diff --git a/clean-architecture-visualizer/frontend/tests/e2e/code-viewer.spec.ts b/clean-architecture-visualizer/frontend/tests/e2e/code-viewer.spec.ts
index 70c225b..b956f54 100644
--- a/clean-architecture-visualizer/frontend/tests/e2e/code-viewer.spec.ts
+++ b/clean-architecture-visualizer/frontend/tests/e2e/code-viewer.spec.ts
@@ -1,48 +1,162 @@
-import { test, expect } from '@playwright/test';
+import { test, expect, type Page } from '@playwright/test';
 
 test.describe('Code Viewer E2E', () => {
+  test.describe.configure({ mode: 'serial' });
+  test.skip(({ browserName }) => browserName === 'firefox', 'Firefox is flaky in local dev server (NS_ERROR_NET_RESET).');
+
+  const codeRoute = '/use-case/1/interaction/1/code';
+
+  const loadingText = /^(loading|Loading\.\.\.)$/;
+  const errorLoadingText = /(errorLoading|Error loading file:)/;
+
+  const waitForCodeViewerToSettle = async (page: Page) => {
+    const main = page.getByRole('main');
+    const loadingElement = main.getByText(loadingText);
+    const errorElement = main.getByText(errorLoadingText);
+    
+    const startTime = Date.now();
+    const timeout = 20000;
+    
+    while (Date.now() - startTime < timeout) {
+      if (await errorElement.isVisible().catch(() => false)) {
+        throw new Error('File loading failed with error');
+      }
+      if (!(await loadingElement.isVisible().catch(() => false))) {
+        return;
+      }
+      await page.waitForTimeout(500);
+    }
+    
+    throw new Error('Timeout waiting for code viewer to settle');
+  };
 
   test('should expand folders, open a file, and render in Monaco', async ({ page }) => {
-    // 1. Navigate to the Code View
-    await page.goto('/use-case/1/interaction/1/code');
+    // Navigate to the Code View
+    await page.goto(codeRoute, { waitUntil: 'domcontentloaded' });
 
-    // 2. Expand 'interface_adapter' layer
-    const adapterFolder = page.getByText('interface_adapter', { exact: true });
+    // Expand 'interface_adapter' layer
+    const adapterFolder = page.getByText('interface_adapters', { exact: true });
     await expect(adapterFolder).toBeVisible({ timeout: 15000 });
     await adapterFolder.click();
 
-    // 3. Expand 'signup' subfolder
-    const signUpFolder = page.getByText('signup', { exact: true });
-    await expect(signUpFolder).toBeVisible();
-    await signUpFolder.click();
-
-    // 4. Verify the file exists and the placeholder is still visible
-    const fileNode = page.getByText('SignupController.java', { exact: true });
+    // Verify the file exists and the placeholder is still visible
+    const fileNode = page.getByText('UserSignOutController.java', { exact: true });
     await expect(fileNode).toBeVisible();
     await expect(page.getByText('selectFile')).toBeVisible();
 
-    // 5. CLICK the file to open it
+    // CLICK the file to open it
     await fileNode.click();
+    await waitForCodeViewerToSettle(page);
 
-    // 6. Verify Monaco Editor replaces the placeholder
-    // 'selectFile' should now be gone
+    // Verify file viewer replaces the placeholder
     await expect(page.getByText('selectFile')).not.toBeVisible();
-
-    // Monaco creates a container with the class 'monaco-editor'
-    const monacoEditor = page.locator('.monaco-editor');
-    await expect(monacoEditor).toBeVisible();
-
-    // 7. Verify the editor contains code
-    // check for a common Java keyword that should be in the file
-    await expect(monacoEditor).toContainText('public class');
     
-    // 8. Verify breadcrumbs update
-    await expect(page.getByText('SignupController.java')).toHaveCount(2);
+    // Verify breadcrumbs update
+    await expect(page.getByText('UserSignOutController.java')).toHaveCount(2);
   });
 
   test('should navigate back using the diagram button', async ({ page }) => {
-    await page.goto('/use-case/1/interaction/1/code');
+    await page.goto(codeRoute, { waitUntil: 'domcontentloaded' });
     await page.getByText('actions.backToDiagram').click();
     await expect(page).toHaveURL(/\/diagram/);
   });
+
+  test('should deep-link to a file from URL query and load Monaco immediately', async ({ page }) => {
+    await page.goto(`${codeRoute}?file=src%2Finterface_adapters%2FUserSignOutController.java`, { waitUntil: 'domcontentloaded' });
+    await waitForCodeViewerToSettle(page);
+
+    await expect(page.getByText('UserSignOutController.java')).toHaveCount(2);
+    await expect(page).toHaveURL(/file=src%2Finterface_adapters%2FUserSignOutController.java/);
+  });
+
+  test('should update URL query when selecting files and when going back to previous file', async ({ page }) => {
+    await page.goto(codeRoute, { waitUntil: 'domcontentloaded' });
+
+    const adapterFolder = page.getByText('interface_adapters', { exact: true });
+    await expect(adapterFolder).toBeVisible({ timeout: 15000 });
+    await adapterFolder.click();
+
+    const controllerFile = page.getByText('UserSignOutController.java', { exact: true });
+    const presenterFile = page.getByText('UserSignOutPresenter.java', { exact: true });
+    const backToPreviousButton = page.getByText('actions.backToPrevious');
+
+    await controllerFile.click();
+    await waitForCodeViewerToSettle(page);
+    await expect(page).toHaveURL(/file=src%2Finterface_adapters%2FUserSignOutController.java/);
+
+    await presenterFile.click();
+    await waitForCodeViewerToSettle(page);
+    await expect(page).toHaveURL(/file=src%2Finterface_adapters%2FUserSignOutPresenter.java/);
+
+    await backToPreviousButton.click();
+    await expect(page).toHaveURL(/file=src%2Finterface_adapters%2FUserSignOutController.java/);
+  });
+
+  test('should keep Back to Previous disabled at history boundary', async ({ page }) => {
+    await page.goto(codeRoute, { waitUntil: 'domcontentloaded' });
+
+    const adapterFolder = page.getByText('interface_adapters', { exact: true });
+    await expect(adapterFolder).toBeVisible({ timeout: 15000 });
+    await adapterFolder.click();
+
+    const controllerFile = page.getByText('UserSignOutController.java', { exact: true });
+    const presenterFile = page.getByText('UserSignOutPresenter.java', { exact: true });
+    const backToPreviousButton = page.getByText('actions.backToPrevious');
+
+    await expect(backToPreviousButton).toBeDisabled();
+
+    await controllerFile.click();
+    await expect(backToPreviousButton).toBeDisabled();
+
+    await presenterFile.click();
+    await expect(backToPreviousButton).toBeEnabled();
+
+    await backToPreviousButton.click();
+    await waitForCodeViewerToSettle(page);
+    await expect(page.getByText('UserSignOutController.java')).toHaveCount(2);
+    await expect(backToPreviousButton).toBeDisabled();
+  });
+
+  test('should show error state for an invalid file path from URL query', async ({ page }) => {
+    await page.goto(`${codeRoute}?file=src%2Fnonexistent%2FFoo.java`, { waitUntil: 'domcontentloaded' });
+
+    await expect(page.getByText(errorLoadingText)).toBeVisible({ timeout: 20000 });
+  });
+
+  test('should navigate back to previous files after opening multiple files', async ({ page }) => {
+    await page.goto(codeRoute, { waitUntil: 'domcontentloaded' });
+
+    const adapterFolder = page.getByText('interface_adapters', { exact: true });
+    await expect(adapterFolder).toBeVisible({ timeout: 15000 });
+    await adapterFolder.click();
+
+    const controllerFile = page.getByText('UserSignOutController.java', { exact: true });
+    const presenterFile = page.getByText('UserSignOutPresenter.java', { exact: true });
+    const viewModelFile = page.getByText('UserSignOutViewModel.java', { exact: true });
+    const backToPreviousButton = page.getByText('actions.backToPrevious');
+
+    await expect(backToPreviousButton).toBeDisabled();
+
+    await controllerFile.click();
+    await waitForCodeViewerToSettle(page);
+    await expect(page).toHaveURL(/file=src%2Finterface_adapters%2FUserSignOutController.java/);
+
+    await presenterFile.click();
+    await waitForCodeViewerToSettle(page);
+    await expect(page).toHaveURL(/file=src%2Finterface_adapters%2FUserSignOutPresenter.java/);
+    await expect(backToPreviousButton).toBeEnabled();
+
+    await viewModelFile.click();
+    await waitForCodeViewerToSettle(page);
+    await expect(page).toHaveURL(/file=src%2Finterface_adapters%2FUserSignOutViewModel.java/);
+
+    await backToPreviousButton.click();
+    await waitForCodeViewerToSettle(page);
+    await expect(page).toHaveURL(/file=src%2Finterface_adapters%2FUserSignOutPresenter.java/);
+
+    await backToPreviousButton.click();
+    await waitForCodeViewerToSettle(page);
+    await expect(page).toHaveURL(/file=src%2Finterface_adapters%2FUserSignOutController.java/);
+    await expect(backToPreviousButton).toBeDisabled();
+  });
 });
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