Open-source alternatives to Nock

Fetch-mock is a testing utility designed to isolate application code from external network dependencies by intercepting and overriding outgoing traffic. It functions as a network request interceptor that captures calls made via the Fetch API, allowing developers to simulate server responses and verify application behavior without requiring a live backend infrastructure. The library distinguishes itself through a unified interface that provides consistent network interception logic across diverse runtime environments, including browsers, service workers, and server-side platforms. By replacing

moco is a multi-protocol mock server and programmable API stub used to simulate HTTP, REST, WebSocket, and Server-Sent Events endpoints. It enables the creation of fake backend services to facilitate development and integration testing without requiring a live server. The project distinguishes itself through its ability to act as a network traffic replay tool, capturing real request-response pairs via a proxy and playing them back locally. It also supports bidirectional communication simulations, including WebSocket server simulation and SSE stream simulation with configurable delivery delays

This project is a multi-protocol API simulation and mocking system designed to replace external dependencies during development and testing. It provides an API mocking server, a network traffic proxy, and specialized simulators for language model services and identity providers. The system distinguishes itself through deep AI simulation capabilities, including the emulation of language model providers and Model Context Protocol servers using JSON-RPC 2.0. It supports multi-turn conversational logic, state tracking for AI chat APIs, and the visualization of agent execution through call graphs

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

OkHttp is an HTTP client for the JVM and Android that enables network communication via synchronous and asynchronous requests. It provides a core identity as a comprehensive networking toolkit featuring a WebSocket client for bidirectional data synchronization, a TLS security toolkit for encrypted communication, and a programmable HTTP mock server for simulating backend responses. The project distinguishes itself through a specialized security and performance architecture. It implements mutual TLS, certificate pinning, and DNS over HTTPS to secure data in transit, while utilizing connection p

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

This library provides a utility for intercepting and simulating HTTP requests within the Axios ecosystem. It functions as an HTTP request interceptor that allows developers to define mock responses for outgoing network traffic, enabling frontend integration testing and development without requiring live backend infrastructure. The tool distinguishes itself by offering granular control over the request lifecycle, including the ability to match specific request patterns and forward unmatched traffic to actual destinations. It supports the simulation of unstable network environments by injecting

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

TUnit is a comprehensive C# testing framework, mocking library, and fluent assertion tool. It utilizes source generation for test discovery and mock creation, ensuring compatibility with Native AOT and IL trimming by eliminating the need for runtime reflection and proxies. The framework provides specialized capabilities for integration testing, including the management of distributed application lifecycles, isolated database schemas, and the correlation of telemetry and logs across process boundaries via OTLP. It also includes an HTTP testing utility to intercept network exchanges and mock AP

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

httplab is an HTTP mock server and interactive API simulator used to analyze incoming request payloads and simulate custom HTTP responses for application testing. It functions as a request inspector that captures and displays real-time details of HTTP headers and bodies sent by clients. The project includes a response management system that imports and stores predefined HTTP response templates using JSON files. This allows for the persistence of request and response configurations to local files for use in reusable testing scenarios. The tool provides capabilities for generating custom HTTP

httplab is an interactive HTTP mock server, request inspection tool, and response simulator. It functions as a web server designed to capture and analyze incoming HTTP requests and headers in real time for debugging purposes. The tool enables the construction and delivery of custom HTTP responses to simulate various server behaviors. Users can forge specific headers and bodies to test how client applications handle different status codes and payloads, with the ability to save these configurations as response templates for consistent reuse across sessions. The project covers a broad surface o

Mockserver is a multi-protocol mock server and API verification proxy used to simulate HTTP, gRPC, and WebSocket endpoints. It functions as a tool for testing client applications without relying on live backend services, providing a system to simulate chat completions and streaming responses for large language model integrations. The project automates behavior by generating request expectations and response behaviors from OpenAPI and Swagger specification files. It differentiates itself through a network traffic simulator that introduces latency, dropped connections, and failures to verify ho

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,

Mock is a JavaScript API mocking tool and network request interceptor designed to decouple front-end development from back-end progress. It functions as an API simulation tool and mock data generator, allowing developers to build user interfaces and high-fidelity prototypes by mimicking the request and response cycle without a live server. The system provides a mechanism for intercepting outgoing HTTP calls and returning simulated data. It enables front-end prototyping by generating synthetic datasets to validate application behavior during automated testing cycles and development. Capabilit

Moto is a cloud service mockery framework and API mock server that simulates AWS infrastructure locally. It allows developers to test cloud-dependent code and verify infrastructure-as-code templates without deploying real resources or incurring costs. The project functions as an SDK interceptor that can patch existing service clients to redirect requests to a local mock environment. It can also be run as a standalone HTTP server, enabling any programming language to interact with the simulated endpoints. The framework covers a vast array of simulated capabilities, including data storage, com

WebMock is a Ruby library for HTTP request stubbing, response simulation, and network interception. It serves as a test assertion framework for verifying outgoing HTTP traffic and a network interception layer that blocks external connections to prevent accidental requests during testing. The project provides capabilities for dynamic response generation and the sequencing of different return values for repeated requests to the same endpoint. It supports simulating network failures, such as timeouts or exceptions, and allows for response replays using raw HTTP data loaded from local files. The

Playwright is a comprehensive browser automation framework designed for end-to-end testing and web workflow automation. It provides a unified API to drive web applications across multiple browser engines, enabling developers to simulate complex user interactions, perform web scraping, and validate application behavior in consistent, isolated environments. The framework distinguishes itself through a web-first testing paradigm that prioritizes stability and resilience. By utilizing an auto-waiting actionability engine and accessibility-tree-based locators, it eliminates common sources of test

GoMock is a mocking framework for the Go programming language.

Cypress is a browser-based testing framework designed for writing and running automated tests directly inside the browser. It serves as an end-to-end testing framework, a frontend component testing tool, and a web application test runner. The project also functions as a headless browser automation tool and a network traffic interceptor. The system differentiates itself by executing test code within the same browser process as the application, allowing for direct access to the DOM. It includes a network traffic interception system to stub and manipulate browser requests, as well as a graphical

Mirage JS is a browser-based mock server and client-side API simulator designed for JavaScript application development. It functions as a mocking tool that intercepts network requests to simulate backend API behavior, allowing developers to build and test frontend interfaces without requiring live backend infrastructure. The tool provides a simulation layer that mimics REST endpoints to facilitate rapid prototyping of user flows and application logic. It enables the creation of fake server responses directly within the web browser to verify how an application handles various data scenarios an

Karate is a Gherkin API testing framework and JVM test orchestrator designed for API validation, workflow automation, and browser automation. It provides a unified environment for executing scriptable logic on the Java Virtual Machine with thread-safe concurrent processing. The project includes a built-in HTTP mock server to simulate backend API behavior by delivering dynamic responses to intercepted requests. It also features a visual regression testing tool that detects interface changes by comparing screenshots against baseline images using pixel-matching analysis. The system supports API

Ava is a test runner for JavaScript and TypeScript designed to execute test suites with a focus on concurrency and isolation. It serves as a concurrent test executor that runs test files in parallel across multiple processes to reduce total runtime and prevent state leakage between suites. The project features a built-in snapshot testing framework that saves large data structures to disk and compares subsequent executions to detect regressions via diffs. It is also compatible with the Test Anything Protocol, allowing it to export results for use with external reporting tools. Its capability

Jasmine is a JavaScript testing framework and test runner designed for behavior-driven development. It provides a comprehensive toolset for writing and executing descriptive test suites that verify code behavior in both web browsers and Node.js environments. The framework is distinguished by its integrated mocking library, which allows for dependency isolation through spies and stubs, as well as the ability to simulate time and timing functions to test asynchronous logic synchronously. It also includes a mechanism to enforce test uniqueness, preventing duplicate names for tests and suites. I

Playwright for Python is a browser automation framework designed for end-to-end testing, web scraping, and user interaction simulation. It functions as a headless browser controller that enables programmatic navigation, data extraction, and the execution of complex workflows across multiple rendering engines. The framework distinguishes itself through an actionability-aware interaction engine that automatically verifies element readiness before performing actions, significantly reducing test flakiness. It utilizes isolated browser contexts to maintain separate storage and cookies for parallel

nextest is a high-performance test execution framework and Rust test runner designed to manage parallelism, retries, and timeouts. It serves as a test recording system that captures execution metadata and outputs into archives for later failure analysis and replay. The project distinguishes itself as a flakiness detection tool, identifying unstable tests through stress loops and automated retry policies. It also functions as a CI test orchestrator, capable of partitioning test suites across multiple workers and exporting results in standardized JUnit XML and JSON formats. The framework provi