The 35 Essential Free & Open Source API Testing Tools for Developers in 2025

Hello fellow developers and tech enthusiasts! As part of the RobotAlp content team, I'm constantly exploring the tools and technologies that shape our digital world. API testing is a topic that frequently comes up in our research, and for good reason – it's absolutely vital for ensuring the web services we build and rely on work correctly and reliably. While there are many commercial options out there, I've noticed a strong interest in the community for powerful, accessible, and budget-friendly solutions.

This led me to dive deep into the current landscape of free and open-source API testing tools. My goal was to sift through the numerous options and compile a comprehensive list of essential tools that developers should be aware of as we navigate 2025. This isn't just about saving on licensing fees; it's about leveraging the innovation, flexibility, and community support that open-source software often brings to the table. So, let's explore some of the top free and open-source tools that can help you build, test, and maintain robust APIs.

Before we jump into the list, it's worth briefly touching upon why API testing is so critical. In today's interconnected digital environment, APIs are the backbone of most applications. They enable different software components to communicate and exchange data. Effective API testing ensures:

• Reliability: Your API behaves as expected under various conditions.
• Functionality: All endpoints, requests, and responses work correctly.
• Performance: Your API can handle the expected load and responds within acceptable timeframes.
• Security: Your API is not vulnerable to common threats.

Quick feedback from API tests is also invaluable in Agile and CI/CD environments, as API tests are generally faster and more stable than UI tests.

A Curated List of Free & Open Source API Testing Tools for 2025

The following tools are categorized to help you find what best suits your needs, from GUI-based clients for interactive testing to CLI tools for automation and specialized frameworks for complex scenarios.

These tools provide a visual interface for crafting requests, sending them to APIs, and inspecting the responses. They are excellent for manual testing, exploration, and debugging.

1.SoapUI (Open Source Edition)

• Description: A long-standing headless functional testing tool for APIs from SmartBear. It offers robust features for API inspection, functional testing, and automation, supporting both SOAP and REST web services.
• Key Features: Scriptable test steps (Groovy), drag-and-drop test creation, complex scenario workflows, asynchronous testing, built-in mock services, data-driven testing support.
• Supported Protocols: SOAP (WSDL), REST (HTTP/HTTPS) for XML and JSON payloads.
• Link: soapui.org (Open Source)

2. Advanced REST Client (ARC)

• Description: An open-source API client designed for testing HTTP APIs, known for its clean UI. It was built to provide the developer community with necessary tools for free.
• Key Features: Simple, distraction-free interface; supports saving requests/projects to cloud services (e.g., Google Drive); environment variables and scripting; built-in viewer for API documentation formats (RAML, OpenAPI).
• Supported Protocols: HTTP/HTTPS (with RESTful JSON or any text payloads).
• Link: advancedrestclient.com

3. Apache JMeter

• Description: A veteran 100% Java open-source tool designed for load and functional testing. While renowned for performance testing, JMeter is also highly capable for functional API tests. It offers a GUI for building test plans and can run headlessly in CI environments.
• Key Features: Extensive protocol support, thread group design for concurrent user simulation, assertions for response validation, rich plugin ecosystem, detailed listeners and reports.
• Supported Protocols: HTTP/S, SOAP, JDBC, FTP, TCP, and more (flexible via plugins).
• Link:jmeter.apache.org

4. BloomRPC (Archived)

• Description: A GUI client for gRPC services, inspired by Postman and GraphQL Playground. Though archived in 2023, it remains a notable tool for gRPC API exploration. It aimed to be the simplest and most efficient tool for gRPC developers.
• Key Features: User-friendly gRPC request construction; ability to load .proto files and invoke RPC methods; real-time display of responses.
• Supported Protocols: gRPC (HTTP/2) – supports unary and server-streaming calls.
• Link: GitHub – bloomrpc (archived)

5. Insomnia (Core Open Source)

• Description: A cross-platform API client known for its sleek UI and rich feature set for REST, GraphQL, WebSockets, gRPC, and more. Kong now maintains it.
• Key Features: Multi-protocol debugging and testing (including GraphQL with schema introspection and real-time APIs like WebSockets/SSE); built-in OpenAPI designer and documentation preview; CLI integration for CI pipelines; environment variables and plugin system. Collaboration features available (cloud or Git sync).
• Supported Protocols: HTTP/HTTPS (REST), GraphQL, WebSocket/SSE, gRPC, any HTTP-based protocol.
• Link:insomnia.rest

6. Hoppscotch (formerly Postwoman)

• Description: A web-based, open-source API development ecosystem that is lightweight, fast, and offers a PWA interface for browser-based API testing.
• Key Features: Entirely browser-based (no installation) with self-hosting option; collections and environment support; real-time WebSocket, Socket.IO, and SSE clients; GraphQL query editor; CLI (hopp-cli) for automation; minimalist UI, keyboard-shortcut friendly. Fully open-source and community-driven.
• Supported Protocols: REST (HTTP/HTTPS), GraphQL, WebSockets, Socket.IO, SSE, MQTT.
• Link: hoppscotch.io

7. Bruno

• Description: A fast, Git-friendly open-source API client designed as an alternative to Postman/Insomnia, focusing on a file-based workflow where requests are stored as plain text (YAML/JSON) for version control. It works offline and emphasizes essentials.
• Key Features: Collections stored as files in your repository (simplifies code review); core Git integration for syncing and collaboration; supports environments, variables, and pre/post-response scripts; clean, minimal UI, works entirely offline (no account/cloud sync needed).
• Supported Protocols: RESTful APIs (HTTP/HTTPS); also supports GraphQL queries and basic WebSocket testing.
• Link: usebruno.com

8. Firecamp

• Description: An open-source API development platform providing "API playgrounds" for multi-protocol testing, marketed as a secure, cost-effective Postman alternative for teams.
• Key Features: Dedicated "playgrounds" for each protocol (e.g., GraphQL IDE, WebSocket client) in a unified interface; built-in team collaboration (shared workspaces); API collections, environment variables, automated test runner, and CLI/CI integration.
• Supported Protocols: REST (HTTP), GraphQL, WebSocket, Socket.IO, SSE.
• Link: firecamp.io

• Description: A newer open-source API client from Insomnia's original author, built with Rust/Tauri for speed, offline use, and Git-friendliness, similar to Bruno.
• Key Features: Extremely fast startup, low resource usage (Rust-based); stores data as plain text for Git versioning; multiple windows/tabs support; scripting with plugins (Node.js based); emphasizes privacy (local-only data, no telemetry).
• Supported Protocols: HTTP/HTTPS (REST), GraphQL, WebSockets & SSE, gRPC.
• Link: yaak.app

10. Scalar

• Description: An open-source API platform that uniquely combines an API client with beautiful, interactive API documentation generation, primarily from OpenAPI/Swagger definitions.
• Key Features: Interactive API console generated from your API spec; first-class OpenAPI support; tooling to host interactive API docs; web-based client, can be self-hosted.
• Supported Protocols: REST (OpenAPI / HTTP); GraphQL support is in progress.
• Link: scalar.com

11. Requestly

• Description: An open-source platform combining a local HTTP Proxy and an API client with collaboration features, usable as a browser extension or desktop app. It allows building, testing, intercepting, and mocking APIs.
• Key Features: Local-first solution with REST API Client (Collections, Environments, History); HTTP Interceptor (block/modify requests); API Mocking (local/cloud-based, static/dynamic responses); 1-click imports from Postman, Insomnia, etc..
• Supported Protocols: HTTP/HTTPS (REST), GraphQL (for mocking).
• Link: requestly.com (Official site, GitHub also available )

12. Swagger UI

• Description: A popular open-source tool that visualizes and allows interaction with API resources based on an OpenAPI specification, without needing extra code.
• Key Features: Auto-generates API documentation from OpenAPI specs; built-in interactive interface for sending API requests and viewing responses; easy integration into development environments and CI/CD pipelines.
• Supported Protocols: REST (OpenAPI / HTTP).
• Link: swagger.io/tools/swagger-ui/

13. HttpMaster Express

• Description: A web development and test tool to automate testing of websites and RESTful web services, allowing API response monitoring. Offers a free "Express" edition.
• Key Features: Customizable API request options; parameter capabilities for dynamic data; request chaining.
• Supported Protocols: REST (HTTP/HTTPS).
• Link: www.httpmaster.net

CLI tools are essential for automation, scripting, and integration into CI/CD pipelines.

14. cURL

• Description: The quintessential command-line tool for transferring data with URLs, ubiquitous for quick API tests from the terminal.
• Key Features: Supports numerous protocols; human-readable syntax for HTTP methods, headers, body data, authentication, etc.; output to files or pipe to other tools; cross-platform, often pre-installed.
• Supported Protocols: HTTP/HTTPS (including HTTP/2), FTP/FTPS, SCP, SFTP, SMTP/SMTPS, LDAP, MQTT, and others. For API testing, HTTP/HTTPS and even gRPC (with HTTP/2) are key.
• Link: curl.se

15. HTTPie

• Description: A user-friendly CLI HTTP client, often dubbed "cURL for humans," with an intuitive syntax and formatted, colorized JSON output by default. (Also now offers a GUI client ).
• Key Features: Expressive command syntax (e.g., http GET example.com param=value); automatic syntax highlighting and formatted JSON; persistent sessions; flag-based authentication.
• Supported Protocols: HTTP/HTTPS (REST and GraphQL endpoints); gRPC via plugins.
• Link: httpie.io

16. Newman

• Description: The open-source command-line Collection Runner for Postman. It allows you to run Postman API tests headlessly in any environment, especially CI/CD pipelines.
• Key Features: Runs Postman collections and environments from JSON files; supports various reporters (CLI, JSON, JUnit, HTML, etc.) for test results; can parallelize or delay requests.
• Supported Protocols: Whatever Postman supports – primarily HTTP/HTTPS APIs (REST, SOAP, GraphQL).
• Link: GitHub – postmanlabs/newman

17. grpcurl

• Description: A command-line tool analogous to cURL, but specifically for interacting with gRPC servers without needing proto-specific client code.
• Key Features: Invokes gRPC methods with simple syntax (server address, service/method, JSON request body); supports server reflection to fetch service definitions; handles TLS and streaming.
• Supported Protocols: gRPC (HTTP/2) – unary and streaming, with/without TLS. Not for REST.
• Link: GitHub – fullstorydev/grpcurl

18. HTTP Prompt

• Description: An interactive CLI tool providing a REPL-like experience for crafting and debugging HTTP requests incrementally with autocomplete.
• Key Features: Autocomplete for HTTP methods, headers, history; supports environment variables and scripts within the prompt; context changes as you cd into URL paths.
• Supported Protocols: HTTP/HTTPS (primarily RESTful endpoints).
• Link: GitHub – http-prompt

19. wscat

• Description: A simple CLI tool from the ws Node.js library for interactively testing WebSocket connections.
• Key Features: Connects to a WebSocket URL and provides an interactive shell to send/receive messages; lightweight, installed via npm.
• Supported Protocols: WebSocket (ws://, wss://).
• Link: GitHub – websockets/wscat

20. Apache Bench (ab)

• Description: A simple command-line tool for benchmarking HTTP servers and APIs, designed for basic load testing.
• Key Features: Simulates high load by sending multiple concurrent requests; provides real-time performance metrics (requests/sec, time/request); supports GET and POST.
• Supported Protocols: HTTP/HTTPS.

CI/CD Integration Friendly Tools & Frameworks

These tools and frameworks are designed for automating API tests and integrating them into continuous integration and delivery pipelines.

21. Karate DSL

• Description: An open-source API test automation framework combining testing, mocking, performance testing, and even UI automation, using a BDD-style (Cucumber-like) DSL that doesn't require programming for basic tests.
• Key Features: Cucumber-like syntax for API calls and assertions; built-in HTTP calls, JSON/XML assertions, API mocking, and UI testing (WebDriver); runs as JUnit tests, generating standard reports for CI; data-driven testing; parallel execution.
• Supported Protocols: HTTP/HTTPS (REST and SOAP); gRPC and GraphQL via community contributions.
• Link: GitHub – karate (karatelabs/karate)

22. Rest-Assured

• Description: A widely used open-source Java DSL for testing RESTful services, bringing a fluent BDD-style syntax (given/when/then) to Java API tests.
• Key Features: Fluent API for request construction and response validation; eliminates boilerplate HTTP connection code; built-in JSON/XML path assertions; supports various authentication mechanisms; integrates with JUnit/TestNG for CI.
• Supported Protocols: HTTP/HTTPS (REST and SOAP); supports JSON and XML payloads.
• Link: rest-assured.io

23. k6

• Description: A modern open-source load testing tool by Grafana Labs, also excellent for API performance checks and automation, using JavaScript for scripting test scenarios.
• Key Features: Code-as-test approach (JS scripts); supports checks and thresholds for performance assertions (e.g., "95% of requests < 500ms"); CLI execution with metrics output to JSON or Grafana; efficient Go engine for high concurrency.
• Supported Protocols: HTTP/HTTPS (REST, GraphQL, WebSockets upgrade testing); gRPC and MQTT via modules.
• Link: k6.io

24. Pact

• Description: An open-source consumer-driven contract testing tool crucial for microservices, ensuring services adhere to a mutual contract (HTTP request/response pairs) to catch breaking API changes early.
• Key Features: Libraries for multiple languages to define contracts in consumer tests; provider service then runs Pact verification; supports asynchronous messaging contract tests; Pact Broker for sharing contracts.
• Supported Protocols: HTTP/HTTPS (mostly REST/JSON); asynchronous messaging (e.g., HTTP->AMQP).
• Link: pact.io

25. Gatling

• Description: An open-source load and performance testing tool written in Scala, well-suited for APIs, allowing high-load simulations with detailed HTML reports.
• Key Features: Load testing as code (fluent Scala DSL); asynchronous and efficient engine; real-time monitoring and detailed metrics; official support for HTTP, WebSockets, SSE, JMS, MQTT; CI integration (Maven/Gradle).
• Supported Protocols: Primarily HTTP/HTTPS; also WebSocket, SSE, JMS, gRPC, MQTT (some via community plugins).
• Link: gatling.io

26. Step CI

• Description: An emerging open-source, self-hosted platform specifically designed for automated API testing (REST, GraphQL, gRPC) in CI pipelines using YAML/JSON/JS config files.
• Key Features: Multiple config formats (YAML, JSON, JS); supports test data or fake data generation; fuzz testing and specialized tests (e.g., ChatGPT API, CO2 usage); concurrent test execution; can autogenerate tests from OpenAPI spec.
• Supported Protocols: REST, GraphQL, gRPC (first-class support).
• Link: stepci.com

27. Locust

• Description: A popular open-source load testing tool that uses Python to define user behavior, often used in CI to test API load handling.
• Key Features: Simple Python classes for user tasks (e.g., API requests via requests library); can spawn thousands of distributed users; web UI for live test monitoring.
• Supported Protocols: HTTP/HTTPS out of the box; can test any protocol by writing Python code (e.g., Locust libraries for WebSockets). Primarily for REST API load tests.
• Link: locust.io

28. Dredd

• Description: An API contract testing tool that validates your API implementation against its specification (OpenAPI/Swagger or API Blueprint), ensuring your API doesn't drift from its documentation.
• Key Features: Language-agnostic CLI tool; iterates through documented endpoints/requests, comparing actual vs. expected responses; CI integration; hooks system for extending tests; supports Swagger/OpenAPI 2.0/3.0 and API Blueprint.
• Supported Protocols: HTTP/HTTPS (for APIs defined by OpenAPI or API Blueprint).
• Link: dredd.org

29. Schemathesis

• Description: A specification-based (OpenAPI or GraphQL) API testing tool that uses property-based testing to generate numerous test cases, including edge cases and invalid data, to fuzz your API and find bugs.
• Key Features: Automated test generation (conforming and violating schema); pytest integration; stateful testing across endpoints; powered by Hypothesis library.
• Supported Protocols: HTTP APIs defined by OpenAPI (3.0, 3.1) or GraphQL schemas.
• Link: schemathesis.io

30. TraceTest

• Description: A unique open-source tool by Kubeshop that uses distributed tracing data (OpenTelemetry) to test APIs, verifying not just the response but also the internal system behavior for a request in microservices.
• Key Features: Trace-based assertions (e.g., assert DB call made, no service errors in trace); OpenTelemetry integration; no need for mocks (tests system as a whole with real data); Web UI and CLI/YAML for tests (CI/CD friendly); analyzes OpenTelemetry instrumentation quality.
• Supported Protocols: Any system emitting OpenTelemetry traces (typically HTTP APIs in microservices); trigger tests via HTTP, gRPC, or trace ID.
• Link: tracetest.io

31. Keploy

• Description: An innovative open-source, developer-centric backend testing tool that generates API tests and data mocks/stubs from real user traffic by recording API calls and DB queries.
• Key Features: Record & Replay of live API calls and DB interactions; auto-generates mocks/stubs for external dependencies; aims to reduce manual test writing; integrates as SDK/agent with minimal code changes for CI.
• Supported Protocols: Mainly HTTP/HTTPS APIs (records incoming HTTP requests); also records SQL queries or other calls if configured. Multi-language support.
• Link: keploy.io

32. SuperTest

• Description: A popular open-source JavaScript library (super-agent driven) for testing Node.js HTTP servers with a fluent API, often used with Jest or Mocha.
• Key Features: Fluent API for requests and expectations; allows in-memory integration testing of Node APIs (faster, isolates failures).
• Supported Protocols: HTTP/HTTPS (focused on testing Node.js HTTP servers).
• Link: GitHub – ladjs/supertest (maintained fork)

33. Mockoon

• Description: An open-source tool for quickly creating mock REST API servers locally without coding, ideal for simulating API responses when the real API is unavailable or under development.
• Key Features: Simple UI for no-code mock API creation; multiple environments and dynamic response logic; simulates status codes, headers, delays; cross-platform, CI/CD integration.
• Supported Protocols: REST APIs (HTTP/HTTPS).
• Link: mockoon.com

34. WireMock

• Description: An open-source API mocking tool for simulating HTTP-based APIs, useful for testing dependencies and edge cases.
• Key Features: Mocks HTTP requests/responses (dynamic stubbing, proxying, record-and-replay); supports various request methods; simulates delays, network faults; supports JSON, XML, SOAP payloads; CI/CD integration.
• Supported Protocols: HTTP/HTTPS.
• Link: wiremock.org 

35. Fiddler Classic (Core Open Source)

• Description: A powerful web debugging proxy that captures, inspects, and modifies HTTP/HTTPS traffic. The core "Classic" version is open-source and useful for API testing and debugging.
• Key Features: Captures and logs HTTP/HTTPS traffic; traffic manipulation tools; mocks API responses, simulates latency; HTTPS decryption.
• Supported Protocols: HTTP/HTTPS.
• Link: telerik.com

The best API testing tool for you will depend on your specific needs, team expertise, project requirements, and the types of APIs you're testing (REST, GraphQL, gRPC, SOAP, etc.). Consider factors like:

• Ease of use and learning curve.
• Protocol support.
• Automation capabilities and CI/CD integration.
• Scripting flexibility and customization options.
• Community support and documentation.
• Specific testing needs like performance, security, or contract testing.

The landscape of free and open-source API testing tools in 2025 is vibrant and offers a wealth of options for developers. Leveraging these tools can significantly enhance the quality, reliability, and performance of your APIs. By automating your API testing, you can get faster feedback, catch bugs earlier in the development cycle, and ultimately deliver better software.

While these tools focus on testing the functionality and performance of your APIs, remember that monitoring their availability and behavior in production is equally crucial. RobotAlp's API Monitoring service can help you ensure your APIs are consistently up and performing as expected once they are live. For more insights and tools, don't forget to check out the RobotAlp Blog.