diff --git a/src/cliproxy/account-safety.ts b/src/cliproxy/account-safety.ts
index 8c9c2136..f7a89cde 100644
--- a/src/cliproxy/account-safety.ts
+++ b/src/cliproxy/account-safety.ts
@@ -16,8 +16,13 @@ import { CLIProxyProvider } from './types';
 import { loadAccountsRegistry, pauseAccount, resumeAccount } from './accounts/registry';
 import { getCcsDir } from '../utils/config-manager';
 
+const ISSUE_509_URL = 'https://github.com/kaitranntt/ccs/issues/509';
+
 /** Providers that use Google OAuth (ban risk when overlapping) */
 const GOOGLE_OAUTH_PROVIDERS: CLIProxyProvider[] = ['gemini', 'agy', 'codex'];
+/** Providers that should display direct CLI warnings for #509 */
+const BAN_WARNING_PROVIDERS: CLIProxyProvider[] = ['gemini', 'agy'];
+const shownBanWarnings = new Set<CLIProxyProvider>();
 
 // --- Auto-pause persistence (crash recovery) ---
 
@@ -161,6 +166,10 @@ export function warnCrossProviderDuplicates(provider: CLIProxyProvider): boolean
   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(
+    '    If provider requests start returning 403/Forbidden, treat it as a possible ban.'
+  );
+  console.error(`    Details: ${ISSUE_509_URL}`);
   console.error('');
 
   for (const [email, providers] of duplicates) {
@@ -188,10 +197,67 @@ export function warnNewAccountConflict(
     `    ${maskEmail(email)} is also registered under: ${conflictingProviders.join(', ')}`
   );
   console.error('    Concurrent usage may cause Google to ban your account.');
+  console.error('    403/Forbidden responses can be an early sign of account disablement.');
   console.error('    Consider pausing the duplicate or using a different account.');
+  console.error(`    Details: ${ISSUE_509_URL}`);
   console.error('');
 }
 
+function isBanWarningProvider(provider: CLIProxyProvider): boolean {
+  return BAN_WARNING_PROVIDERS.includes(provider);
+}
+
+/**
+ * Show one-time warning for known OAuth ban risk providers.
+ */
+export function warnOAuthBanRisk(provider: CLIProxyProvider): void {
+  if (!isBanWarningProvider(provider) || shownBanWarnings.has(provider)) return;
+
+  shownBanWarnings.add(provider);
+  console.error('');
+  console.error(warn('Account safety warning (#509)'));
+  console.error(
+    '    Using the same Google account in both "ccs gemini" and "ccs agy" can trigger suspension.'
+  );
+  console.error(
+    '    If you see 403/Forbidden during provider calls, treat it as likely account disable/ban.'
+  );
+  console.error(
+    '    Use separate Google accounts per provider and stop retrying blocked accounts.'
+  );
+  console.error(`    Details: ${ISSUE_509_URL}`);
+  console.error('');
+}
+
+/**
+ * Detect whether an error message contains a likely 403/Forbidden ban signal.
+ */
+export function isPossible403BanSignal(errorMessage: string): boolean {
+  const lower = errorMessage.toLowerCase();
+  return lower.includes('403') || lower.includes('forbidden');
+}
+
+/**
+ * Show targeted warning when OAuth provider errors include 403/Forbidden.
+ * Returns true when warning was emitted.
+ */
+export function warnPossible403Ban(provider: CLIProxyProvider, errorMessage: string): boolean {
+  if (!isBanWarningProvider(provider) || !isPossible403BanSignal(errorMessage)) {
+    return false;
+  }
+
+  console.error('');
+  console.error(warn(`Account safety: ${provider} returned 403/Forbidden`));
+  console.error(
+    '    For gemini/agy flows this often means the Google account was blocked/disabled.'
+  );
+  console.error('    Stop retries for this account and switch to a different account/provider.');
+  console.error(`    Details: ${ISSUE_509_URL}`);
+  console.error(`    Error: "${truncate(errorMessage, 160)}"`);
+  console.error('');
+  return true;
+}
+
 // --- Enforcement: auto-pause/restore ---
 
 /**
diff --git a/src/cliproxy/auth/oauth-handler.ts b/src/cliproxy/auth/oauth-handler.ts
index dc651216..f05316d2 100644
--- a/src/cliproxy/auth/oauth-handler.ts
+++ b/src/cliproxy/auth/oauth-handler.ts
@@ -44,7 +44,12 @@ 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';
+import {
+  checkNewAccountConflict,
+  warnNewAccountConflict,
+  warnOAuthBanRisk,
+  warnPossible403Ban,
+} from '../account-safety';
 
 /**
  * Prompt user to add another account
@@ -278,7 +283,9 @@ async function handlePasteCallbackMode(
     });
 
     if (!startResponse.ok) {
+      const startError = `OAuth start failed with status ${startResponse.status}`;
       console.log(fail('Failed to start OAuth flow'));
+      warnPossible403Ban(provider, startError);
       return null;
     }
 
@@ -380,7 +387,10 @@ async function handlePasteCallbackMode(
     };
 
     if (!callbackResponse.ok || callbackData.status === 'error') {
-      console.log(fail(callbackData.error || 'OAuth callback failed'));
+      const callbackError =
+        callbackData.error || `OAuth callback failed with status ${callbackResponse.status}`;
+      console.log(fail(callbackError));
+      warnPossible403Ban(provider, callbackError);
       return null;
     }
 
@@ -417,6 +427,7 @@ export async function triggerOAuth(
   options: OAuthOptions = {}
 ): Promise<AccountInfo | null> {
   const oauthConfig = getOAuthConfig(provider);
+  warnOAuthBanRisk(provider);
   const { verbose = false, add = false, fromUI = false, noIncognito = true } = options;
   let { nickname } = options;
   const resolvedKiroMethod =
diff --git a/src/cliproxy/executor/index.ts b/src/cliproxy/executor/index.ts
index 3ea5b1c7..edc701e4 100644
--- a/src/cliproxy/executor/index.ts
+++ b/src/cliproxy/executor/index.ts
@@ -67,6 +67,7 @@ import { checkOrJoinProxy, registerProxySession, setupCleanupHandlers } from './
 import { parseThinkingOverride } from './thinking-arg-parser';
 import {
   warnCrossProviderDuplicates,
+  warnOAuthBanRisk,
   cleanupStaleAutoPauses,
   enforceProviderIsolation,
   restoreAutoPausedAccounts,
@@ -187,6 +188,7 @@ export async function execClaudeWithCLIProxy(
 
   const providerConfig = getProviderConfig(provider);
   log(`Provider: ${providerConfig.displayName}`);
+  warnOAuthBanRisk(provider);
 
   // Check remote proxy if configured
   let useRemoteProxy = false;
diff --git a/src/cliproxy/executor/retry-handler.ts b/src/cliproxy/executor/retry-handler.ts
index 514af33f..7149cdad 100644
--- a/src/cliproxy/executor/retry-handler.ts
+++ b/src/cliproxy/executor/retry-handler.ts
@@ -10,7 +10,7 @@
 
 import { fail, warn, info } from '../../utils/ui';
 import { CLIProxyProvider } from '../types';
-import { handleBanDetection } from '../account-safety';
+import { handleBanDetection, warnPossible403Ban } from '../account-safety';
 import { CompositeTierConfig } from '../../config/unified-config-types';
 
 /**
@@ -59,6 +59,7 @@ export async function handleTokenExpiration(
       if (account) {
         handleBanDetection(provider, account.id, tokenResult.error);
       }
+      warnPossible403Ban(provider, tokenResult.error);
     }
 
     // Token expired and refresh failed - trigger re-auth
diff --git a/ui/src/pages/cliproxy.tsx b/ui/src/pages/cliproxy.tsx
index 3640565f..583b3355 100644
--- a/ui/src/pages/cliproxy.tsx
+++ b/ui/src/pages/cliproxy.tsx
@@ -8,9 +8,10 @@ import { useState, useMemo } from 'react';
 import { useQueryClient } from '@tanstack/react-query';
 import { Button } from '@/components/ui/button';
 import { Badge } from '@/components/ui/badge';
+import { Alert, AlertDescription, AlertTitle } from '@/components/ui/alert';
 import { ScrollArea } from '@/components/ui/scroll-area';
 import { Skeleton } from '@/components/ui/skeleton';
-import { Check, X, RefreshCw, Sparkles, Zap, GitBranch, Trash2 } from 'lucide-react';
+import { Check, X, RefreshCw, Sparkles, Zap, GitBranch, Trash2, AlertTriangle } from 'lucide-react';
 import { QuickSetupWizard } from '@/components/quick-setup-wizard';
 import { AddAccountDialog } from '@/components/account/add-account-dialog';
 import { ProviderEditor } from '@/components/cliproxy/provider-editor';
@@ -243,6 +244,10 @@ export function CliproxyPage() {
   const parentAuthForVariant = selectedVariantData
     ? providers.find((p) => p.provider === selectedVariantData.provider)
     : undefined;
+  const warningProvider = (selectedVariantData?.provider || selectedStatus?.provider || '')
+    .toLowerCase()
+    .trim();
+  const showAccountSafetyWarning = warningProvider === 'gemini' || warningProvider === 'agy';
 
   const handleRefresh = () => {
     queryClient.invalidateQueries({ queryKey: ['cliproxy'] });
@@ -389,6 +394,30 @@ export function CliproxyPage() {
 
       {/* Right Panel */}
       <div className="flex-1 flex flex-col min-w-0 bg-background">
+        {showAccountSafetyWarning && (
+          <div className="px-4 pt-4">
+            <Alert variant="warning" className="py-2">
+              <AlertTriangle className="h-4 w-4" />
+              <AlertTitle>Account Safety Warning (Issue #509)</AlertTitle>
+              <AlertDescription>
+                Using the same Google account in both <code className="font-mono">ccs gemini</code>{' '}
+                and <code className="font-mono">ccs agy</code> can trigger suspension. If CLI shows{' '}
+                <code className="font-mono">403 Forbidden</code>, treat it as likely account
+                disable/ban and switch accounts.{' '}
+                <a
+                  href="https://github.com/kaitranntt/ccs/issues/509"
+                  target="_blank"
+                  rel="noreferrer"
+                  className="underline font-medium"
+                >
+                  Read issue #509
+                </a>
+                .
+              </AlertDescription>
+            </Alert>
+          </div>
+        )}
+
         {selectedVariantData && parentAuthForVariant ? (
           // Variant selected - show ProviderEditor with variant profile name
           <ProviderEditor