diff --git a/src/cliproxy/auth/oauth-process.ts b/src/cliproxy/auth/oauth-process.ts
index ba157784..829d911c 100644
--- a/src/cliproxy/auth/oauth-process.ts
+++ b/src/cliproxy/auth/oauth-process.ts
@@ -217,13 +217,67 @@ function displayUrlFromStderr(
   }
 }
 
+const ANSI_ESCAPE_REGEX = /\x1b\[[0-9;]*m/g;
+
+export function extractLikelyAuthFailureFromStderr(
+  provider: CLIProxyProvider,
+  stderrData: string
+): string | null {
+  // Keep this scoped to ghcp to avoid over-classifying other providers.
+  if (provider !== 'ghcp') {
+    return null;
+  }
+
+  if (!stderrData.trim()) {
+    return null;
+  }
+
+  const normalizedLines = stderrData
+    .split('\n')
+    .map((line) => line.replace(ANSI_ESCAPE_REGEX, '').trim())
+    .filter(Boolean)
+    .map((line) => {
+      const messageIndex = line.indexOf('msg="');
+      if (messageIndex >= 0) {
+        const message = line
+          .slice(messageIndex + 5)
+          .replace(/"$/, '')
+          .trim();
+        if (message) {
+          return message;
+        }
+      }
+      return line;
+    });
+
+  const prioritizedPatterns = [
+    /github copilot authentication failed:\s*(.+)/i,
+    /authentication failed:\s*(.+)/i,
+    /failed to verify copilot access[^:]*:\s*(.+)/i,
+    /failed to save auth:\s*(.+)/i,
+  ];
+
+  for (let i = normalizedLines.length - 1; i >= 0; i--) {
+    const line = normalizedLines[i];
+    for (const pattern of prioritizedPatterns) {
+      const match = line.match(pattern);
+      if (match?.[1]?.trim()) {
+        return match[1].trim().slice(0, 240);
+      }
+    }
+  }
+
+  return null;
+}
+
 /** Handle token not found after successful process exit */
 async function handleTokenNotFound(
   provider: CLIProxyProvider,
   callbackPort: number | null,
   tokenDir: string,
   nickname: string | undefined,
-  verbose: boolean
+  verbose: boolean,
+  failureReason?: string
 ): Promise<AccountInfo | null> {
   // Kiro-specific: Try auto-import from Kiro IDE
   if (provider === 'kiro') {
@@ -248,6 +302,18 @@ async function handleTokenNotFound(
 
   // Default behavior for other providers
   console.log('');
+
+  if (failureReason) {
+    console.log(fail('Authentication completed but token was not persisted'));
+    console.log(`    ${failureReason}`);
+    console.log('');
+    console.log('This usually means provider-side authorization was accepted,');
+    console.log('but CLIProxy failed a post-auth verification or token save step.');
+    console.log('');
+    console.log(`Try: ccs ${provider} --auth --verbose`);
+    return null;
+  }
+
   console.log(fail('Token not found after authentication'));
   console.log('');
   console.log('The browser showed success but callback was not received.');
@@ -470,11 +536,13 @@ export function executeOAuthProcess(options: OAuthProcessOptions): Promise<Accou
 
           resolve(registerAccountFromToken(provider, tokenDir, nickname));
         } else {
+          const failureReason = extractLikelyAuthFailureFromStderr(provider, state.stderrData);
+
           // Emit device code failure event for UI
           if (isDeviceCodeFlow && state.deviceCodeDisplayed) {
             deviceCodeEvents.emit('deviceCode:failed', {
               sessionId: state.sessionId,
-              error: 'Token not found after authentication',
+              error: failureReason || 'Token not found after authentication',
             });
           }
 
@@ -484,7 +552,8 @@ export function executeOAuthProcess(options: OAuthProcessOptions): Promise<Accou
             callbackPort,
             tokenDir,
             nickname,
-            verbose
+            verbose,
+            failureReason || undefined
           );
           resolve(account);
         }
diff --git a/src/web-server/routes/cliproxy-auth-routes.ts b/src/web-server/routes/cliproxy-auth-routes.ts
index 940f9e7d..17d761e2 100644
--- a/src/web-server/routes/cliproxy-auth-routes.ts
+++ b/src/web-server/routes/cliproxy-auth-routes.ts
@@ -112,6 +112,13 @@ export function getStartUrlUnsupportedReason(
   return null;
 }
 
+export function getStartAuthFailureMessage(provider: CLIProxyProvider): string {
+  if (provider === 'ghcp') {
+    return 'Authentication failed, was cancelled, or GitHub Copilot verification did not complete. Ensure the account has an active Copilot subscription and retry.';
+  }
+  return 'Authentication failed or was cancelled';
+}
+
 /**
  * GET /api/cliproxy/auth - Get auth status for built-in CLIProxy profiles
  * Also fetches CLIProxyAPI stats to update lastUsedAt for active providers
@@ -503,7 +510,9 @@ router.post('/:provider/start', async (req: Request, res: Response): Promise<voi
         },
       });
     } else {
-      res.status(400).json({ error: 'Authentication failed or was cancelled' });
+      res.status(400).json({
+        error: getStartAuthFailureMessage(provider as CLIProxyProvider),
+      });
     }
   } catch (error) {
     respondInternalError(res, error, 'Failed to start OAuth flow.');
diff --git a/tests/unit/cliproxy/oauth-process-error-parser.test.ts b/tests/unit/cliproxy/oauth-process-error-parser.test.ts
new file mode 100644
index 00000000..84fe5331
--- /dev/null
+++ b/tests/unit/cliproxy/oauth-process-error-parser.test.ts
@@ -0,0 +1,35 @@
+import { describe, expect, it } from 'bun:test';
+import { extractLikelyAuthFailureFromStderr } from '../../../src/cliproxy/auth/oauth-process';
+
+describe('oauth-process stderr parsing', () => {
+  it('ignores non-ghcp providers', () => {
+    const stderr =
+      'time="2026-03-03T10:00:00Z" level=error msg="GitHub Copilot authentication failed: example"';
+
+    expect(extractLikelyAuthFailureFromStderr('qwen', stderr)).toBeNull();
+  });
+
+  it('extracts copilot verification failures from logrus lines', () => {
+    const stderr =
+      'time="2026-03-03T10:00:00Z" level=error msg="GitHub Copilot authentication failed: github-copilot: failed to verify Copilot access - you may not have an active Copilot subscription: 403 Forbidden"';
+
+    expect(extractLikelyAuthFailureFromStderr('ghcp', stderr)).toBe(
+      'github-copilot: failed to verify Copilot access - you may not have an active Copilot subscription: 403 Forbidden'
+    );
+  });
+
+  it('extracts generic authentication failure lines', () => {
+    const stderr = 'level=error msg="Authentication failed: state mismatch"';
+
+    expect(extractLikelyAuthFailureFromStderr('ghcp', stderr)).toBe('state mismatch');
+  });
+
+  it('caps extracted message length to prevent noisy broadcasts', () => {
+    const longSuffix = 'x'.repeat(400);
+    const stderr = `level=error msg="Authentication failed: ${longSuffix}"`;
+
+    const parsed = extractLikelyAuthFailureFromStderr('ghcp', stderr);
+    expect(parsed).not.toBeNull();
+    expect((parsed as string).length).toBe(240);
+  });
+});
diff --git a/tests/unit/web-server/cliproxy-auth-routes.test.ts b/tests/unit/web-server/cliproxy-auth-routes.test.ts
index 9982c133..9314f32c 100644
--- a/tests/unit/web-server/cliproxy-auth-routes.test.ts
+++ b/tests/unit/web-server/cliproxy-auth-routes.test.ts
@@ -1,5 +1,8 @@
 import { describe, expect, it } from 'bun:test';
-import { getStartUrlUnsupportedReason } from '../../../src/web-server/routes/cliproxy-auth-routes';
+import {
+  getStartAuthFailureMessage,
+  getStartUrlUnsupportedReason,
+} from '../../../src/web-server/routes/cliproxy-auth-routes';
 
 describe('cliproxy-auth-routes start-url guard', () => {
   it('rejects device code providers', () => {
@@ -27,3 +30,16 @@ describe('cliproxy-auth-routes start-url guard', () => {
     expect(getStartUrlUnsupportedReason('claude')).toBeNull();
   });
 });
+
+describe('cliproxy-auth-routes start failure messaging', () => {
+  it('returns ghcp-specific guidance for Copilot verification failures', () => {
+    expect(getStartAuthFailureMessage('ghcp')).toContain(
+      'GitHub Copilot verification did not complete'
+    );
+  });
+
+  it('keeps generic failure text for other providers', () => {
+    expect(getStartAuthFailureMessage('gemini')).toBe('Authentication failed or was cancelled');
+    expect(getStartAuthFailureMessage('kiro')).toBe('Authentication failed or was cancelled');
+  });
+});