccount(provider, accountId);
+    }
+    console.error(
+      info(
+        `Restored ${session.accounts.length} auto-paused account(s) from crashed ${session.initiator} session`
+      )
+    );
+  }
+
+  if (alive.length !== data.sessions.length) {
+    saveAutoPaused({ sessions: alive });
+  }
+}
+
+/**
+ * Enforce provider isolation by auto-pausing conflicting accounts in other providers.
+ * Records paused accounts for crash recovery and session exit restore.
+ * Returns number of accounts paused.
+ */
+export function enforceProviderIsolation(provider: CLIProxyProvider): number {
+  if (!GOOGLE_OAUTH_PROVIDERS.includes(provider)) return 0;
+
+  // If another provider session is actively managing isolation, just warn
+  const data = loadAutoPaused();
+  const otherActive = data.sessions.filter((s) => s.initiator !== provider && isPidAlive(s.pid));
+  if (otherActive.length > 0) return 0;
+
+  const registry = loadAccountsRegistry();
+  const currentAccounts = registry.providers[provider];
+  if (!currentAccounts) return 0;
+
+  // Collect active emails for current provider
+  const myEmails = new Set<string>();
+  for (const [, account] of Object.entries(currentAccounts.accounts)) {
+    if (account.email && !account.paused) {
+      myEmails.add(account.email.toLowerCase());
+    }
+  }
+  if (myEmails.size === 0) return 0;
+
+  // Find conflicting accounts in other Google OAuth providers
+  const toPause: Array<{ provider: CLIProxyProvider; accountId: string }> = [];
+
+  for (const other of GOOGLE_OAUTH_PROVIDERS) {
+    if (other === provider) continue;
+    const otherAccounts = registry.providers[other];
+    if (!otherAccounts) continue;
+
+    for (const [accountId, account] of Object.entries(otherAccounts.accounts)) {
+      if (account.email && !account.paused && myEmails.has(account.email.toLowerCase())) {
+        toPause.push({ provider: other, accountId });
+      }
+    }
+  }
+
+  if (toPause.length === 0) return 0;
+
+  // Pause conflicting accounts
+  for (const { provider: p, accountId } of toPause) {
+    pauseAccount(p, accountId);
+  }
+
+  // Record for crash recovery (re-read to reduce concurrent write race window)
+  const freshData = loadAutoPaused();
+  freshData.sessions = freshData.sessions.filter((s) => s.initiator !== provider);
+  freshData.sessions.push({
+    initiator: provider,
+    pid: process.pid,
+    pausedAt: new Date().toISOString(),
+    accounts: toPause,
+  });
+  saveAutoPaused(freshData);
+
+  console.error('');
+  console.error(info(`Account safety: auto-paused ${toPause.length} conflicting account(s)`));
+  for (const { provider: p, accountId } of toPause) {
+    const acct = registry.providers[p]?.accounts[accountId];
+    const display = acct?.email ? maskEmail(acct.email) : accountId;
+    console.error(`    ${display} (${p})`);
+  }
+  console.error('    Will restore on session exit.');
+  console.error('');
+
+  return toPause.length;
+}
+
+/**
+ * Restore accounts that were auto-paused by this session.
+ * Called on session exit (process 'exit' event).
+ * Skips accounts re-paused after enforcement (e.g., by ban handler).
+ */
+export function restoreAutoPausedAccounts(provider: CLIProxyProvider): void {
+  const data = loadAutoPaused();
+  const mySession = data.sessions.find((s) => s.initiator === provider && s.pid === process.pid);
+  if (!mySession) return;
+
+  const registry = loadAccountsRegistry();
+
+  for (const { provider: p, accountId } of mySession.accounts) {
+    // Don't restore if account was re-paused after enforcement (e.g., ban detected)
+    const account = registry.providers[p]?.accounts[accountId];
+    if (account?.pausedAt && account.pausedAt > mySession.pausedAt) {
+      continue;
+    }
+    resumeAccount(p, accountId);
+  }
+
+  data.sessions = data.sessions.filter((s) => !(s.initiator === provider && s.pid === process.pid));
+  saveAutoPaused(data);
+}
+
+// Error patterns that indicate Google has disabled/banned an account
+const BAN_PATTERNS = [
+  'disabled in this account',
+  'violation of terms of service',
+  'account has been disabled',
+  'account is disabled',
+  'account has been suspended',
+  'account has been banned',
+];
+
+/**
+ * Check if an error message indicates an account ban/disable.
+ */
+export function isBanResponse(errorMessage: string): boolean {
+  const lower = errorMessage.toLowerCase();
+  return BAN_PATTERNS.some((pattern) => lower.includes(pattern));
+}
+
+/**
+ * Handle detected account ban by auto-pausing the affected account.
+ * Returns true if account was paused.
+ */
+export function handleBanDetection(
+  provider: CLIProxyProvider,
+  accountId: string,
+  errorMessage: string
+): boolean {
+  if (!isBanResponse(errorMessage)) return false;
+
+  console.error('');
+  console.error(warn('Account safety: account appears disabled by Google'));
+  console.error(`    Account "${accountId}" (${provider}) returned:`);
+  console.error(`    "${truncate(errorMessage, 120)}"`);
+  console.error('');
+  console.error(info('Auto-pausing this account to prevent further issues.'));
+  console.error(`    Resume later: ccs ${provider} --resume ${accountId}`);
+  console.error('');
+
+  return pauseAccount(provider, accountId);
+}
+
+/** Mask email for privacy in terminal output */
+export function maskEmail(email: string): string {
+  const [local, domain] = email.split('@');
+  if (!local || !domain) return email;
+  return `${local.slice(0, 3)}***@${domain}`;
+}
+
+/** Truncate string with ellipsis */
+function truncate(str: string, maxLen: number): string {
+  return str.length > maxLen ? str.slice(0, maxLen - 3) + '...' : str;
+}
diff --git a/src/cliproxy/auth/oauth-handler.ts b/src/cliproxy/auth/oauth-handler.ts
index f8fe82b6..29e47e98 100644
--- a/src/cliproxy/auth/oauth-handler.ts
+++ b/src/cliproxy/auth/oauth-handler.ts
@@ -39,6 +39,7 @@ import { getProviderTokenDir, isAuthenticated, registerAccountFromToken } from '
 import { executeOAuthProcess } from './oauth-process';
 import { importKiroToken } from './kiro-import';
 import { getProxyTarget, buildProxyUrl, buildManagementHeaders } from '../proxy-target-resolver';
+import { checkNewAccountConflict, warnNewAccountConflict } from '../account-safety';
 
 /**
  * Prompt user to add another account
@@ -379,7 +380,17 @@ async function handlePasteCallbackMode(
     }
 
     console.log(ok('Authentication successful!'));
-    return registerAccountFromToken(provider, tokenDir, nickname);
+    const account = registerAccountFromToken(provider, tokenDir, nickname);
+
+    // Account safety: check for cross-provider conflicts
+    if (account?.email) {
+      const conflicts = checkNewAccountConflict(provider, account.email);
+      if (conflicts) {
+        warnNewAccountConflict(account.email, conflicts);
+      }
+    }
+
+    return account;
   } catch (error) {
     if (verbose) {
       console.log(fail(`Error: ${(error as Error).message}`));
@@ -543,6 +554,14 @@ export async function triggerOAuth(
     console.log('       Or enable "Kiro: Use normal browser" in: ccs config');
   }
 
+  // Account safety: check for cross-provider conflicts
+  if (account?.email) {
+    const conflicts = checkNewAccountConflict(provider, account.email);
+    if (conflicts) {
+      warnNewAccountConflict(account.email, conflicts);
+    }
+  }
+
   return account;
 }
 
diff --git a/src/cliproxy/executor/index.ts b/src/cliproxy/executor/index.ts
index 807ca8b1..098a7356 100644
--- a/src/cliproxy/executor/index.ts
+++ b/src/cliproxy/executor/index.ts
@@ -63,6 +63,12 @@ import {
   handleQuotaCheck,
 } from './retry-handler';
 import { checkOrJoinProxy, registerProxySession, setupCleanupHandlers } from './session-bridge';
+import {
+  warnCrossProviderDuplicates,
+  cleanupStaleAutoPauses,
+  enforceProviderIsolation,
+  restoreAutoPausedAccounts,
+} from '../account-safety';
 import { getWebSearchHookEnv } from '../../utils/websearch-manager';
 
 /** Default executor configuration */
@@ -507,6 +513,20 @@ export async function execClaudeWithCLIProxy(
     await handleQuotaCheck(provider);
   }
 
+  // 3c. Account safety: enforce cross-provider isolation
+  if (!skipLocalAuth) {
+    cleanupStaleAutoPauses();
+    const isolated = enforceProviderIsolation(provider);
+    if (isolated === 0) {
+      // No enforcement — still warn about duplicates for awareness
+      warnCrossProviderDuplicates(provider);
+    } else {
+      process.on('exit', () => {
+        restoreAutoPausedAccounts(provider);
+      });
+    }
+  }
+
   // 4. First-run model configuration
   if (supportsModelConfig(provider) && !skipLocalAuth) {
     await configureProviderModel(provider, false, cfg.customSettingsPath);
diff --git a/src/cliproxy/executor/retry-handler.ts b/src/cliproxy/executor/retry-handler.ts
index 83480f0a..d3bb5996 100644
--- a/src/cliproxy/executor/retry-handler.ts
+++ b/src/cliproxy/executor/retry-handler.ts
@@ -10,6 +10,7 @@
 
 import { fail, warn, info } from '../../utils/ui';
 import { CLIProxyProvider } from '../types';
+import { handleBanDetection } from '../account-safety';
 
 /**
  * Check if error is network-related
@@ -50,6 +51,15 @@ export async function handleTokenExpiration(
   const tokenResult = await ensureTokenValid(provider, verbose);
 
   if (!tokenResult.valid) {
+    // Check if this is an account ban/disable before generic error
+    if (tokenResult.error) {
+      const { getDefaultAccount } = await import('../account-manager');
+      const account = getDefaultAccount(provider);
+      if (account) {
+        handleBanDetection(provider, account.id, tokenResult.error);
+      }
+    }
+
     // Token expired and refresh failed - trigger re-auth
     console.error(warn('OAuth token expired and refresh failed'));
     if (tokenResult.error) {
diff --git a/tests/unit/cliproxy/account-safety.test.ts b/tests/unit/cliproxy/account-safety.test.ts
new file mode 100644
index 00000000..48c7234e
--- /dev/null
+++ b/tests/unit/cliproxy/account-safety.test.ts
@@ -0,0 +1,621 @@
+/**
+ * Account Safety Guards Unit Tests
+ *
+ * Tests ban detection, email masking, cross-provider duplicate detection,
+ * enforcement lifecycle, and crash recovery.
+ */
+
+import { describe, it, expect, beforeEach, afterEach } from 'bun:test';
+import * as fs from 'fs';
+import * as os from 'os';
+import * as path from 'path';
+import {
+  isBanResponse,
+  maskEmail,
+  detectCrossProviderDuplicates,
+  enforceProviderIsolation,
+  cleanupStaleAutoPauses,
+  restoreAutoPausedAccounts,
+  checkNewAccountConflict,
+  handleBanDetection,
+  warnCrossProviderDuplicates,
+} from '../../../src/cliproxy/account-safety';
+
+// --- Test isolation: use temp CCS_HOME ---
+
+let tmpDir: string;
+let origCcsHome: string | undefined;
+
+beforeEach(() => {
+  tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'ccs-test-safety-'));
+  origCcsHome = process.env.CCS_HOME;
+  process.env.CCS_HOME = tmpDir;
+});
+
+afterEach(() => {
+  if (origCcsHome !== undefined) {
+    process.env.CCS_HOME = origCcsHome;
+  } else {
+    delete process.env.CCS_HOME;
+  }
+  fs.rmSync(tmpDir, { recursive: true, force: true });
+});
+
+// CCS_HOME appends .ccs — all paths go through getCcsDir() = CCS_HOME/.ccs
+function ccsDir(): string {
+  return path.join(tmpDir, '.ccs');
+}
+
+// --- Helper: write accounts registry ---
+
+function writeRegistry(providers: Record<string, unknown>): void {
+  const registryDir = path.join(ccsDir(), 'cliproxy');
+  fs.mkdirSync(registryDir, { recursive: true });
+  fs.writeFileSync(
+    path.join(registryDir, 'accounts.json'),
+    JSON.stringify({ version: 1, providers }, null, 2)
+  );
+}
+
+// --- Helper: write auto-paused file ---
+
+function writeAutoPaused(sessions: unknown[]): void {
+  const dir = path.join(ccsDir(), 'cliproxy');
+  fs.mkdirSync(dir, { recursive: true });
+  fs.writeFileSync(path.join(dir, 'auto-paused.json'), JSON.stringify({ sessions }, null, 2));
+}
+
+function readAutoPaused(): { sessions: unknown[] } {
+  const filePath = path.join(ccsDir(), 'cliproxy', 'auto-paused.json');
+  if (!fs.existsSync(filePath)) return { sessions: [] };
+  return JSON.parse(fs.readFileSync(filePath, 'utf-8'));
+}
+
+// --- Helper: write dummy token files ---
+
+function writeTokenFile(filename: string, paused = false): void {
+  const dir = paused
+    ? path.join(ccsDir(), 'cliproxy', 'auth-paused')
+    : path.join(ccsDir(), 'cliproxy', 'auth');
+  fs.mkdirSync(dir, { recursive: true });
+  fs.writeFileSync(path.join(dir, filename), JSON.stringify({ type: 'test' }));
+}
+
+// ========================================
+// isBanResponse
+// ========================================
+
+describe('isBanResponse', () => {
+  it('should detect "disabled in this account"', () => {
+    expect(isBanResponse('API access disabled in this account')).toBe(true);
+  });
+
+  it('should detect "violation of terms of service"', () => {
+    expect(isBanResponse('Your account was flagged for violation of terms of service')).toBe(true);
+  });
+
+  it('should detect "account has been suspended"', () => {
+    expect(isBanResponse('This account has been suspended by Google')).toBe(true);
+  });
+
+  it('should be case-insensitive', () => {
+    expect(isBanResponse('ACCOUNT HAS BEEN DISABLED')).toBe(true);
+  });
+
+  it('should return false for normal errors', () => {
+    expect(isBanResponse('Rate limit exceeded')).toBe(false);
+    expect(isBanResponse('Internal server error')).toBe(false);
+    expect(isBanResponse('Network timeout')).toBe(false);
+  });
+
+  it('should return false for empty string', () => {
+    expect(isBanResponse('')).toBe(false);
+  });
+});
+
+// ========================================
+// maskEmail
+// ========================================
+
+describe('maskEmail', () => {
+  it('should mask standard email', () => {
+    expect(maskEmail('user@example.com')).toBe('use***@example.com');
+  });
+
+  it('should handle short local part', () => {
+    expect(maskEmail('ab@example.com')).toBe('ab***@example.com');
+  });
+
+  it('should handle single char local part', () => {
+    expect(maskEmail('a@example.com')).toBe('a***@example.com');
+  });
+
+  it('should return input if no @ sign', () => {
+    expect(maskEmail('not-an-email')).toBe('not-an-email');
+  });
+
+  it('should return input if empty string', () => {
+    expect(maskEmail('')).toBe('');
+  });
+});
+
+// ========================================
+// detectCrossProviderDuplicates
+// ========================================
+
+describe('detectCrossProviderDuplicates', () => {
+  it('should return empty map when no duplicates', () => {
+    writeRegistry({
+      gemini: {
+        default: 'user1@gmail.com',
+        accounts: {
+          'user1@gmail.com': {
+            email: 'user1@gmail.com',
+            tokenFile: 'gemini-user1.json',
+          },
+        },
+      },
+      agy: {
+        default: 'user2@gmail.com',
+        accounts: {
+          'user2@gmail.com': {
+            email: 'user2@gmail.com',
+            tokenFile: 'agy-user2.json',
+          },
+        },
+      },
+    });
+
+    const dupes = detectCrossProviderDuplicates();
+    expect(dupes.size).toBe(0);
+  });
+
+  it('should detect same email across providers', () => {
+    writeRegistry({
+      gemini: {
+        default: 'shared@gmail.com',
+        accounts: {
+          'shared@gmail.com': {
+            email: 'shared@gmail.com',
+            tokenFile: 'gemini-shared.json',
+          },
+        },
+      },
+      agy: {
+        default: 'shared@gmail.com',
+        accounts: {
+          'shared@gmail.com': {
+            email: 'shared@gmail.com',
+            tokenFile: 'agy-shared.json',
+          },
+        },
+      },
+    });
+
+    const dupes = detectCrossProviderDuplicates();
+    expect(dupes.size).toBe(1);
+    expect(dupes.get('shared@gmail.com')).toEqual(['gemini', 'agy']);
+  });
+
+  it('should skip paused accounts', () => {
+    writeRegistry({
+      gemini: {
+        default: 'shared@gmail.com',
+        accounts: {
+          'shared@gmail.com': {
+            email: 'shared@gmail.com',
+            tokenFile: 'gemini-shared.json',
+            paused: true,
+          },
+        },
+      },
+      agy: {
+        default: 'shared@gmail.com',
+        accounts: {
+          'shared@gmail.com': {
+            email: 'shared@gmail.com',
+            tokenFile: 'agy-shared.json',
+          },
+        },
+      },
+    });
+
+    const dupes = detectCrossProviderDuplicates();
+    expect(dupes.size).toBe(0);
+  });
+
+  it('should be case-insensitive on email', () => {
+    writeRegistry({
+      gemini: {
+        default: 'User@Gmail.com',
+        accounts: {
+          'User@Gmail.com': {
+            email: 'User@Gmail.com',
+            tokenFile: 'gemini-user.json',
+          },
+        },
+      },
+      agy: {
+        default: 'user@gmail.com',
+        accounts: {
+          'user@gmail.com': {
+            email: 'user@gmail.com',
+            tokenFile: 'agy-user.json',
+          },
+        },
+      },
+    });
+
+    const dupes = detectCrossProviderDuplicates();
+    expect(dupes.size).toBe(1);
+  });
+});
+
+// ========================================
+// checkNewAccountConflict
+// ========================================
+
+describe('checkNewAccountConflict', () => {
+  it('should return null for non-Google provider', () => {
+    const result = checkNewAccountConflict('kiro' as never, 'user@gmail.com');
+    expect(result).toBeNull();
+  });
+
+  it('should return null when no conflict', () => {
+    writeRegistry({
+      gemini: {
+        default: 'other@gmail.com',
+        accounts: {
+          'other@gmail.com': {
+            email: 'other@gmail.com',
+            tokenFile: 'gemini-other.json',
+          },
+        },
+      },
+    });
+
+    const result = checkNewAccountConflict('agy', 'new@gmail.com');
+    expect(result).toBeNull();
+  });
+
+  it('should return conflicting providers', () => {
+    writeRegistry({
+      gemini: {
+        default: 'shared@gmail.com',
+        accounts: {
+          'shared@gmail.com': {
+            email: 'shared@gmail.com',
+            tokenFile: 'gemini-shared.json',
+          },
+        },
+      },
+    });
+
+    const result = checkNewAccountConflict('agy', 'shared@gmail.com');
+    expect(result).toEqual(['gemini']);
+  });
+
+  it('should return null when email is undefined', () => {
+    const result = checkNewAccountConflict('agy', undefined);
+    expect(result).toBeNull();
+  });
+});
+
+// ========================================
+// cleanupStaleAutoPauses
+// ========================================
+
+describe('cleanupStaleAutoPauses', () => {
+  it('should do nothing when no sessions', () => {
+    // No auto-paused.json exists
+    cleanupStaleAutoPauses();
+    // Should not throw
+  });
+
+  it('should remove sessions with dead PIDs', () => {
+    // Use PID 999999999 which is almost certainly dead
+    writeAutoPaused([
+      {
+        initiator: 'gemini',
+        pid: 999999999,
+        pausedAt: new Date().toISOString(),
+        accounts: [{ provider: 'agy', accountId: 'test@gmail.com' }],
+      },
+    ]);
+
+    // Write registry with the paused account so resumeAccount can find it
+    writeRegistry({
+      agy: {
+        default: 'test@gmail.com',
+        accounts: {
+          'test@gmail.com': {
+            email: 'test@gmail.com',
+            tokenFile: 'agy-test.json',
+            paused: true,
+            pausedAt: new Date().toISOString(),
+          },
+        },
+      },
+    });
+    writeTokenFile('agy-test.json', true);
+
+    cleanupStaleAutoPauses();
+
+    const data = readAutoPaused();
+    expect(data.sessions.length).toBe(0);
+  });
+
+  it('should keep sessions with alive PIDs', () => {
+    const alivePid = process.pid; // Current process is alive
+
+    writeAutoPaused([
+      {
+        initiator: 'gemini',
+        pid: alivePid,
+        pausedAt: new Date().toISOString(),
+        accounts: [{ provider: 'agy', accountId: 'test@gmail.com' }],
+      },
+    ]);
+
+    cleanupStaleAutoPauses();
+
+    const data = readAutoPaused();
+    expect(data.sessions.length).toBe(1);
+  });
+});
+
+// ========================================
+// enforceProviderIsolation
+// ========================================
+
+describe('enforceProviderIsolation', () => {
+  it('should return 0 for non-Google provider', () => {
+    const result = enforceProviderIsolation('kiro' as never);
+    expect(result).toBe(0);
+  });
+
+  it('should return 0 when no conflicting accounts', () => {
+    writeRegistry({
+      gemini: {
+        default: 'user1@gmail.com',
+        accounts: {
+          'user1@gmail.com': {
+            email: 'user1@gmail.com',
+            tokenFile: 'gemini-user1.json',
+          },
+        },
+      },
+      agy: {
+        default: 'user2@gmail.com',
+        accounts: {
+          'user2@gmail.com': {
+            email: 'user2@gmail.com',
+            tokenFile: 'agy-user2.json',
+          },
+        },
+      },
+    });
+    writeTokenFile('gemini-user1.json');
+    writeTokenFile('agy-user2.json');
+
+    const result = enforceProviderIsolation('gemini');
+    expect(result).toBe(0);
+  });
+
+  it('should pause conflicting accounts and record session', () => {
+    writeRegistry({
+      gemini: {
+        default: 'shared@gmail.com',
+        accounts: {
+          'shared@gmail.com': {
+            email: 'shared@gmail.com',
+            tokenFile: 'gemini-shared.json',
+          },
+        },
+      },
+      agy: {
+        default: 'shared@gmail.com',
+        accounts: {
+          'shared@gmail.com': {
+            email: 'shared@gmail.com',
+            tokenFile: 'agy-shared.json',
+          },
+        },
+      },
+    });
+    writeTokenFile('gemini-shared.json');
+    writeTokenFile('agy-shared.json');
+
+    const result = enforceProviderIsolation('gemini');
+    expect(result).toBe(1);
+
+    // Verify auto-paused.json was written
+    const data = readAutoPaused();
+    expect(data.sessions.length).toBe(1);
+    expect(data.sessions[0].initiator).toBe('gemini');
+    expect(data.sessions[0].pid).toBe(process.pid);
+  });
+});
+
+// ========================================
+// restoreAutoPausedAccounts
+// ========================================
+
+describe('restoreAutoPausedAccounts', () => {
+  it('should do nothing when no session exists', () => {
+    restoreAutoPausedAccounts('gemini');
+    // Should not throw
+  });
+
+  it('should skip accounts re-paused after enforcement', () => {
+    const enforcementTime = '2024-01-01T00:00:00.000Z';
+    const laterTime = '2024-01-01T01:00:00.000Z';
+
+    writeAutoPaused([
+      {
+        initiator: 'gemini',
+        pid: process.pid,
+        pausedAt: enforcementTime,
+        accounts: [{ provider: 'agy', accountId: 'banned@gmail.com' }],
+      },
+    ]);
+
+    writeRegistry({
+      agy: {
+        default: 'banned@gmail.com',
+        accounts: {
+          'banned@gmail.com': {
+            email: 'banned@gmail.com',
+            tokenFile: 'agy-banned.json',
+            paused: true,
+            pausedAt: laterTime, // Re-paused AFTER enforcement (e.g., ban)
+          },
+        },
+      },
+    });
+    writeTokenFile('agy-banned.json', true);
+
+    restoreAutoPausedAccounts('gemini');
+
+    // Account should NOT be restored because it was re-paused later
+    const registry = JSON.parse(
+      fs.readFileSync(path.join(ccsDir(), 'cliproxy', 'accounts.json'), 'utf-8')
+    );
+    expect(registry.providers.agy.accounts['banned@gmail.com'].paused).toBe(true);
+  });
+});
+
+// ========================================
+// handleBanDetection
+// ========================================
+
+describe('handleBanDetection', () => {
+  it('should pause account when ban error detected', () => {
+    writeRegistry({
+      gemini: {
+        default: 'user@gmail.com',
+        accounts: {
+          'user@gmail.com': {
+            email: 'user@gmail.com',
+            tokenFile: 'gemini-user.json',
+          },
+        },
+      },
+    });
+    writeTokenFile('gemini-user.json');
+
+    const result = handleBanDetection(
+      'gemini',
+      'user@gmail.com',
+      'API access disabled in this account'
+    );
+
+    expect(result).toBe(true);
+
+    // Verify account was paused in registry
+    const registry = JSON.parse(
+      fs.readFileSync(path.join(ccsDir(), 'cliproxy', 'accounts.json'), 'utf-8')
+    );
+    expect(registry.providers.gemini.accounts['user@gmail.com'].paused).toBe(true);
+  });
+
+  it('should return false for non-ban errors', () => {
+    writeRegistry({
+      gemini: {
+        default: 'user@gmail.com',
+        accounts: {
+          'user@gmail.com': {
+            email: 'user@gmail.com',
+            tokenFile: 'gemini-user.json',
+          },
+        },
+      },
+    });
+    writeTokenFile('gemini-user.json');
+
+    const result = handleBanDetection('gemini', 'user@gmail.com', 'Rate limit exceeded');
+
+    expect(result).toBe(false);
+
+    // Verify account was NOT paused
+    const registry = JSON.parse(
+      fs.readFileSync(path.join(ccsDir(), 'cliproxy', 'accounts.json'), 'utf-8')
+    );
+    expect(registry.providers.gemini.accounts['user@gmail.com'].paused).toBeUndefined();
+  });
+});
+
+// ========================================
+// warnCrossProviderDuplicates
+// ========================================
+
+describe('warnCrossProviderDuplicates', () => {
+  it('should return true when duplicates exist', () => {
+    writeRegistry({
+      gemini: {
+        default: 'shared@gmail.com',
+        accounts: {
+          'shared@gmail.com': {
+            email: 'shared@gmail.com',
+            tokenFile: 'gemini-shared.json',
+          },
+        },
+      },
+      agy: {
+        default: 'shared@gmail.com',
+        accounts: {
+          'shared@gmail.com': {
+            email: 'shared@gmail.com',
+            tokenFile: 'agy-shared.json',
+          },
+        },
+      },
+    });
+
+    const result = warnCrossProviderDuplicates('gemini');
+    expect(result).toBe(true);
+  });
+
+  it('should return false when no duplicates', () => {
+    writeRegistry({
+      gemini: {
+        default: 'user1@gmail.com',
+        accounts: {
+          'user1@gmail.com': {
+            email: 'user1@gmail.com',
+            tokenFile: 'gemini-user1.json',
+          },
+        },
+      },
+      agy: {
+        default: 'user2@gmail.com',
+        accounts: {
+          'user2@gmail.com': {
+            email: 'user2@gmail.com',
+            tokenFile: 'agy-user2.json',
+          },
+        },
+      },
+    });
+
+    const result = warnCrossProviderDuplicates('gemini');
+    expect(result).toBe(false);
+  });
+
+  it('should return false for non-Google providers', () => {
+    writeRegistry({
+      kiro: {
+        default: 'user@example.com',
+        accounts: {
+          'user@example.com': {
+            email: 'user@example.com',
+            tokenFile: 'kiro-user.json',
+          },
+        },
+      },
+    });
+
+    const result = warnCrossProviderDuplicates('kiro' as never);
+    expect(result).toBe(false);
+  });
+});