diff --git a/packages/cli/src/ui/components/AuthDialog.test.tsx b/packages/cli/src/ui/components/AuthDialog.test.tsx
index d9f1bd2c..2850762f 100644
--- a/packages/cli/src/ui/components/AuthDialog.test.tsx
+++ b/packages/cli/src/ui/components/AuthDialog.test.tsx
@@ -18,6 +18,7 @@ describe('AuthDialog', () => {
   beforeEach(() => {
     originalEnv = { ...process.env };
     process.env.GEMINI_API_KEY = '';
+    process.env.GEMINI_DEFAULT_AUTH_TYPE = '';
     vi.clearAllMocks();
   });
 
@@ -59,28 +60,165 @@ describe('AuthDialog', () => {
     );
   });
 
-  it('should detect GEMINI_API_KEY environment variable', () => {
-    process.env.GEMINI_API_KEY = 'foobar';
+  describe('GEMINI_API_KEY environment variable', () => {
+    it('should detect GEMINI_API_KEY environment variable', () => {
+      process.env.GEMINI_API_KEY = 'foobar';
 
-    const settings: LoadedSettings = new LoadedSettings(
-      {
-        settings: {
-          selectedAuthType: undefined,
+      const settings: LoadedSettings = new LoadedSettings(
+        {
+          settings: {
+            selectedAuthType: undefined,
+          },
+          path: '',
         },
-        path: '',
-      },
-      {
-        settings: {},
-        path: '',
-      },
-      [],
-    );
+        {
+          settings: {},
+          path: '',
+        },
+        [],
+      );
 
-    const { lastFrame } = render(
-      <AuthDialog onSelect={() => {}} settings={settings} />,
-    );
+      const { lastFrame } = render(
+        <AuthDialog onSelect={() => {}} settings={settings} />,
+      );
 
-    expect(lastFrame()).toContain('Existing API key detected (GEMINI_API_KEY)');
+      expect(lastFrame()).toContain(
+        'Existing API key detected (GEMINI_API_KEY)',
+      );
+    });
+
+    it('should not show the GEMINI_API_KEY message if GEMINI_DEFAULT_AUTH_TYPE is set to something else', () => {
+      process.env.GEMINI_API_KEY = 'foobar';
+      process.env.GEMINI_DEFAULT_AUTH_TYPE = AuthType.LOGIN_WITH_GOOGLE;
+
+      const settings: LoadedSettings = new LoadedSettings(
+        {
+          settings: {
+            selectedAuthType: undefined,
+          },
+          path: '',
+        },
+        {
+          settings: {},
+          path: '',
+        },
+        [],
+      );
+
+      const { lastFrame } = render(
+        <AuthDialog onSelect={() => {}} settings={settings} />,
+      );
+
+      expect(lastFrame()).not.toContain(
+        'Existing API key detected (GEMINI_API_KEY)',
+      );
+    });
+
+    it('should show the GEMINI_API_KEY message if GEMINI_DEFAULT_AUTH_TYPE is set to use api key', () => {
+      process.env.GEMINI_API_KEY = 'foobar';
+      process.env.GEMINI_DEFAULT_AUTH_TYPE = AuthType.USE_GEMINI;
+
+      const settings: LoadedSettings = new LoadedSettings(
+        {
+          settings: {
+            selectedAuthType: undefined,
+          },
+          path: '',
+        },
+        {
+          settings: {},
+          path: '',
+        },
+        [],
+      );
+
+      const { lastFrame } = render(
+        <AuthDialog onSelect={() => {}} settings={settings} />,
+      );
+
+      expect(lastFrame()).toContain(
+        'Existing API key detected (GEMINI_API_KEY)',
+      );
+    });
+  });
+
+  describe('GEMINI_DEFAULT_AUTH_TYPE environment variable', () => {
+    it('should select the auth type specified by GEMINI_DEFAULT_AUTH_TYPE', () => {
+      process.env.GEMINI_DEFAULT_AUTH_TYPE = AuthType.LOGIN_WITH_GOOGLE;
+
+      const settings: LoadedSettings = new LoadedSettings(
+        {
+          settings: {
+            selectedAuthType: undefined,
+          },
+          path: '',
+        },
+        {
+          settings: {},
+          path: '',
+        },
+        [],
+      );
+
+      const { lastFrame } = render(
+        <AuthDialog onSelect={() => {}} settings={settings} />,
+      );
+
+      // This is a bit brittle, but it's the best way to check which item is selected.
+      expect(lastFrame()).toContain('● Login with Google');
+    });
+
+    it('should fall back to default if GEMINI_DEFAULT_AUTH_TYPE is not set', () => {
+      const settings: LoadedSettings = new LoadedSettings(
+        {
+          settings: {
+            selectedAuthType: undefined,
+          },
+          path: '',
+        },
+        {
+          settings: {},
+          path: '',
+        },
+        [],
+      );
+
+      const { lastFrame } = render(
+        <AuthDialog onSelect={() => {}} settings={settings} />,
+      );
+
+      // Default is LOGIN_WITH_GOOGLE
+      expect(lastFrame()).toContain('● Login with Google');
+    });
+
+    it('should show an error and fall back to default if GEMINI_DEFAULT_AUTH_TYPE is invalid', () => {
+      process.env.GEMINI_DEFAULT_AUTH_TYPE = 'invalid-auth-type';
+
+      const settings: LoadedSettings = new LoadedSettings(
+        {
+          settings: {
+            selectedAuthType: undefined,
+          },
+          path: '',
+        },
+        {
+          settings: {},
+          path: '',
+        },
+        [],
+      );
+
+      const { lastFrame } = render(
+        <AuthDialog onSelect={() => {}} settings={settings} />,
+      );
+
+      expect(lastFrame()).toContain(
+        'Invalid value for GEMINI_DEFAULT_AUTH_TYPE: "invalid-auth-type"',
+      );
+
+      // Default is LOGIN_WITH_GOOGLE
+      expect(lastFrame()).toContain('● Login with Google');
+    });
   });
 
   it('should prevent exiting when no auth method is selected and show error message', async () => {
diff --git a/packages/cli/src/ui/components/AuthDialog.tsx b/packages/cli/src/ui/components/AuthDialog.tsx
index d46feb0b..ae076ee7 100644
--- a/packages/cli/src/ui/components/AuthDialog.tsx
+++ b/packages/cli/src/ui/components/AuthDialog.tsx
@@ -18,18 +18,47 @@ interface AuthDialogProps {
   initialErrorMessage?: string | null;
 }
 
+function parseDefaultAuthType(
+  defaultAuthType: string | undefined,
+): AuthType | null {
+  if (
+    defaultAuthType &&
+    Object.values(AuthType).includes(defaultAuthType as AuthType)
+  ) {
+    return defaultAuthType as AuthType;
+  }
+  return null;
+}
+
 export function AuthDialog({
   onSelect,
   settings,
   initialErrorMessage,
 }: AuthDialogProps): React.JSX.Element {
-  const [errorMessage, setErrorMessage] = useState<string | null>(
-    initialErrorMessage
-      ? initialErrorMessage
-      : process.env.GEMINI_API_KEY
-        ? 'Existing API key detected (GEMINI_API_KEY). Select "Gemini API Key" option to use it.'
-        : null,
-  );
+  const [errorMessage, setErrorMessage] = useState<string | null>(() => {
+    if (initialErrorMessage) {
+      return initialErrorMessage;
+    }
+
+    const defaultAuthType = parseDefaultAuthType(
+      process.env.GEMINI_DEFAULT_AUTH_TYPE,
+    );
+
+    if (process.env.GEMINI_DEFAULT_AUTH_TYPE && defaultAuthType === null) {
+      return (
+        `Invalid value for GEMINI_DEFAULT_AUTH_TYPE: "${process.env.GEMINI_DEFAULT_AUTH_TYPE}". ` +
+        `Valid values are: ${Object.values(AuthType).join(', ')}.`
+      );
+    }
+
+    if (
+      process.env.GEMINI_API_KEY &&
+      (!defaultAuthType || defaultAuthType === AuthType.USE_GEMINI)
+    ) {
+      return 'Existing API key detected (GEMINI_API_KEY). Select "Gemini API Key" option to use it.';
+    }
+    return null;
+  });
   const items = [
     {
       label: 'Login with Google',
@@ -55,6 +84,13 @@ export function AuthDialog({
       return item.value === settings.merged.selectedAuthType;
     }
 
+    const defaultAuthType = parseDefaultAuthType(
+      process.env.GEMINI_DEFAULT_AUTH_TYPE,
+    );
+    if (defaultAuthType) {
+      return item.value === defaultAuthType;
+    }
+
     if (process.env.GEMINI_API_KEY) {
       return item.value === AuthType.USE_GEMINI;
     }