Contract Testing

Contract Testing

When testing API contracts or microservices:

1. DEFINE consumer expectations (what consumers actually need)
2. VERIFY provider fulfills contracts (Pact verification)
3. DETECT breaking changes before deployment (CI/CD integration)
4. VERSION APIs semantically (breaking = major bump)
5. MAINTAIN backward compatibility for supported versions

Quick Contract Testing Steps:

• Consumer: Define expected request/response pairs
• Provider: Verify against all consumer contracts
• CI/CD: Block deploys that break contracts
• Versioning: Document supported versions and deprecation

Critical Success Factors:

• Consumers own the contract (they define what they need)
• Provider must pass all consumer contracts before deploy
• Breaking changes require coordination, not surprise

Quick Reference Card

When to Use

• Microservices communication
• Third-party API integrations
• Distributed team coordination
• Preventing breaking changes

Consumer-Driven Contract Flow

Consumer → Defines Expectations → Contract
                    ↓
Provider → Verifies Contract → Pass/Fail
                    ↓
CI/CD → Blocks Breaking Changes

Breaking vs Non-Breaking Changes

| Change Type | Breaking? | Semver | |-------------|-----------|--------| | Remove field | ✅ Yes | Major | | Rename field | ✅ Yes | Major | | Change type | ✅ Yes | Major | | Add optional field | ❌ No | Minor | | Add new endpoint | ❌ No | Minor | | Bug fix | ❌ No | Patch |

Tools

| Tool | Best For | |------|----------| | Pact | Consumer-driven contracts | | OpenAPI/Swagger | API-first design | | JSON Schema | Schema validation | | GraphQL | Schema-first contracts |

---

Consumer Contract (Pact)

// Consumer defines what it needs
const { Pact } = require('@pact-foundation/pact');

describe('Order API Consumer', () => {
  const provider = new Pact({
    consumer: 'CheckoutUI',
    provider: 'OrderService'
  });

  beforeAll(() => provider.setup());
  afterAll(() => provider.finalize());

  it('creates an order', async () => {
    await provider.addInteraction({
      state: 'products exist',
      uponReceiving: 'a create order request',
      withRequest: {
        method: 'POST',
        path: '/orders',
        body: { productId: 'abc', quantity: 2 }
      },
      willRespondWith: {
        status: 201,
        body: {
          orderId: like('order-123'),  // Any string matching pattern
          total: like(19.99)           // Any number
        }
      }
    });

    const response = await orderClient.create({ productId: 'abc', quantity: 2 });
    expect(response.orderId).toBeDefined();
  });
});

---

Provider Verification

// Provider verifies it fulfills all consumer contracts
const { Verifier } = require('@pact-foundation/pact');

describe('Order Service Provider', () => {
  it('fulfills all consumer contracts', async () => {
    await new Verifier({
      provider: 'OrderService',
      providerBaseUrl: 'http://localhost:3000',
      pactUrls: ['./pacts/checkoutui-orderservice.json'],
      stateHandlers: {
        'products exist': async () => {
          await db.products.create({ id: 'abc', price: 9.99 });
        }
      }
    }).verifyProvider();
  });
});

---

Breaking Change Detection

// Agent detects breaking changes
await Task("Contract Validation", {
  currentContract: 'openapi-v2.yaml',
  previousContract: 'openapi-v1.yaml',
  detectBreaking: true,
  calculateSemver: true,
  generateMigrationGuide: true
}, "qe-api-contract-validator");

// Output:
// Breaking changes found: 2
// - Removed field: order.discount
// - Type change: order.total (number → string)
// Recommended version: 3.0.0 (major bump)

---

CI/CD Integration

name: Contract Tests
on: [push]

jobs:
  consumer-tests:
    steps:
      - run: npm run test:contract
      - name: Publish Pacts
        run: npx pact-broker publish ./pacts --broker-base-url $PACT_BROKER

  provider-verification:
    needs: consumer-tests
    steps:
      - name: Verify Provider
        run: npm run verify:contracts
      - name: Can I Deploy?
        run: npx pact-broker can-i-deploy --pacticipant OrderService --version $VERSION

---

Agent Coordination Hints

Memory Namespace

aqe/contract-testing/
├── contracts/*           - Current contracts
├── breaking-changes/*    - Detected breaking changes
├── versioning/*          - Version compatibility matrix
└── verification-results/* - Provider verification history

Fleet Coordination

const contractFleet = await FleetManager.coordinate({
  strategy: 'contract-testing',
  agents: [
    'qe-api-contract-validator',  // Validation, breaking detection
    'qe-test-generator',          // Generate contract tests
    'qe-security-scanner'         // API security
  ],
  topology: 'sequential'
});

---

Agent CLI & Advanced Patterns

For v3 agent-specific commands (aqe contract ...), GraphQL contracts, event contracts, and Pact Broker integration, see [references/agent-commands.md](references/agent-commands.md).

Related Skills

• [api-testing-patterns](../api-testing-patterns/) - API testing strategies
• [shift-left-testing](../shift-left-testing/) - Early contract validation
• [cicd-pipeline-qe-orchestrator](../cicd-pipeline-qe-orchestrator/) - Pipeline integration

---

Remember

Consumers own the contract. They define what they need; providers must fulfill it. Breaking changes require major version bumps and coordination. CI/CD blocks deploys that break contracts. Use Pact for consumer-driven, OpenAPI for API-first.

With Agents: Agents validate contracts, detect breaking changes with semver recommendations, and generate migration guides. Use agents to maintain contract compliance at scale.

Gotchas

• Pact broker URL must be configured before running — agent will generate tests that silently skip verification without it
• Consumer tests pass locally but fail in CI when provider states aren't set up — always verify both sides
• Adding a required field to a response is a BREAKING change even though provider tests pass — consumer didn't expect it
• Agent may generate contracts from API docs instead of actual consumer usage — contracts must reflect real consumer needs
• GraphQL contract testing requires schema stitching awareness — fragments may reference types from other services

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: proffesor-for-testing
• Source: proffesor-for-testing/agentic-qe
• License: MIT
• Homepage: https://agentic-qe.dev/

Install and usage instructions live in the source repository linked above.