E2E Behavior Validation for Frontend Modifications

Core Principle: Test Product Behavior, Not UI States

CRITICAL: Tests must verify that product features WORK correctly, not just that UI elements render.

What NOT to test (UI States):

• ❌ "Dropdown opens when clicked"
• ❌ "Modal appears after button click"
• ❌ "Loading spinner shows during request"
• ❌ "Form fields are visible"
• ❌ "Sidebar collapses"

What TO test (Product Behavior):

• ✅ "Selecting an LLM provider configures the agent to use that provider"
• ✅ "Creating a new agent persists it and shows in the agents list"
• ✅ "Running a tool with parameters returns the expected output"
• ✅ "Chat messages stream correctly and maintain conversation context"
• ✅ "Workflow execution triggers tools in the correct order"

Prerequisites

Requires Playwright MCP server. If the browser_navigate tool is unavailable, instruct the user to add it:

claude mcp add playwright -- npx @playwright/mcp@latest

Step 1: Understand the Feature Intent

Before writing ANY test, answer these questions:

1. What user problem does this feature solve?
2. What is the expected outcome when the feature works correctly?
3. What data flows through the system? (user input → API → state → UI)
4. What should persist after page reload?
5. What downstream effects should this action have?

Document these answers as comments in your test file.

Step 2: Build and Start

pnpm build:cli
cd packages/playground/e2e/kitchen-sink && pnpm dev

Verify server at http://localhost:4111

Step 3: Map Feature to Behavior Tests

Feature-to-Test Mapping Guide

Step 4: Write Behavior-Focused Tests

Test Structure Template

import { test, expect, Page } from '@playwright/test';
import { resetStorage } from '../__utils__/reset-storage';
import { selectFixture } from '../__utils__/select-fixture';
import { nanoid } from 'nanoid';

/**
 * FEATURE: [Name of feature]
 * USER STORY: As a user, I want to [action] so that [outcome]
 * BEHAVIOR UNDER TEST: [Specific behavior being validated]
 */

test.describe('[Feature Name] - Behavior Tests', () => {
  let page: Page;

  test.beforeEach(async ({ browser }) => {
    const context = await browser.newContext();
    page = await context.newPage();
  });

  test.afterEach(async () => {
    await resetStorage(page);
  });

  test('should [verb describing behavior] when [trigger condition]', async () => {
    // ARRANGE: Set up preconditions
    // - Navigate to the feature
    // - Configure any required state
    // ACT: Perform the user action that triggers the behavior
    // ASSERT: Verify the OUTCOME, not the UI state
    // - Check data persistence
    // - Verify downstream effects
    // - Confirm API calls made correctly
  });
});

Behavior Test Patterns

Pattern 1: Configuration Affects Behavior

test('selecting LLM provider should use that provider for agent responses', async () => {
  // ARRANGE
  await page.goto('/agents/my-agent/chat');

  // Intercept API to verify provider
  let capturedProvider: string | null = null;
  await page.route('**/api/chat', route => {
    const body = JSON.parse(route.request().postData() || '{}');
    capturedProvider = body.provider;
    route.continue();
  });

  // ACT: Select a different provider
  await page.getByTestId('provider-selector').click();
  await page.getByRole('option', { name: 'OpenAI' }).click();

  // Send a message to trigger the agent
  await page.getByTestId('chat-input').fill('Hello');
  await page.getByTestId('send-button').click();

  // ASSERT: Verify the selected provider was used
  await expect.poll(() => capturedProvider).toBe('openai');
});

Pattern 2: Data Persistence

test('created agent should persist after page reload', async () => {
  // ARRANGE
  await page.goto('/agents');
  const agentName = `Test Agent ${nanoid()}`;

  // ACT: Create new agent
  await page.getByTestId('create-agent-button').click();
  await page.getByTestId('agent-name-input').fill(agentName);
  await page.getByTestId('save-agent-button').click();

  // Wait for creation to complete
  await expect(page.getByText(agentName)).toBeVisible();

  // ASSERT: Verify persistence
  await page.reload();
  await expect(page.getByText(agentName)).toBeVisible({ timeout: 10000 });
});

Pattern 3: Tool Execution Produces Correct Output

test('weather tool should return formatted weather data', async () => {
  // ARRANGE
  await selectFixture(page, 'weather-success');
  await page.goto('/tools/weather-tool');

  // ACT: Execute tool with parameters
  await page.getByTestId('param-city'
how to use e2e-tests-studio
CursorClaude CodeClineCodexWindsurfGooseGitHub CopilotVS CodeGemini CLIKiloOpencodeKimi CLIRoo ClineAiderContinueZedBolt.newLovablev0Replit AgentSweepSmol DeveloperDevinCavemanAmpAntigravity
How to use e2e-tests-studio on Cursor
AI-first code editor with Composer
1
Prerequisites
Before installing skills in Cursor, ensure your development environment meets these requirements:
›Cursor installed and configured on your development machine
›Node.js version 16.0+ with npm package manager (verify with node --version)
›Active project directory or workspace where you want to add e2e-tests-studio
2
Execute installation command
Execute the skills CLI command in your project's root directory to begin installation:
$npx skills add https://github.com/mastra-ai/mastra --skill e2e-tests-studiocopy
The skills CLI fetches e2e-tests-studio from GitHub repository mastra-ai/mastra and configures it for Cursor.
3
Select Cursor when prompted
The CLI will show a list of available agents. Use arrow keys to navigate and space to select Cursor:
◆ Which agents do you want to install to?
│
│ ── Universal (.agents/skills) ── always included ────
│   • Amp
│   • Antigravity
│   • Cline
│   • Codex
│ ●Cursor(selected)
│   • Cursor
│   • Windsurf
4
Verify installation
Confirm successful installation by checking the skill directory location:
.cursor/skills/e2e-tests-studio
Reload or restart Cursor to activate e2e-tests-studio. Access the skill through slash commands (e.g., /e2e-tests-studio) or your agent's skill management interface.
⚠
Security & Verification Notice
We perform automated surface-level scans (Gen AI Scanner, Socket, Snyk) during installation. These checks detect common vulnerabilities but do not guarantee complete security. Always review skill source code and verify the publisher's reputation before production use.
Skills execute code in your development environment. Always verify the publisher's identity, review recent commits, and test in isolated environments before production deployment.
Additional Resources
›View source code on GitHub›Skills CLI documentation›Learn more about Cursor›What are agent skills?