Pact Contract Builder
Build consumer-driven contract test JSON for the Pact framework. Define consumer and provider names, add HTTP interactions with request/response pairs and provider states, then generate and download a valid Pact JSON contract file. Supports Pact Spec v2, v3, and v4. All generation runs in your browser.
Frequently Asked Questions
What is consumer-driven contract testing?▼
Consumer-driven contract testing (CDCT) is an approach where the consumer of an API defines the contract — what requests it makes and what responses it expects. The provider then verifies it can satisfy those contracts. This ensures API changes don't silently break consumers, catching integration bugs before they reach production.
What is Pact and how does it work?▼
Pact is the most popular CDCT framework. The consumer runs tests against a mock server that records interactions into a Pact file (JSON contract). That file is shared with the provider (via Pact Broker or directly), and the provider verifies it can satisfy all interactions. Both sides are tested independently, avoiding the need for a shared test environment.
What is a provider state?▼
A provider state describes the precondition the provider needs to be in for a given interaction to succeed — for example, 'a user with id 123 exists'. When the provider runs verification, it reads the provider state and sets up the necessary data before replaying the request. Without provider states, the provider has no way to know how to prepare for the test.
What is the difference between Pact v2, v3, and v4?▼
Pact v2 is the most widely supported spec and covers basic request/response contracts. Pact v3 adds support for message-based interactions (Kafka, SNS, queues) and provider states as parameters. Pact v4 adds support for asynchronous interactions, plugins, and combined HTTP + message contracts in a single file. For REST APIs, v2 or v3 is sufficient.
How do I use the generated Pact contract?▼
Save the JSON file and point your Pact consumer test framework at it (e.g. in Jest: PactV2.fromFile('./contract.json')). For provider verification, publish the contract to a Pact Broker or share the file directly, then run provider tests with the Pact verifier library for your language (Ruby, Go, Java, JS, etc.).
What is a Pact Broker?▼
A Pact Broker is a shared server that stores and retrieves Pact contract files. It lets teams share contracts between consumer and provider CI pipelines automatically, tracks contract compatibility by version, and can block deployments if contracts are broken. PactFlow (pactflow.io) is the hosted SaaS version; you can also self-host the open-source pact-broker.
Related Products
on GumroadDevToolkit Starter Kit
A ready-to-deploy developer tools website with 12 built-in tools. Astro + Preact + Tailwind. Deploy to Cloudflare Pages for free. Built-in SEO, dark mode, and ad-ready layout.
Buy on Gumroad →TypeScript Quick Reference Toolkit
5 practical reference files for TypeScript developers: cheatsheet, TS vs JS comparison, 20 utility types with real examples, tsconfig best practices template, and 15 common mistakes guide. Print it. Keep it on your desk.
Buy on Gumroad →Developer Productivity Bundle
Stop rebuilding the same setup every project. 51 VSCode snippets, 40 Git aliases, 5 Docker Compose stacks, 5 GitHub Actions workflows, 5 README templates, and a dotfiles kit. Set up a professional dev environment in under an hour.
Buy on Gumroad →You Might Also Need
OpenAPI Validator
Paste your OpenAPI 3.x spec (YAML or JSON) to validate structure, find missing required fields, detect duplicate paths, check for missing operationIds, and get errors and warnings with path references.
Open free tool →
API Test Case Generator
Generate Jest and Supertest test cases from your API endpoint definition. Define method, path, auth, and fields to get a complete copy-ready test suite.
Open free tool →
API Response Mock Generator
Generate realistic mock API responses with custom status codes, headers, and JSON body. Quick templates for 200, 401, 404, 429, 500 and more. Runs in your browser.
Open free tool →
More Free Tools
Related Articles
API Testing Best Practices 2026: Postman vs Insomnia vs Bruno
Complete guide to API testing in 2026. Compare Postman, Insomnia, and Bruno, learn best practices for REST and GraphQL testing, and build automation pipelines that catch regressions before production.
MSW 2.0: The Best Way to Mock APIs in Frontend Tests
A complete guide to Mock Service Worker (MSW) 2.0 — setup, request handlers, Vitest and Jest integration, Storybook mocking, and best practices for realistic API mocking in frontend tests.
API Testing Best Practices: A Complete Guide for Developers
Learn API testing best practices for 2026—from unit tests to contract testing, authentication testing, performance testing, and CI/CD integration. Includes free tools and real examples.
Base64 Encode/Decode Explained: When and How to Use It
Understand when to use Base64 encoding and when to avoid it. This guide explains the Base64 algorithm, practical use cases, code examples in JavaScript, Python, Go, and more.
Best Python Developer Tools in 2026: IDEs, Linters, Testing, and More
The complete guide to Python developer tools in 2026. Best Python IDEs, linters, formatters, testing frameworks, virtual environment managers, and productivity tools every Python developer should know.
Enjoying this tool? Get more like it.
Weekly developer tool guides, productivity playbooks & AI tricks. Free. No spam.
Unlock AI-Powered Dev Tools
- ⚡ AI Code Review, Doc Generator & SQL Builder
- ⚡ All premium templates & early access
- ⚡ Member discounts on Gumroad products
$9/mo after trial · Cancel anytime
Want the full toolkit?
Get DevPlaybook Pro
Every template, guide, boilerplate, and automation script in one bundle. 13 premium products — grab them all at once and save big.
MIT licensed · Instant download · No subscription
See what's included → Browse all deals →