Development & Testing

Pact Contract Testing

Verify API contracts between consumers and providers with Pact, ensuring microservices stay compatible as they evolve.

Claude Code Codex Cursor Copilot

Overview

Pact is a contract testing framework that verifies the integration between API consumers and providers without requiring both services to run simultaneously. The consumer writes a test that defines what it expects from the provider, generating a "pact" (contract). The provider then verifies it can fulfill this contract. AI agents can generate both consumer tests and provider verification setups.

Your AI agent can write consumer-driven contract tests that define expected request/response pairs, configure the Pact mock server for consumer tests, set up provider verification with state handlers, and publish pacts to a Pact Broker for team coordination. The agent understands the consumer-driven contract testing pattern and can implement it correctly for REST and message-based APIs.

Pact is essential for microservice architectures where services evolve independently. Instead of running expensive end-to-end tests that require all services to be up, you verify contracts independently in each service's CI pipeline. Your AI agent can configure the Pact Broker, set up webhooks for triggered verification, and implement the can-i-deploy check before releasing new versions.

Who Is This For?

Microservice teams verifying API compatibility without end-to-end tests
Backend developers defining provider states for contract verification
Frontend teams ensuring backend changes do not break their API expectations
Platform engineers setting up Pact Broker for centralized contract management

Installation

Setup for Claude Code

npm install -D @pact-foundation/pact
Claude Code generates consumer tests and provider verifications

Configuration

// consumer.spec.ts
import { PactV4 } from "@pact-foundation/pact";

const provider = new PactV4({
  consumer: "frontend",
  provider: "user-service",
});

test("get user by ID", async () => {
  await provider
    .addInteraction()
    .given("user 1 exists")
    .uponReceiving("a request for user 1")
    .withRequest("GET", "/api/users/1")
    .willRespondWith(200, (b) => b.jsonBody({ id: 1, name: "Alice" }))
    .executeTest(async (mockServer) => {
      const res = await fetch(`${mockServer.url}/api/users/1`);
      expect(res.status).toBe(200);
    });
});