Api contract testing

Skill secondsky/claude-skills/plugins/api-contract-testing/skills/api-contract-testing

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Install

npx -y skills add secondsky/claude-skills --skill api-contract-testing

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Verifies API contracts between services using consumer-driven contracts, schema validation, and tools like Pact. Use when testing microservices communication, preventing breaking changes, or validating OpenAPI specifications.

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SKILL.md

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API Contract Testing

Verify that APIs honor their contracts between consumers and providers without requiring full integration tests.

Key Concepts

Term	Definition
Consumer	Service that calls an API
Provider	Service that exposes an API
Contract	Agreed request/response format
Pact	Consumer-driven contract testing tool
Schema	Structure definition (OpenAPI, JSON Schema)
Broker	Central repository for contracts

Pact Consumer Test (TypeScript)

import { PactV3, MatchersV3 } from '@pact-foundation/pact';

const provider = new PactV3({
  consumer: 'OrderService',
  provider: 'UserService'
});

describe('User API Contract', () => {
  it('returns user by ID', async () => {
    await provider
      .given('user 123 exists')
      .uponReceiving('a request for user 123')
      .withRequest({ method: 'GET', path: '/users/123' })
      .willRespondWith({
        status: 200,
        body: MatchersV3.like({
          id: '123',
          name: MatchersV3.string('John'),
          email: MatchersV3.email('[email protected]')
        })
      })
      .executeTest(async (mockServer) => {
        const response = await fetch(`${mockServer.url}/users/123`);
        expect(response.status).toBe(200);
      });
  });

  it('returns 404 for non-existent user', async () => {
    await provider
      .given('user does not exist')
      .uponReceiving('a request for non-existent user')
      .withRequest({ method: 'GET', path: '/users/999' })
      .willRespondWith({
        status: 404,
        body: MatchersV3.like({
          error: { code: 'NOT_FOUND', message: MatchersV3.string() }
        })
      })
      .executeTest(async (mockServer) => {
        const response = await fetch(`${mockServer.url}/users/999`);
        expect(response.status).toBe(404);
      });
  });
});

Provider Verification

import { Verifier } from '@pact-foundation/pact';

new Verifier({
  provider: 'UserService',
  providerBaseUrl: 'http://localhost:3000',
  pactBrokerUrl: process.env.PACT_BROKER_URL,
  publishVerificationResult: true,
  providerVersion: process.env.GIT_SHA,
  stateHandlers: {
    'user 123 exists': async () => {
      await db.users.create({ id: '123', name: 'John' });
    },
    'user does not exist': async () => {
      await db.users.deleteAll();
    }
  }
}).verifyProvider();

OpenAPI Validation (Express)

const OpenApiValidator = require('express-openapi-validator');

app.use(OpenApiValidator.middleware({
  apiSpec: './openapi.yaml',
  validateRequests: true,
  validateResponses: true,
  validateSecurity: true
}));

Additional Implementations

Python JSON Schema: See references/python-json-schema.md
Java REST Assured: See references/java-rest-assured.md
Pact Broker CI/CD: See references/pact-broker-cicd.md

Best Practices

Do:

Test from consumer perspective
Use matchers for flexible matching
Validate structure, not specific values
Version contracts explicitly
Test error responses
Run tests in CI pipeline
Test backward compatibility

Don't:

Test business logic in contracts
Hard-code specific values
Skip error scenarios
Ignore versioning
Deploy without verification

Tools

Pact - Multi-language consumer-driven contracts
Spring Cloud Contract - JVM ecosystem
OpenAPI/Swagger - Schema-first validation
Dredd - API blueprint testing
Spectral - OpenAPI linting