diff --git a/apps/frontend/src/app/app.config.ts b/apps/frontend/src/app/app.config.ts
index 0a16893..005e57a 100644
--- a/apps/frontend/src/app/app.config.ts
+++ b/apps/frontend/src/app/app.config.ts
@@ -9,6 +9,7 @@ import { routes } from './app.routes';
 import { csrfInterceptor } from './core/csrf.interceptor';
 import { sessionExpiryInterceptor } from './core/session-expiry.interceptor';
 import { titleStrategyProvider } from './core/title.strategy';
+import { backendErrorToastInterceptor } from './core/backend-error-toast.interceptor';
 
 registerLocaleData(localeDe);
 
@@ -16,7 +17,9 @@ export const appConfig: ApplicationConfig = {
   providers: [
     provideBrowserGlobalErrorListeners(),
     { provide: LOCALE_ID, useValue: 'de-DE' },
-    provideHttpClient(withInterceptors([csrfInterceptor, sessionExpiryInterceptor])),
+    provideHttpClient(
+      withInterceptors([csrfInterceptor, sessionExpiryInterceptor, backendErrorToastInterceptor]),
+    ),
     provideRouter(routes, withComponentInputBinding()),
     titleStrategyProvider,
   ],
diff --git a/apps/frontend/src/app/core/backend-error-toast.interceptor.spec.ts b/apps/frontend/src/app/core/backend-error-toast.interceptor.spec.ts
new file mode 100644
index 0000000..76f5342
--- /dev/null
+++ b/apps/frontend/src/app/core/backend-error-toast.interceptor.spec.ts
@@ -0,0 +1,60 @@
+import { provideHttpClient, withInterceptors } from '@angular/common/http';
+import { HttpClient } from '@angular/common/http';
+import { HttpTestingController, provideHttpClientTesting } from '@angular/common/http/testing';
+import { TestBed } from '@angular/core/testing';
+import { ToastService } from '../shared/ui';
+import { backendErrorToastInterceptor } from './backend-error-toast.interceptor';
+
+describe('backendErrorToastInterceptor', () => {
+  it('shows the backend message, validation details and request ID', () => {
+    const show = vi.fn();
+    TestBed.configureTestingModule({
+      providers: [
+        provideHttpClient(withInterceptors([backendErrorToastInterceptor])),
+        provideHttpClientTesting(),
+        { provide: ToastService, useValue: { show } },
+      ],
+    });
+    const http = TestBed.inject(HttpClient);
+    const controller = TestBed.inject(HttpTestingController);
+
+    http.post('/api/projects/project-1/rooms', {}).subscribe({ error: () => undefined });
+    controller.expectOne('/api/projects/project-1/rooms').flush(
+      {
+        status: 400,
+        code: 'VALIDATION_FAILED',
+        message: 'Die Eingaben sind ungültig.',
+        requestId: 'request-123',
+        validation: [{ field: 'name', messages: ['Name fehlt.'] }],
+      },
+      { status: 400, statusText: 'Bad Request' },
+    );
+
+    expect(show).toHaveBeenCalledWith({
+      tone: 'danger',
+      title: 'Anfrage fehlgeschlagen',
+      message: 'Die Eingaben sind ungültig. name: Name fehlt.',
+      requestId: 'request-123',
+    });
+    controller.verify();
+  });
+
+  it('does not show an error for the expected anonymous session check', () => {
+    const show = vi.fn();
+    TestBed.configureTestingModule({
+      providers: [
+        provideHttpClient(withInterceptors([backendErrorToastInterceptor])),
+        provideHttpClientTesting(),
+        { provide: ToastService, useValue: { show } },
+      ],
+    });
+    const http = TestBed.inject(HttpClient);
+    const controller = TestBed.inject(HttpTestingController);
+
+    http.get('/api/auth/me').subscribe({ error: () => undefined });
+    controller.expectOne('/api/auth/me').flush({}, { status: 401, statusText: 'Unauthorized' });
+
+    expect(show).not.toHaveBeenCalled();
+    controller.verify();
+  });
+});
diff --git a/apps/frontend/src/app/core/backend-error-toast.interceptor.ts b/apps/frontend/src/app/core/backend-error-toast.interceptor.ts
new file mode 100644
index 0000000..fd1e00d
--- /dev/null
+++ b/apps/frontend/src/app/core/backend-error-toast.interceptor.ts
@@ -0,0 +1,58 @@
+import { HttpErrorResponse, type HttpInterceptorFn } from '@angular/common/http';
+import { inject } from '@angular/core';
+import type { ApiErrorBody } from '@boilerplate/api-client';
+import { catchError, throwError } from 'rxjs';
+import { ToastService } from '../shared/ui';
+
+export const backendErrorToastInterceptor: HttpInterceptorFn = (request, next) => {
+  const toasts = inject(ToastService);
+  return next(request).pipe(
+    catchError((error: unknown) => {
+      if (error instanceof HttpErrorResponse && isBackendRequest(request.url)) {
+        const body = apiErrorBody(error.error);
+        if (!isExpectedAnonymousCheck(request.method, request.url, error.status)) {
+          toasts.show({
+            tone: 'danger',
+            title: error.status >= 500 ? 'Serverfehler' : 'Anfrage fehlgeschlagen',
+            message: messageFor(error, body),
+            ...(body?.requestId ? { requestId: body.requestId } : {}),
+          });
+        }
+      }
+      return throwError(() => error);
+    }),
+  );
+};
+
+function isBackendRequest(url: string): boolean {
+  return new URL(url, document.baseURI).pathname.startsWith('/api/');
+}
+
+function isExpectedAnonymousCheck(method: string, url: string, status: number): boolean {
+  return (
+    method === 'GET' && status === 401 && new URL(url, document.baseURI).pathname === '/api/auth/me'
+  );
+}
+
+function apiErrorBody(value: unknown): ApiErrorBody | undefined {
+  if (
+    typeof value !== 'object' ||
+    value === null ||
+    !('message' in value) ||
+    typeof value.message !== 'string' ||
+    !('requestId' in value) ||
+    typeof value.requestId !== 'string'
+  ) {
+    return undefined;
+  }
+  return value as ApiErrorBody;
+}
+
+function messageFor(error: HttpErrorResponse, body: ApiErrorBody | undefined): string {
+  if (error.status === 0) return 'Das Backend ist derzeit nicht erreichbar.';
+  const details = body?.validation
+    ?.flatMap((entry) => entry.messages.map((message) => `${entry.field}: ${message}`))
+    .join(' · ');
+  if (body && details) return `${body.message} ${details}`;
+  return body?.message ?? 'Die Anfrage konnte nicht verarbeitet werden.';
+}