MCP Apps Best Practices: Security, UX, and Performance Guidelines

Building MCP Apps that users trust and love requires more than just functional code. Whether you're creating your first interactive component or scaling a suite of tools, following established best practices ensures your apps are secure, intuitive, and fast.

This guide covers the three pillars of production-ready MCP Apps: security, user experience, and performance.

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Security Best Practices

Security isn't optional—it's foundational. MCP Apps run within AI assistants and often handle sensitive user data.

1. Validate All User Inputs

Never trust input coming from the LLM or user. Always sanitize and validate:

// Bad: Directly using user input
const query = userInput; // Risky!

// Good: Validation with Zod
import { z } from 'zod';

const SearchSchema = z.object({
  query: z.string().min(1).max(100).trim(),
  limit: z.number().int().min(1).max(50).default(10),
});

const result = SearchSchema.safeParse(userInput);
if (!result.success) {
  return { error: 'Invalid input', details: result.error };
}

2. Implement Proper Authentication

Use secure token handling:

• Store tokens securely: Never expose API keys in client-side code
• Use environment variables: Load secrets from .env files
• Implement token rotation: Support refresh tokens where possible
• Scope permissions: Request minimal required permissions

// Secure API client setup
const apiClient = new ApiClient({
  apiKey: process.env.API_KEY, // Never hardcode
  baseUrl: process.env.API_BASE_URL,
});

3. Sanitize Output Data

Prevent data leakage and injection:

• Remove sensitive fields before returning data
• Escape HTML if rendering user content
• Log safely (no passwords, tokens, or PII)

// Sanitize response data
function sanitizeUserData(user: User) {
  const { password, apiKey, ...safeUser } = user;
  return safeUser;
}

4. Security Checklist

Before publishing any MCP App:

• Input validation on all endpoints
• No hardcoded secrets or API keys
• Proper error handling (don't leak stack traces)
• Rate limiting implemented
• CORS configured correctly
• Dependencies audited (npm audit)
• No sensitive data in logs

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UX Best Practices

Great MCP Apps feel natural within the AI conversation flow. They should enhance, not interrupt.

1. Follow Progressive Disclosure

Don't overwhelm users. Start simple, then expand:

// Progressive form fields
const renderForm = (step: number) => {
  if (step === 1) {
    return <BasicInfoFields />;
  }
  if (step === 2) {
    return <AdvancedOptions />;
  }
  return <Confirmation />;
};

2. Provide Clear Feedback

Users should always know what's happening:

• Show loading states for async operations
• Confirm successful actions
• Explain errors in plain language
• Use progress indicators for multi-step flows

// Clear feedback pattern
const handleSubmit = async () => {
  setStatus('loading');
  try {
    await saveData();
    setStatus('success');
    showToast('Settings saved successfully!');
  } catch (error) {
    setStatus('error');
    showToast('Failed to save: ' + error.message);
  }
};

3. Design for Context

MCP Apps exist inside conversations. Respect that context:

• Keep it compact: Don't take over the entire screen
• Support quick actions: Allow common tasks in 1-2 clicks
• Handle interruptions gracefully: Save state if user switches away
• Use consistent patterns: Match the AI assistant's UI conventions

4. Accessibility First

Ensure everyone can use your apps:

• Use semantic HTML elements
• Include proper ARIA labels
• Support keyboard navigation
• Test with screen readers
• Maintain sufficient color contrast

// Accessible button example
<button
  aria-label="Close settings panel"
  onClick={handleClose}
  className="p-2 rounded hover:bg-gray-100"
>
  <XIcon aria-hidden="true" />
</button>

5. UX Patterns That Work

Pattern	When to Use	Example
Inline editing	Quick changes	Toggle switches, text inputs
Modal dialogs	Important decisions	Confirm delete, warnings
Expandable sections	Optional details	Advanced settings
Tabbed interface	Related content groups	Settings categories
Cards	Browseable items	App directory, file list

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Performance Best Practices

Slow apps frustrate users. Optimize for speed at every layer.

1. Optimize Bundle Size

Keep your MCP App lightweight:

// Use dynamic imports for heavy components
const HeavyChart = dynamic(() => import('./HeavyChart'), {
  loading: () => <Skeleton />,
});

// Tree-shake unused imports
import { specificFunction } from 'library'; // Good
import * as Library from 'library'; // Avoid - imports everything

2. Implement Efficient Caching

Reduce redundant API calls:

// React Query for smart caching
import { useQuery } from '@tanstack/react-query';

const useUserData = (userId: string) => {
  return useQuery({
    queryKey: ['user', userId],
    queryFn: () => fetchUser(userId),
    staleTime: 5 * 60 * 1000, // 5 minutes
    cacheTime: 30 * 60 * 1000, // 30 minutes
  });
};

3. Debounce and Throttle

Prevent excessive updates:

// Debounce search input
import { useDebounce } from 'use-debounce';

const [searchTerm, setSearchTerm] = useState('');
const [debouncedTerm] = useDebounce(searchTerm, 300);

// Only search after user stops typing
useEffect(() => {
  if (debouncedTerm) {
    performSearch(debouncedTerm);
  }
}, [debouncedTerm]);

4. Optimize Images and Assets

// Use Next.js Image for optimization
import Image from 'next/image';

<Image
  src="/app-icon.png"
  alt="App Icon"
  width={64}
  height={64}
  priority // For above-fold images
/>

5. Performance Targets

Aim for these metrics:

Metric	Target	Why It Matters
First Contentful Paint	< 1.5s	Users see something quickly
Time to Interactive	< 3.5s	Users can interact
Bundle size	< 200KB	Fast download on slow connections
API response	< 500ms	Feels instantaneous

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Testing Your MCP App

Unit Tests

Test individual components:

import { render, screen } from '@testing-library/react';
import { UserCard } from './UserCard';

test('renders user name', () => {
  render(<UserCard name="John Doe" email="john@example.com" />);
  expect(screen.getByText('John Doe')).toBeInTheDocument();
});

Integration Tests

Test the complete flow:

test('user can complete checkout', async () => {
  render(<CheckoutApp />);
  
  await userEvent.type(screen.getByLabelText('Card'), '4242424242424242');
  await userEvent.click(screen.getByText('Pay'));
  
  expect(await screen.findByText('Payment successful')).toBeInTheDocument();
});

Manual Testing Checklist

• Works in Claude Desktop
• Works in ChatGPT
• Works in VS Code
• Handles edge cases (empty states, errors)
• Responsive design tested
• Keyboard navigation works
• Screen reader compatible

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Deployment Checklist

Before going live:

Code Quality

• TypeScript compiles without errors
• ESLint passes
• Prettier formatting applied
• All tests passing
• No console errors

Security

• Environment variables configured
• API keys rotated
• Rate limiting enabled
• Error messages don't leak sensitive info

Documentation

• README with setup instructions
• Environment variable documentation
• Usage examples
• Troubleshooting guide

Monitoring

• Error tracking configured (Sentry, LogRocket)
• Analytics implemented
• Health check endpoint
• Logging strategy in place

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Common Mistakes to Avoid

1. Ignoring Error States

Always handle errors gracefully:

// Bad: Silent failures
const data = await fetchData(); // What if this fails?

// Good: Error handling
try {
  const data = await fetchData();
  setData(data);
} catch (error) {
  setError('Failed to load data. Please try again.');
  console.error('Fetch error:', error);
}

2. Over-Engineering

Start simple. Add complexity only when needed.

3. Forgetting Mobile

Test on mobile devices—many users interact with AI assistants on phones.

4. Poor State Management

Keep state close to where it's used. Avoid prop drilling.

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Conclusion

Building great MCP Apps is about balancing three priorities:

1. Security — Protect user data and your infrastructure
2. UX — Create intuitive, accessible experiences
3. Performance — Deliver fast, responsive interactions

Follow this guide, use the checklists, and continuously iterate based on user feedback.

Next Steps

• Submit your MCP App to the directory
• Join our Discord for community support
• Read our Getting Started Guide if you're new to MCP development

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Have questions or suggestions? Reach out on Twitter or GitHub.