diff --git a/src/cliproxy/account-manager.ts b/src/cliproxy/account-manager.ts
index fc450c7b..29ce11ba 100644
--- a/src/cliproxy/account-manager.ts
+++ b/src/cliproxy/account-manager.ts
@@ -47,6 +47,8 @@ export interface AccountInfo {
   pausedAt?: string;
   /** Account tier: free or paid (Pro/Ultra combined) */
   tier?: AccountTier;
+  /** GCP Project ID (Antigravity only) - read-only, fetched from auth token */
+  projectId?: string;
 }
 
 /** Provider accounts configuration */
@@ -293,7 +295,8 @@ export function registerAccount(
   provider: CLIProxyProvider,
   tokenFile: string,
   email?: string,
-  nickname?: string
+  nickname?: string,
+  projectId?: string
 ): AccountInfo {
   const registry = loadAccountsRegistry();
 
@@ -350,7 +353,7 @@ export function registerAccount(
   const isFirstAccount = Object.keys(providerAccounts.accounts).length === 0;
 
   // Create or update account
-  providerAccounts.accounts[accountId] = {
+  const accountMeta: Omit<AccountInfo, 'id' | 'provider' | 'isDefault'> = {
     email,
     nickname: accountNickname,
     tokenFile,
@@ -358,6 +361,13 @@ export function registerAccount(
     lastUsedAt: new Date().toISOString(),
   };
 
+  // Include projectId for Antigravity accounts
+  if (provider === 'agy' && projectId) {
+    accountMeta.projectId = projectId;
+  }
+
+  providerAccounts.accounts[accountId] = accountMeta;
+
   // Set as default if first account
   if (isFirstAccount) {
     providerAccounts.default = accountId;
@@ -374,6 +384,7 @@ export function registerAccount(
     tokenFile,
     createdAt: providerAccounts.accounts[accountId].createdAt,
     lastUsedAt: providerAccounts.accounts[accountId].lastUsedAt,
+    projectId: providerAccounts.accounts[accountId].projectId,
   };
 }
 
@@ -629,6 +640,20 @@ export function discoverExistingAccounts(): void {
       // Skip if token file already registered (under any accountId)
       const existingTokenFiles = Object.values(providerAccounts.accounts).map((a) => a.tokenFile);
       if (existingTokenFiles.includes(file)) {
+        // Token file exists - check if we need to update projectId for agy accounts
+        const projectIdValue =
+          typeof data.project_id === 'string' && data.project_id.trim()
+            ? data.project_id.trim()
+            : null;
+        if (provider === 'agy' && projectIdValue) {
+          const existingEntry = Object.entries(providerAccounts.accounts).find(
+            ([, meta]) => meta.tokenFile === file
+          );
+          // Update if missing or changed
+          if (existingEntry && existingEntry[1].projectId !== projectIdValue) {
+            existingEntry[1].projectId = projectIdValue;
+          }
+        }
         continue;
       }
 
@@ -671,13 +696,24 @@ export function discoverExistingAccounts(): void {
       // Register account with auto-generated nickname
       // Use mtime as lastUsedAt (when token was last modified = last auth/refresh)
       const lastModified = stats.mtime || stats.birthtime || new Date();
-      providerAccounts.accounts[accountId] = {
+      const accountMeta: Omit<AccountInfo, 'id' | 'provider' | 'isDefault'> = {
         email,
         nickname: generateNickname(email),
         tokenFile: file,
         createdAt: stats.birthtime?.toISOString() || new Date().toISOString(),
         lastUsedAt: lastModified.toISOString(),
       };
+
+      // Read project_id for Antigravity accounts (read-only field from auth token)
+      const discoveredProjectId =
+        typeof data.project_id === 'string' && data.project_id.trim()
+          ? data.project_id.trim()
+          : null;
+      if (provider === 'agy' && discoveredProjectId) {
+        accountMeta.projectId = discoveredProjectId;
+      }
+
+      providerAccounts.accounts[accountId] = accountMeta;
     } catch {
       // Skip invalid files
       continue;
@@ -693,7 +729,7 @@ export function discoverExistingAccounts(): void {
     if (!freshRegistry.providers[prov]) {
       freshRegistry.providers[prov] = discovered;
     } else {
-      // Merge accounts, preferring fresh registry's existing entries
+      // Merge accounts, preferring fresh registry's existing entries but updating projectId
       const freshProviderAccounts = freshRegistry.providers[prov];
       if (!freshProviderAccounts) continue;
       for (const [id, meta] of Object.entries(discovered.accounts)) {
@@ -703,6 +739,9 @@ export function discoverExistingAccounts(): void {
           if (!freshProviderAccounts.default || freshProviderAccounts.default === 'default') {
             freshProviderAccounts.default = id;
           }
+        } else if (meta.projectId && !freshProviderAccounts.accounts[id].projectId) {
+          // Update existing account with projectId if discovered from auth file
+          freshProviderAccounts.accounts[id].projectId = meta.projectId;
         }
       }
     }
diff --git a/src/cliproxy/auth/token-manager.ts b/src/cliproxy/auth/token-manager.ts
index 436539cc..16e79c78 100644
--- a/src/cliproxy/auth/token-manager.ts
+++ b/src/cliproxy/auth/token-manager.ts
@@ -229,8 +229,15 @@ export function registerAccountFromToken(
     const content = fs.readFileSync(tokenPath, 'utf-8');
     const data = JSON.parse(content);
     const email = data.email || undefined;
+    const projectId = data.project_id || undefined;
 
-    return registerAccount(provider, newestFile, email, nickname || generateNickname(email));
+    return registerAccount(
+      provider,
+      newestFile,
+      email,
+      nickname || generateNickname(email),
+      projectId
+    );
   } catch {
     return null;
   }
diff --git a/ui/src/components/cliproxy/provider-editor/account-item.tsx b/ui/src/components/cliproxy/provider-editor/account-item.tsx
index 031a01b3..2aec131d 100644
--- a/ui/src/components/cliproxy/provider-editor/account-item.tsx
+++ b/ui/src/components/cliproxy/provider-editor/account-item.tsx
@@ -25,6 +25,8 @@ import {
   Pause,
   Play,
   AlertCircle,
+  AlertTriangle,
+  FolderCode,
 } from 'lucide-react';
 import {
   cn,
@@ -167,6 +169,54 @@ export function AccountItem({
                 </Badge>
               )}
             </div>
+            {/* Project ID for Antigravity accounts - read-only */}
+            {account.provider === 'agy' && (
+              <div className="flex items-center gap-1.5 mt-1">
+                {account.projectId ? (
+                  <TooltipProvider>
+                    <Tooltip>
+                      <TooltipTrigger asChild>
+                        <div className="flex items-center gap-1 text-xs text-muted-foreground">
+                          <FolderCode className="w-3 h-3" aria-hidden="true" />
+                          <span
+                            className={cn(
+                              'font-mono max-w-[180px] truncate',
+                              privacyMode && PRIVACY_BLUR_CLASS
+                            )}
+                            title={account.projectId}
+                          >
+                            {account.projectId}
+                          </span>
+                        </div>
+                      </TooltipTrigger>
+                      <TooltipContent side="bottom">
+                        <p className="text-xs">GCP Project ID (read-only)</p>
+                      </TooltipContent>
+                    </Tooltip>
+                  </TooltipProvider>
+                ) : (
+                  <TooltipProvider>
+                    <Tooltip>
+                      <TooltipTrigger asChild>
+                        <div className="flex items-center gap-1 text-xs text-amber-600 dark:text-amber-500">
+                          <AlertTriangle className="w-3 h-3" aria-label="Warning" />
+                          <span>Project ID: N/A</span>
+                        </div>
+                      </TooltipTrigger>
+                      <TooltipContent side="bottom" className="max-w-[250px]">
+                        <div className="text-xs space-y-1">
+                          <p className="font-medium text-amber-600">Missing Project ID</p>
+                          <p>
+                            This may cause errors. Remove the account and re-add it to fetch the
+                            project ID.
+                          </p>
+                        </div>
+                      </TooltipContent>
+                    </Tooltip>
+                  </TooltipProvider>
+                )}
+              </div>
+            )}
             {account.lastUsedAt && (
               <div className="flex items-center gap-1 text-xs text-muted-foreground mt-0.5">
                 <Clock className="w-3 h-3" />
diff --git a/ui/src/components/monitoring/auth-monitor/components/provider-card.tsx b/ui/src/components/monitoring/auth-monitor/components/provider-card.tsx
index d7faf5f9..56faedbc 100644
--- a/ui/src/components/monitoring/auth-monitor/components/provider-card.tsx
+++ b/ui/src/components/monitoring/auth-monitor/components/provider-card.tsx
@@ -3,10 +3,11 @@
  */
 
 import type React from 'react';
-import { ChevronRight } from 'lucide-react';
+import { ChevronRight, AlertTriangle } from 'lucide-react';
 import { cn, STATUS_COLORS } from '@/lib/utils';
 import { PROVIDER_COLORS } from '@/lib/provider-config';
 import { ProviderIcon } from '@/components/shared/provider-icon';
+import { Tooltip, TooltipContent, TooltipProvider, TooltipTrigger } from '@/components/ui/tooltip';
 import type { ProviderStats } from '../types';
 import { getSuccessRate, cleanEmail } from '../utils';
 import { InlineStatsBadge } from './inline-stats-badge';
@@ -102,16 +103,35 @@ export function ProviderCard({
         </div>
       </div>
 
-      {/* Account color dots */}
-      <div className="flex gap-1 mt-3">
-        {stats.accounts.slice(0, 5).map((acc) => (
-          <div
-            key={acc.id}
-            className="w-2 h-2 rounded-full"
-            style={{ backgroundColor: acc.color }}
-            title={privacyMode ? '••••••' : cleanEmail(acc.email)}
-          />
-        ))}
+      {/* Account color dots with warning for agy accounts missing projectId */}
+      <div className="flex gap-1 mt-3 items-center">
+        {stats.accounts.slice(0, 5).map((acc) => {
+          const isMissingProjectId = stats.provider === 'agy' && !acc.projectId;
+          return (
+            <div key={acc.id} className="relative">
+              <div
+                className="w-2 h-2 rounded-full"
+                style={{ backgroundColor: acc.color }}
+                title={privacyMode ? '••••••' : cleanEmail(acc.email)}
+              />
+              {isMissingProjectId && (
+                <TooltipProvider>
+                  <Tooltip>
+                    <TooltipTrigger asChild>
+                      <AlertTriangle
+                        className="absolute -top-1 -right-1 w-2.5 h-2.5 text-amber-500"
+                        aria-label="Missing Project ID"
+                      />
+                    </TooltipTrigger>
+                    <TooltipContent side="top" className="text-xs">
+                      Missing Project ID - re-add account to fix
+                    </TooltipContent>
+                  </Tooltip>
+                </TooltipProvider>
+              )}
+            </div>
+          );
+        })}
         {stats.accounts.length > 5 && (
           <span className="text-[10px] text-muted-foreground ml-1">
             +{stats.accounts.length - 5}
diff --git a/ui/src/components/monitoring/auth-monitor/hooks.ts b/ui/src/components/monitoring/auth-monitor/hooks.ts
index 1640f037..dd19b8bc 100644
--- a/ui/src/components/monitoring/auth-monitor/hooks.ts
+++ b/ui/src/components/monitoring/auth-monitor/hooks.ts
@@ -99,6 +99,7 @@ export function useAuthMonitorData(): AuthMonitorData {
           failureCount: failure,
           lastUsedAt: realStats?.lastUsedAt ?? account.lastUsedAt,
           color: ACCOUNT_COLORS[colorIndex % ACCOUNT_COLORS.length],
+          projectId: account.projectId,
         };
         accountsList.push(row);
         providerData.accounts.push(row);
diff --git a/ui/src/components/monitoring/auth-monitor/types.ts b/ui/src/components/monitoring/auth-monitor/types.ts
index 1eb61953..408b1e71 100644
--- a/ui/src/components/monitoring/auth-monitor/types.ts
+++ b/ui/src/components/monitoring/auth-monitor/types.ts
@@ -12,6 +12,8 @@ export interface AccountRow {
   failureCount: number;
   lastUsedAt?: string;
   color: string;
+  /** GCP Project ID (Antigravity only) - read-only */
+  projectId?: string;
 }
 
 export interface ProviderStats {
diff --git a/ui/src/lib/api-client.ts b/ui/src/lib/api-client.ts
index 22d67bb0..95ce900c 100644
--- a/ui/src/lib/api-client.ts
+++ b/ui/src/lib/api-client.ts
@@ -83,6 +83,8 @@ export interface OAuthAccount {
   pausedAt?: string;
   /** Account tier: free or paid (Pro/Ultra combined) */
   tier?: 'free' | 'paid' | 'unknown';
+  /** GCP Project ID (Antigravity only) - read-only */
+  projectId?: string;
 }
 
 export interface AuthStatus {