Improve error messages on failed onboarding (#17357)

packages/cli/src/core/auth.ts

@@ -8,6 +8,7 @@ import {
   type AuthType,
   type Config,
   getErrorMessage,
+  ValidationRequiredError,
 } from '@google/gemini-cli-core';
 
 /**
@@ -29,6 +30,11 @@ export async function performInitialAuth(
     // The console.log is intentionally left out here.
     // We can add a dedicated startup message later if needed.
   } catch (e) {
+    if (e instanceof ValidationRequiredError) {
+      // Don't treat validation required as a fatal auth error during startup.
+      // This allows the React UI to load and show the ValidationDialog.
+      return null;
+    }
     return `Failed to login. Message: ${getErrorMessage(e)}`;
   }