HTTP Request Mocking Libraries

nock is a Node.js HTTP mocking library and request interceptor. It captures outgoing network traffic to specific hosts and paths, returning predefined responses to decouple applications from external servers during automated testing. The project functions as an API expectation framework to verify that specific calls were made with correct parameters. It includes a traffic recorder that captures real network interactions as fixtures for playback and a network simulation tool to introduce artificial latency or trigger network errors. Its capabilities cover request matching via hostnames, paths

Pollyjs is an HTTP mocking and recording library designed to capture and replay network interactions to create deterministic tests. It functions as a request interceptor and stubbing framework that records real API responses into files, ensuring consistent test results across different environments without relying on live network servers. The tool utilizes pluggable network adapters to support various environments and employs a state-based mode switching system to toggle between recording, replaying, and stubbing behaviors. It uses pattern-based request matching and persistent interaction sto

MSW is a JavaScript API mocking library and integration testing tool designed to intercept network requests at the network level. It allows for the definition of mock data and status codes using routing syntax to simulate server responses without requiring changes to the application source code. The project utilizes a service worker to proxy API calls in the browser, providing a mechanism for isolated frontend development and testing. It employs platform-specific network adapters to maintain a consistent mocking interface across both browser and Node.js environments. The library covers a ran

WireMock is a suite of tools for service virtualization, providing mock servers for HTTP APIs and WebSocket communication. It serves as an HTTP API mock server and request verifier, allowing for the simulation of responses and the interception of network requests to test client applications without a live backend. The project differentiates itself through a service virtualization model that records real network traffic and replays it as stubs to mimic external service behavior. It includes a network fault injector to simulate response delays and connectivity issues, and a stateful scenario ma

VCR is a Ruby library that records and replays HTTP interactions during test runs, storing them in serialized cassette files. It captures real HTTP requests and responses, then serves those recorded responses instead of making actual network calls, enabling fast and deterministic test suites that work offline. The library provides configurable request matching, allowing comparisons based on method, URI, host, path, body, or headers to find the correct recorded response. It supports scheduled cassette re-recording to automatically refresh stored interactions at a configurable interval, keeping

Mockoon is an HTTP API simulation platform and design tool used to create fake API endpoints that simulate server responses without a backend. It functions as an OpenAPI mock server tool and a JSON API mocking utility, allowing for the rapid iteration of API contracts and the generation of response templates. The project distinguishes itself through a combination of stateful mocking, using a local JSON database to simulate CRUD operations, and the ability to run as a headless API mock server via a command line interface. It supports real-time collaborative editing and AI-assisted prototyping

Karate is an end-to-end testing framework and API test automation tool used to validate REST and SOAP APIs, drive browser interactions, and measure system performance. It functions as a unified solution for executing automated tests across both API backends and web frontends. The framework utilizes a custom domain specific language and Gherkin-style feature mapping to define requests and assertions. It includes an API mocking server to simulate backend responses, enabling development and integration testing without live dependencies. The tool covers a broad range of functional testing catego

Moya is a network abstraction layer for Swift that provides a structured framework for defining and executing REST API requests. It functions as a type-safe API client that decouples network endpoint definitions from the underlying implementation details to prevent configuration errors and URL typos. The project distinguishes itself by using protocol-based endpoint definitions and a provider-coordinated execution model. It includes a system for mapping network responses to strongly typed objects and features a dedicated tool for generating type-safe network interface files from external REST

This project is a comprehensive Python network request framework designed for both synchronous and asynchronous HTTP communication. It provides a high-performance client capable of executing non-blocking requests within event-driven applications, while also supporting standard blocking calls for simpler scripts. The library is built to operate natively across diverse asynchronous runtimes, automatically detecting and utilizing the underlying event loop for concurrency. What distinguishes this library is its modular architecture, which decouples request construction from network execution thro

Typhoeus is a Ruby wrapper for libcurl that functions as a session-based HTTP client. It provides an interface for making both synchronous and asynchronous network requests. The project acts as a parallel request manager, using a managed queue to execute multiple network requests concurrently. It further distinguishes itself as a mocking tool for stubbing requests with predefined responses and as a caching layer that stores responses to avoid redundant network calls. The library covers a broad range of capabilities including session cookie management, response body streaming for large files,

Sinon is a JavaScript test double library and mocking framework used to isolate code and verify behavior during unit testing. It provides a collection of spies, stubs, and mocks that replace real function implementations and object methods with controlled substitutes. The project includes a specialized fake timer library for overriding the system clock, allowing the simulation of time progression to trigger timeouts and intervals without waiting for real-time delays. It also features a test sandbox mechanism to group multiple fakes together, ensuring the original state is restored between tes