diff --git a/packages/core-app-api/src/apis/implementations/AuthErrorApi/SignInAuthErrorApi.test.ts b/packages/core-app-api/src/apis/implementations/AuthErrorApi/SignInAuthErrorApi.test.ts
new file mode 100644
index 0000000000..c4d6fe83de
--- /dev/null
+++ b/packages/core-app-api/src/apis/implementations/AuthErrorApi/SignInAuthErrorApi.test.ts
@@ -0,0 +1,94 @@
+/*
+ * Copyright 2024 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { DiscoveryApi } from '@backstage/core-plugin-api';
+import { serializeError } from '@backstage/errors';
+import { SignInAuthErrorApi } from './SignInAuthErrorApi';
+
+describe('SignInAuthErrorApi', () => {
+  const mockDiscoveryApi = {
+    getBaseUrl: jest.fn(),
+  } as unknown as jest.Mocked<DiscoveryApi>;
+
+  beforeEach(() => {
+    jest.resetAllMocks();
+  });
+
+  it('should create an instance of SignInAuthErrorApi', async () => {
+    const api = SignInAuthErrorApi.create({
+      discovery: mockDiscoveryApi,
+    });
+
+    mockDiscoveryApi.getBaseUrl.mockResolvedValue(
+      'http://localhost:7007/api/auth',
+    );
+
+    expect(api).toBeInstanceOf(SignInAuthErrorApi);
+  });
+
+  it('should return an error when the cookie returns an error object', async () => {
+    const errorObject = {
+      name: 'TestError',
+      message: 'This is a test error',
+    };
+    const serializedError = serializeError(errorObject);
+    mockDiscoveryApi.getBaseUrl.mockResolvedValue(
+      'http://localhost:7000/api/auth',
+    );
+
+    global.fetch = jest.fn().mockResolvedValue({
+      ok: true,
+      json: jest.fn().mockResolvedValue(serializedError),
+    } as unknown as Response);
+
+    const api = SignInAuthErrorApi.create({ discovery: mockDiscoveryApi });
+    const error = await api.getSignInAuthError();
+
+    expect(mockDiscoveryApi.getBaseUrl).toHaveBeenCalledWith('auth');
+    expect(fetch).toHaveBeenCalledWith(
+      'http://localhost:7000/api/auth/.backstage/error',
+      {
+        credentials: 'include',
+      },
+    );
+    expect(error).toBeInstanceOf(Error);
+    expect((error as Error).name).toEqual(errorObject.name);
+    expect((error as Error).message).toEqual(errorObject.message);
+  });
+
+  it('should return undefined when the backend does not return an error object', async () => {
+    mockDiscoveryApi.getBaseUrl.mockResolvedValue(
+      'http://localhost:7000/api/auth',
+    );
+
+    global.fetch = jest.fn().mockResolvedValue({
+      ok: true,
+      json: jest.fn().mockResolvedValue(undefined),
+    } as unknown as Response);
+
+    const api = SignInAuthErrorApi.create({ discovery: mockDiscoveryApi });
+    const error = await api.getSignInAuthError();
+
+    expect(mockDiscoveryApi.getBaseUrl).toHaveBeenCalledWith('auth');
+    expect(fetch).toHaveBeenCalledWith(
+      'http://localhost:7000/api/auth/.backstage/error',
+      {
+        credentials: 'include',
+      },
+    );
+    expect(error).toBeUndefined();
+  });
+});
diff --git a/plugins/auth-backend/src/service/createCookieAuthErrorMiddleware.test.ts b/plugins/auth-backend/src/service/createCookieAuthErrorMiddleware.test.ts
new file mode 100644
index 0000000000..1d270a953f
--- /dev/null
+++ b/plugins/auth-backend/src/service/createCookieAuthErrorMiddleware.test.ts
@@ -0,0 +1,67 @@
+/*
+ * Copyright 2024 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import express from 'express';
+import request from 'supertest';
+import { createCookieAuthErrorMiddleware } from './createCookieAuthErrorMiddleware';
+
+const AUTH_ERROR_COOKIE = 'auth-error';
+
+describe('createCookieAuthErrorMiddleware', () => {
+  let app: express.Express;
+
+  beforeEach(() => {
+    app = express();
+    app.use(express.json());
+    app.use(express.urlencoded({ extended: true }));
+
+    // Mock cookie parser middleware
+    app.use((req, _, next) => {
+      req.cookies = {};
+      const cookieHeader = req.headers.cookie;
+      if (cookieHeader) {
+        const cookies = cookieHeader.split(';');
+        cookies.forEach(cookie => {
+          const [name, ...rest] = cookie.split('=');
+          req.cookies[name.trim()] = decodeURIComponent(rest.join('='));
+        });
+      }
+      next();
+    });
+
+    app.use(
+      createCookieAuthErrorMiddleware(
+        'http://localhost:3000',
+        'http://localhost:7000',
+      ),
+    );
+  });
+
+  it('should return cookie content if error cookie exists', async () => {
+    const error = 'test';
+    const res = await request(app)
+      .get('/.backstage/error')
+      .set('Cookie', `${AUTH_ERROR_COOKIE}=${encodeURIComponent(error)}`);
+
+    expect(res.status).toBe(200);
+    expect(res.body).toEqual('test');
+  });
+
+  it('should return 404 if error cookie does not exist', async () => {
+    const res = await request(app).get('/.backstage/error');
+    expect(res.status).toBe(404);
+  });
+});
diff --git a/plugins/auth-backend/src/service/createCookieAuthErrorMiddleware.ts b/plugins/auth-backend/src/service/createCookieAuthErrorMiddleware.ts
index 325126d831..30724fc3cd 100644
--- a/plugins/auth-backend/src/service/createCookieAuthErrorMiddleware.ts
+++ b/plugins/auth-backend/src/service/createCookieAuthErrorMiddleware.ts
@@ -44,7 +44,7 @@ export function createCookieAuthErrorMiddleware(
       });
       res.status(200).json(error);
     } else {
-      res.status(404);
+      res.status(404).end();
     }
   });