HTTP Mocking - FrontMCP

MCP tools often make HTTP requests to external APIs. The HTTP mocking system intercepts fetch() calls, enabling fully offline testing of your MCP server.

Why HTTP Mocking?

Consider a tool that fetches weather data:

@Tool({ name: 'fetch-weather' })
async getWeather(ctx: ToolContext<{ city: string }>) {
  // This fetch call needs to be mocked for offline testing
  const response = await fetch(`https://api.weather.com/${ctx.input.city}`);
  const data = await response.json();
  return ctx.text(`Weather: ${data.temperature}°F`);
}

Without HTTP mocking:

Tests fail offline
Tests depend on external API availability
Tests may hit rate limits
Tests are slow due to network latency
Tests can’t simulate error conditions

Basic Usage

import { httpMock, httpResponse } from '@frontmcp/testing';

test('mock external API', async ({ mcp }) => {
  // Create an HTTP interceptor
  const interceptor = httpMock.interceptor();

  // Mock a GET request
  interceptor.get('https://api.weather.com/london', {
    body: { temperature: 72, conditions: 'sunny' },
  });

  // Call the tool - HTTP request is intercepted
  const result = await mcp.tools.call('fetch-weather', { city: 'london' });
  expect(result).toBeSuccessful();
  expect(result.text()).toContain('72');

  // Verify mock was used
  interceptor.assertDone();

  // Clean up (important!)
  interceptor.restore();
});

URL Matching

Exact URL Match

interceptor.get('https://api.example.com/users/1', { body: { id: 1 } });

Partial URL Match

URLs are matched if they contain the specified string:

// Matches any URL containing '/users'
interceptor.get('/users', { body: [{ id: 1 }, { id: 2 }] });

Regular Expression

// Match URLs with pattern
interceptor.get(/api\.example\.com\/users\/\d+/, { body: { id: 1 } });

// Match any user ID
interceptor.get(/\/users\/[a-z0-9-]+/, { body: { id: 'matched' } });

Custom Matcher Function

interceptor.mock({
  match: {
    url: (url) => url.startsWith('https://api.') && url.includes('/v2/'),
  },
  response: { body: { version: 2 } },
});

HTTP Methods

Convenience Methods

interceptor.get('/api/data', { body: { result: 'get' } });
interceptor.post('/api/data', { body: { result: 'post' } });
interceptor.put('/api/data', { body: { result: 'put' } });
interceptor.delete('/api/data', { body: { result: 'delete' } });

// Match any method
interceptor.any('/api/data', { body: { result: 'any' } });

Full Mock Definition

interceptor.mock({
  match: {
    url: '/api/users',
    method: 'POST',          // Single method
    // method: ['POST', 'PUT'], // Multiple methods
  },
  response: { status: 201, body: { id: 1 } },
});

Request Matching

Match by Headers

interceptor.mock({
  match: {
    url: '/api/protected',
    headers: {
      'authorization': /^Bearer /,
      'content-type': 'application/json',
    },
  },
  response: { body: { authenticated: true } },
});

Match by Request Body

// Exact match
interceptor.mock({
  match: {
    url: '/api/users',
    method: 'POST',
    body: { name: 'John', email: '[email protected]' },
  },
  response: { status: 201, body: { id: 1 } },
});

// Partial match - only checks specified keys
interceptor.mock({
  match: {
    url: '/api/users',
    method: 'POST',
    body: { name: 'John' }, // Matches if body contains name: 'John'
  },
  response: { status: 201, body: { id: 1 } },
});

// Custom matcher
interceptor.mock({
  match: {
    url: '/api/users',
    body: (body) => body.age >= 18,
  },
  response: { body: { eligible: true } },
});

Response Helpers

httpResponse Utilities

import { httpResponse } from '@frontmcp/testing';

// JSON response (auto-sets Content-Type)
interceptor.get('/api/data', httpResponse.json({ id: 1 }));

// Text response
interceptor.get('/api/text', httpResponse.text('Hello World'));

// HTML response
interceptor.get('/page', httpResponse.html('<h1>Welcome</h1>'));

// Error responses
interceptor.get('/api/missing', httpResponse.notFound());
interceptor.get('/api/auth', httpResponse.unauthorized());
interceptor.get('/api/forbidden', httpResponse.forbidden());
interceptor.get('/api/error', httpResponse.serverError('Database down'));

// Custom error
interceptor.get('/api/rate-limit', httpResponse.error(429, 'Too many requests'));

// Delayed response
interceptor.get('/api/slow', httpResponse.delayed({ data: 'result' }, 500));

Full Response Object

interceptor.mock({
  match: { url: '/api/custom' },
  response: {
    status: 201,
    statusText: 'Created',
    headers: {
      'x-request-id': 'abc123',
      'cache-control': 'no-cache',
    },
    body: { id: 1, created: true },
    delay: 100, // ms
  },
});

One-Time Mocks

Create mocks that only match a limited number of times:

test('simulate retry scenario', async ({ mcp }) => {
  const interceptor = httpMock.interceptor();

  // First request fails
  interceptor.mock({
    match: { url: '/api/flaky' },
    response: httpResponse.serverError('Temporary failure'),
    times: 1,
  });

  // Subsequent requests succeed
  interceptor.mock({
    match: { url: '/api/flaky' },
    response: { body: { success: true } },
  });

  // Tool with retry logic
  const result = await mcp.tools.call('resilient-fetch', {});
  expect(result).toBeSuccessful();

  interceptor.restore();
});

Call Tracking

Track Mock Usage

test('verify HTTP calls', async ({ mcp }) => {
  const interceptor = httpMock.interceptor();

  const handle = interceptor.post('/api/users', { body: { id: 1 } });

  await mcp.tools.call('create-user', { name: 'John', email: '[email protected]' });

  // Check call count
  expect(handle.callCount()).toBe(1);

  // Get call details
  const calls = handle.calls();
  expect(calls[0].url).toContain('/api/users');
  expect(calls[0].method).toBe('POST');
  expect(calls[0].headers['content-type']).toBe('application/json');
  expect(calls[0].body).toEqual({ name: 'John', email: '[email protected]' });

  interceptor.restore();
});

Wait for Calls

test('wait for async operations', async ({ mcp }) => {
  const interceptor = httpMock.interceptor();
  const handle = interceptor.get('/api/webhook', { body: { received: true } });

  // Trigger async operation that makes HTTP call
  await mcp.tools.call('trigger-webhook', {});

  // Wait for up to 5 seconds for 3 calls
  const calls = await handle.waitForCalls(3, 5000);
  expect(calls).toHaveLength(3);

  interceptor.restore();
});

Passthrough Mode

Allow unmatched requests to reach the real network:

test('partial mocking', async ({ mcp }) => {
  const interceptor = httpMock.interceptor();

  // Only mock specific endpoint
  interceptor.get('/api/mocked', { body: { mocked: true } });

  // Allow other requests through
  interceptor.allowPassthrough(true);

  // This uses the mock
  const result1 = await fetch('/api/mocked');
  expect(await result1.json()).toEqual({ mocked: true });

  // This hits the real API
  const result2 = await fetch('https://real-api.com/data');
  // ... real response

  interceptor.restore();
});

By default, unmatched requests throw an error. Only enable passthrough when you specifically need some requests to hit real endpoints.

Verification

Assert All Mocks Used

test('verify all mocks used', async ({ mcp }) => {
  const interceptor = httpMock.interceptor();

  interceptor.get('/api/users', { body: [] }, { times: 1 });
  interceptor.get('/api/orders', { body: [] }, { times: 1 });

  await mcp.tools.call('fetch-all-data', {});

  // Throws if any mock with `times` wasn't used
  interceptor.assertDone();

  interceptor.restore();
});

Check Pending Mocks

test('check unused mocks', async ({ mcp }) => {
  const interceptor = httpMock.interceptor();

  interceptor.get('/api/users', { body: [] }, { times: 1 });
  interceptor.get('/api/orders', { body: [] }, { times: 1 });

  await mcp.tools.call('fetch-users-only', {});

  // Check which mocks weren't used
  const pending = interceptor.pending();
  expect(pending).toHaveLength(1);
  expect(pending[0].match.url).toBe('/api/orders');

  interceptor.restore();
});

Cleanup

Always call interceptor.restore() to restore the original fetch function. Failing to do so will affect other tests.

Pattern: Using try/finally

test('with cleanup', async ({ mcp }) => {
  const interceptor = httpMock.interceptor();

  try {
    interceptor.get('/api/data', { body: { result: true } });
    await mcp.tools.call('fetch-data', {});
  } finally {
    interceptor.restore();
  }
});

Pattern: beforeEach/afterEach

let interceptor: ReturnType<typeof httpMock.interceptor>;

beforeEach(() => {
  interceptor = httpMock.interceptor();
});

afterEach(() => {
  interceptor.restore();
});

test('test 1', async ({ mcp }) => {
  interceptor.get('/api/data', { body: { test: 1 } });
  // ...
});

test('test 2', async ({ mcp }) => {
  interceptor.get('/api/data', { body: { test: 2 } });
  // ...
});

Global Disable

afterAll(() => {
  // Disable all HTTP mocking globally
  httpMock.disable();
});

Real-World Examples

Testing OpenAPI Adapter

test('openapi adapter calls external API', async ({ mcp }) => {
  const interceptor = httpMock.interceptor();

  interceptor.get('https://api.example.com/openapi.json', {
    body: {
      openapi: '3.0.0',
      paths: {
        '/users': {
          get: { operationId: 'getUsers' },
        },
      },
    },
  });

  interceptor.get('https://api.example.com/users', {
    body: [
      { id: 1, name: 'John' },
      { id: 2, name: 'Jane' },
    ],
  });

  const result = await mcp.tools.call('openapi:getUsers', {});
  expect(result).toBeSuccessful();
  expect(result.json()).toHaveLength(2);

  interceptor.restore();
});

Testing Error Handling

test('handles API errors gracefully', async ({ mcp }) => {
  const interceptor = httpMock.interceptor();

  interceptor.get('/api/data', httpResponse.serverError('Database unavailable'));

  const result = await mcp.tools.call('fetch-with-fallback', {});

  // Tool should handle error and return fallback
  expect(result).toBeSuccessful();
  expect(result.json()).toEqual({ fallback: true });

  interceptor.restore();
});

Testing Rate Limiting

test('respects rate limits', async ({ mcp }) => {
  const interceptor = httpMock.interceptor();

  // First request succeeds
  interceptor.mock({
    match: { url: '/api/data' },
    response: { body: { data: 'first' } },
    times: 1,
  });

  // Second request rate limited
  interceptor.mock({
    match: { url: '/api/data' },
    response: httpResponse.error(429, 'Rate limit exceeded'),
  });

  const result1 = await mcp.tools.call('fetch-data', {});
  expect(result1).toBeSuccessful();

  const result2 = await mcp.tools.call('fetch-data', {});
  expect(result2).toBeError();

  interceptor.restore();
});

Best Practices

Do:

Always call restore() in cleanup
Use httpResponse helpers for common responses
Verify mocks were used with assertDone()
Mock at the appropriate level of detail

Don’t:

Forget cleanup (breaks other tests)
Enable passthrough without good reason
Over-mock (test real HTTP handling sometimes)
Hard-code response data that should be dynamic

Get Started

Servers

Authentication

Extensibility

Adapters

Plugins

Testing

Guides

Deployment

​Why HTTP Mocking?

​Basic Usage

​URL Matching

​Exact URL Match

​Partial URL Match

​Regular Expression

​Custom Matcher Function

​HTTP Methods

​Convenience Methods

​Full Mock Definition

​Request Matching

​Match by Headers

​Match by Request Body

​Response Helpers

​httpResponse Utilities

​Full Response Object

​One-Time Mocks

​Call Tracking

​Track Mock Usage

​Wait for Calls

​Passthrough Mode

​Verification

​Assert All Mocks Used

​Check Pending Mocks

​Cleanup

​Pattern: Using try/finally

​Pattern: beforeEach/afterEach

​Global Disable

​Real-World Examples

​Testing OpenAPI Adapter

​Testing Error Handling

​Testing Rate Limiting

​Best Practices

Why HTTP Mocking?

Basic Usage

URL Matching

Exact URL Match

Partial URL Match

Regular Expression

Custom Matcher Function

HTTP Methods

Convenience Methods

Full Mock Definition

Request Matching

Match by Headers

Match by Request Body

Response Helpers

httpResponse Utilities

Full Response Object

One-Time Mocks

Call Tracking

Track Mock Usage

Wait for Calls

Passthrough Mode

Verification

Assert All Mocks Used

Check Pending Mocks

Cleanup

Pattern: Using try/finally

Pattern: beforeEach/afterEach

Global Disable

Real-World Examples

Testing OpenAPI Adapter

Testing Error Handling

Testing Rate Limiting

Best Practices