diff --git a/src/cliproxy/account-manager.ts b/src/cliproxy/account-manager.ts
index fc450c7b..8906994b 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;
@@ -671,13 +681,20 @@ 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)
+      if (provider === 'agy' && data.project_id) {
+        accountMeta.projectId = data.project_id;
+      }
+
+      providerAccounts.accounts[accountId] = accountMeta;
     } catch {
       // Skip invalid files
       continue;
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..d61ea74d 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 +169,48 @@ 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" />
+                          <span className={cn('font-mono', privacyMode && PRIVACY_BLUR_CLASS)}>
+                            {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" />
+                          <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/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 {