10 Best API Testing Tools for Developers

APIs power modern applications – they enable different systems to communicate and deliver seamless user experiences. In fact, APIs drive over half of today’s internet traffic, and their importance is only growin With the rise of API-first development (74% of teams are API-first in 2024), thorough API testing is essential. The right API testing tools help developers ensure functionality, performance, and security before deploying services to customers. In this guide, we’ll compare free, open-source, and professional API testing solutions, covering automation, mobile support, cloud capabilities, and more. Read on to find the best API testing tool for your project.

Choosing an API testing tool depends on your requirements. Some tools are free API testing software (e.g. open-source projects), while others are professional API testing solutions with enterprise features. There are web-based online API test apps as well as desktop clients, some even supporting mobile API testing tools integration. Key factors include support for REST, SOAP, and GraphQL, test automation and CI/CD integration, reporting, and security testing. Throughout this article, we’ll mention tools that fit categories like free open source API test software, automation API testing software, cloud API testing apps, cross-platform API tools, and secure API testing tools. Whether you need a lightweight web client or an advanced API development platform, one of these ten tools has you covered.

1. Postman

Screenshot of Postman’s API client interface (an API testing tool) making a GET request – illustrating how developers use Postman to test APIs.

Postman is a versatile and widely-used API client and testing platform. It provides an intuitive GUI and supports the full API lifecycle: designing, sending requests, automating tests, and monitoring. Postman’s collaborative features – including Collections and Workspaces – let teams organize, share, and reuse API tests across projects. It supports all major API types (REST, SOAP, GraphQL, and even gRPC) and authentication schemes. Some key features include:

• API Requests & Automation: Easily send HTTP requests and write tests using JavaScript. Postman collections can be run in a CI/CD pipeline via Newman (Postman’s CLI) for automated testing.
• Mock Servers & Monitoring: Quickly set up mock servers to simulate APIs before production, and monitor API performance with scheduled checks by API testing tools.
• Collaboration & Version Control: Share environments and collections in team workspaces, with role-based access and Git integration.

Postman’s strengths lie in its ease of use and community. It’s accessible for beginners yet powerful enough for advanced users. According to Nordic APIs, Postman’s GUI “makes API testing approachable” and its large community provides extensive plugins and integrations. For example, Postman now supports AI-enhanced documentation and in-depth reporting. Some drawbacks are that very complex workflows can become hard to manage, and it’s less focused on load/performance testing. Overall, Postman remains a top automation API testing tool for developers of all levels API testing tools.

2. SoapUI (ReadyAPI)

SoapUI (and its commercial sibling ReadyAPI) is a veteran API testing tool known for comprehensive testing capabilities. SoapUI is open-source and excels at testing SOAP, REST, and other web services. Key points include:

• Functional, Load & Security Testing: SoapUI supports functional testing (with assertions and scripting), as well as load and security testing (e.g. SQL injection, XSS) – especially in the ReadyAPI (SmartBear) suite
• Scripting & Assertions: It uses Groovy scripting for advanced validations and custom logic, making it very flexible for complex scenarios.
• Mocking & Simulation: You can create mock services to simulate APIs and drive development/testing before endpoints are live.

SoapUI’s drag-and-drop GUI can create tests by importing WSDL/OpenAPI definitions or recording live traffic. It integrates with CI tools (Jenkins, Maven, etc.) for continuous testing. The open-source SoapUI handles most basic needs, but enterprise teams often upgrade to ReadyAPI for advanced features (distributed testing, data-driven loops). On the downside, SoapUI’s interface can feel outdated and has a learning curve. As Nordic APIs notes, its strengths are “comprehensive support for API paradigms (REST, SOAP) and strong scripting with Groovy”. In short, SoapUI/ReadyAPI is a professional API testing solution ideal for thorough testing of SOAP/REST services API testing tools.

3. Apache JMeter

Apache JMeter is a powerful open-source tool originally built for load testing, but also widely used for functional API testing. JMeter is Java-based, cross-platform, and completely free. It shines when you need to simulate high traffic and measure performance of REST or SOAP APIs. Notable aspects include API testing tools.

• Load/Performance Testing: JMeter lets you configure thousands of virtual users and stress-test endpoints, measuring response times, throughput, and resource usage. It’s great for verifying scalability under load.
• Functional Testing & Scripting: You can also build functional tests in JMeter, using samplers, assertions, and parameterization. It supports CSV data files for data-driven testing. Scripting in Java/Groovy enables complex logic if needed.
• Extensible & CI-Friendly: A rich plugin ecosystem expands JMeter’s capabilities, and it can run headless via CLI (suitable for CI pipelines) API testing tools.

JMeter has an older-style interface and can be cumbersome for elaborate flows, but its robustness and openness are unparalleled. Because it’s open-source, many teams include JMeter in their toolbox for free performance and API checks. As one blog notes, JMeter “is free, open source, and cross-platform” with a proven track record for testing. For developers needing an open source API test software that also handles performance, JMeter is a top choice API testing tools.

4. REST Assured

REST Assured is a Java library (not a standalone GUI) tailored for testing RESTful APIs. It’s favored by developers who prefer writing tests in code. REST Assured provides a fluent API for building HTTP requests and validating responses. Key highlights:

• Java DSL & Integration: You write API tests in JUnit/TestNG using a concise, expressive syntax (e.g. given().when().get("/users").then().statusCode(200)). It integrates seamlessly with existing Java projects API testing tools.
• Built-in Parsing & Validation: It has built-in support to parse JSON and XML, and easily assert contents via JSON path or XML path. Schema validation and detailed assertions help ensure API contracts are met.
• Automation & CI: Since it’s code-based, REST Assured tests can run as part of your CI/CD pipeline like any unit test, providing fast feedback on API changes.

REST Assured is free and open-source, but requires Java expertise. It lacks a GUI – everything is code. This makes it ideal for backend developers who want version-controlled tests and deep integration with Java build tools. For Java shops, REST Assured is a “dream” for API testing. The trade-off is that non-Java teams or those wanting a user interface may prefer other tools. Overall, REST Assured is a powerful automation API testing software when you want code-centric testing by API testing tools.

5. Katalon Studio

Katalon Studio is a unified automation platform that supports API, Web, Mobile, and desktop testing. It combines open-source engines (Selenium/Appium) with a low-code interface. For APIs, Katalon offers:

• All-in-One Testing: You can test REST, SOAP, and GraphQL APIs alongside UI tests in a single project. This is great for end-to-end scenarios.
• Built-in Test Recorder: A click-and-record feature captures API calls from a browser or HTTP client to automatically generate test scripts. This speeds up creating new tests.
• Data-Driven and CI Support: Katalon lets you parameterize tests with external data (CSV/Excel) and integrates with Jenkins, Git, and other DevOps tools for continuous execution API testing tools.

Katalon’s friendly UI and step-by-step setup make it accessible. It also offers mobile API testing (validating APIs used by mobile apps) since it supports iOS and Android testing. The platform includes test reporting and analytics out of the box. However, advanced features require a paid license, and some find the resource usage heavy. According to Katalon’s site, it’s an “all-in-one API testing toolkit” that streamlines tests across platforms. For teams seeking a mobile API testing tool that bridges front-end and back-end tests, Katalon Studio is a strong contender API testing tools.

6. Karate DSL

Karate DSL is an open-source API testing framework that uses the Cucumber/Gherkin syntax for behavior-driven testing. What sets Karate apart is that it combines API tests, performance tests, and mocks into one tool. Highlights include:

• Readable Gherkin Syntax: Write tests in natural language steps without extra glue code. Given When method GET Then status 200. This makes tests easier to understand and maintain.
• All-in-One Framework: Karate has built-in JSON/XML assertions, data-driven testing, parallel execution, and even API mocking. You get a lot of functionality “out of the box” API testing tools.
• Integration with Java: While tests are written in .feature files, you can still invoke them from JUnit if needed. It also supports embedding Java code snippets API testing tools.

Karate DSL is free and programmatic, ideal for teams already using BDD-style workflows. Its parallel execution and built-in mocks make it efficient. According to Nordic APIs, Karate “combines API testing, performance testing, and mocking into one sleek package”. The learning curve is moderate (you don’t write raw code but still need to learn its Gherkin dialect), and debugging can be tricky. For teams wanting a single cross-platform API tool that covers multiple testing needs with minimal coding, Karate DSL is an excellent choice API testing tools.

7. Hoppscotch

Hoppscotch (formerly Postwoman) is a free, open-source web-based API testing app. It runs entirely in the browser and offers a lightweight alternative to desktop clients like Postman. Key features API testing tools.

• Web-Based API Client: No installation needed – just open hoppscotch.io. It supports REST, WebSocket, SSE, and GraphQL out of the box. Great for quick testing from any device API testing tools.
• Collaborative Workspaces: (Newest versions are adding) you can save and share collections of requests and environments online.
• Minimalist UI: Hoppscotch’s interface is clean and easy. It provides history of requests, easy parameter editing, and code snippets in multiple languages API testing tools.

Because it’s browser-native, Hoppscotch is truly cross-platform – you can test APIs from Windows, macOS, Linux, or even mobile browsers. It’s excellent for on-the-fly testing or educational purposes. As one source notes, it was “built from scratch by considering accessibility” and focuses on essential functionality. The drawbacks: it has fewer advanced features (no built-in automation or CI integration), and since it’s browser-based, it’s not ideal for very large test suites. Still, for a free API testing software or quick manual testing, Hoppscotch is an impressive, no-cost option API testing tools.

8. Assertible

Assertible is a cloud service for automated API testing and monitoring. It’s designed for continuous integration and uptime checks. Highlights:

• Easy Test Setup: Define tests via a GUI by importing Swagger/OpenAPI or Postman collections. Then run and schedule them against different environments API testing tools.
• Automated Monitoring: Assertible runs tests on every build or on a schedule, and can post alerts (e.g. to Slack) if an API fails. It keeps detailed logs and charts API testing tools.
• Integration with DevOps: It connects with GitHub (run tests on push), CI tools, and collaboration apps API testing tools.

Assertible focuses on simple, repeatable tests (e.g. status codes, JSON contents) and is cloud-hosted. It’s great for teams that need professional API testing solutions without managing servers. According to Global App Testing, Assertible “tests and monitors web services across deployments”, with dynamic variables and mock endpoints. It’s easy to start, but less suited for very complex or on-prem scenarios. If your goal is automated API test monitoring in a secure, cloud-based environment, Assertible is worth considering.

9. Apigee

Apigee (by Google Cloud) is an enterprise API management platform that includes robust testing features. While primarily an API gateway and management tool, Apigee offers API testing tools.

• Mock Services & Test Tools: You can create mock API proxies and set up test cases based on your API definitions (OpenAPI specs).
• Performance & Load Testing: Apigee lets you simulate traffic and measure API latency, enabling automated performance tests API testing tools.
• API Monitoring & Analytics: Continuously monitor API performance, response times, and errors in real time. Built-in dashboards help identify issues before users notice API testing tools.

Apigee is ideal for organizations that need full lifecycle API management (design, deploy, secure, monitor) along with testing. It’s a “complete API platform” – you can run functional tests and see analytics within the gateway itself. The downside is that it’s a commercial, cloud-hosted platform (pricing and complexity are considerations). For large teams, however, Apigee provides an all-in-one cloud API testing app and governance layer to ensure API quality at scale.

10. Swagger (OpenAPI) Tools

Swagger refers to a suite of tools built around the OpenAPI (Swagger) specification. While Swagger is often thought of for API design and documentation, it also aids testing:

• Interactive Documentation (Swagger UI): Automatically generates a web interface from your OpenAPI spec where you can send test requests to each endpoint. This is a quick way to manually test APIs against the defined contract.
• Mock Servers: Tools like Swagger Codegen can generate mock implementations from your API schema, allowing frontend devs to start integration before the backend is complete API testing tools.
• Contract Testing: Because Swagger enforces a spec, tests can validate that the API’s actual responses match the documented schemas and examples API testing tools.

Using Swagger tools helps ensure that APIs are well-documented and conform to their specs, which is critical for reliable API development. When an API’s OpenAPI definition is kept up-to-date, testers and developers have a single source of truth. In practice, teams often generate tests or sanity checks from the API spec. For example, Swagger’s ecosystem includes ready-made libraries and CLI tools to drive simple validation tests. Overall, embracing Swagger/OpenAPI is an “advanced API development tool” strategy that improves testability and collaboration.

Conclusion

API testing tools are essential for building high-quality, reliable applications. The ten tools above range from free, open-source options (like Hoppscotch, JMeter, Karate) to enterprise-grade platforms (like ReadyAPI and Apigee). They support manual exploration, automated testing, performance/load testing, and security checks. Key features to look for in any tool include support for your API protocols (REST, SOAP, GraphQL, etc.), ease of use or automation capability, CI/CD integration, and reporting.

In 2025, the best API testing tools not only validate functionality but also integrate with DevOps workflows. Tools like Postman and Swagger enhance collaboration and documentation, while libraries like REST Assured or Karate allow developers to keep tests in code. Cloud services such as Assertible and BlazeMeter (not in our top 10 but noteworthy) offer hosted testing and monitoring. Whatever your needs, using a dedicated API testing tool ensures that your APIs are secure, performant, and reliable. Evaluate the options above against your project requirements (and your budget), and start testing early and often. Happy testing!

FAQs

Q: What is an API testing tool?
A: An API testing tool is software that automates the process of sending requests to an API and verifying its responses. These tools check that APIs behave correctly (functional testing), handle load (performance testing), and are free from vulnerabilities (security testing). Unlike user-interface (UI) tests, API tests exercise the underlying endpoints directly. Good API testing tools support multiple protocols (REST, SOAP, GraphQL, etc.), allow scripting of tests, and integrate with CI/CD pipelines for continuous testing. They help catch bugs early, improve software quality, and ensure APIs are reliable and secure

Q: Which API testing tool is best for beginners?
A: For beginners, Postman and Katalon Studio are often recommended. Postman’s graphical interface and extensive documentation make it easy to learn basic API requests and tests. Hoppscotch (web-based) is also beginner-friendly with a clean UI. Katalon Studio provides low-code record-and-playback features, lowering the barrier for non-programmers. These tools allow new users to get started quickly without writing much code, while still offering powerful features as you advance.

Q: Are there free or open-source API testing tools?
A: Yes. Many top API testing tools are free or open-source. Examples include Apache JMeter, REST Assured, Karate DSL, and Hoppscotch. JMeter is open-source (Apache license) and excellent for load and API testing. REST Assured and Karate DSL are free libraries for code-based testing. Hoppscotch is a free web app (MIT licensed) for quick tests. SoapUI even has an open-source version for basic functional testing. Using these free tools can be especially cost-effective for startups or small teams.

Q: How do I choose the best API testing tool for my project?
A: Consider the following when choosing:

• API Types & Protocols: Ensure the tool supports your API’s style (REST, SOAP, GraphQL, gRPC, etc.).
• Testing Needs: Do you need just functional tests, or also load, performance, and security testing? Tools like JMeter handle performance well, while SoapUI/ReadyAPI cover security and load.
• Automation & CI/CD: For continuous integration, pick a tool with command-line or API support (e.g. Newman for Postman, or code libraries like REST Assured).
• Ease of Use: Teams with less coding experience may prefer GUI tools (Postman, Katalon), whereas development-heavy teams might use code-based tools (REST Assured, Karate).
• Collaboration: If multiple team members need to share tests, tools with workspaces or version control integration (Postman Collections, Swagger/OpenAPI) are helpful.
• Budget: Open-source tools eliminate license cost, but paid tools may offer advanced features and vendor support. Balance features against your budget and project scale.

Q: Can I use API testing tools in CI/CD pipelines?
A: Absolutely. Most modern API testing tools integrate with CI/CD. For example, Postman collections can run via Newman in Jenkins or GitHub Actions, and JMeter tests can execute in headless mode on any OS. REST Assured or Karate tests (since they are code) can be part of automated test suites. Tools like Assertible and BlazeMeter provide plugins or APIs to trigger tests during builds. By automating API tests on every code change, you get instant feedback and catch regressions early.

Q: Are there tools for API security testing specifically?
A: Yes. Some API testing platforms include security testing. For instance, ReadyAPI (SmartBear) has features for injecting security scans into API tests. OWASP ZAP (while not solely an API tool) can be configured to proxy and scan APIs for vulnerabilities. StackHawk (not in our top 10 list) focuses on API security in CI/CD. In practice, use dedicated security tools alongside functional API testing. Secure design (using Swagger/OpenAPI) and tools like Apigee (with built-in security features) also help ensure APIs are robust against attacks

Q: How do I test an API on mobile devices?
A: If your API serves a mobile app, you test it the same way as any API. Tools like Postman, JMeter, or REST Assured will send requests to the API endpoints just as a mobile client would. For end-to-end mobile testing (including the app’s UI), tools like Appium integrate with API calls. Katalon Studio and Postman can run on Windows/macOS, but you can trigger their tests from mobile device builds via CI. The key is to simulate the API calls the mobile app makes, which is fully supported by the tools above. Katalon also allows testing mobile app flows and validating backend calls in one suite.

Q: What is an “OpenAPI” or Swagger in relation to testing?
A: OpenAPI (formerly Swagger) is a specification for defining REST API contracts in YAML or JSON. By documenting your API in OpenAPI, you get machine-readable schemas. Tools like Swagger UI or the Postman importer let you turn these specs into interactive documentation or test suites. Testing with OpenAPI means you can automatically validate responses against the defined schemas, and generate tests from examples. Many testing tools use OpenAPI definitions to simplify setup. In other words, Swagger/OpenAPI isn’t a testing tool itself, but it enhances testing by providing a standardized contract

Q: How can I share API tests with my team?
A: Collaboration features vary by tool. Postman’s shared workspaces allow teams to co-edit and version API collections. SoapUI Pro (ReadyAPI) integrates with Git and lets users share projects. Hoppscotch plans to add workspace sharing. If using code-based tools (REST Assured, Karate), store test code in a shared repository (Git/GitHub). Documenting your API with Swagger/OpenAPI and storing it in source control also lets everyone reuse the spec for testing. Finally, cloud tools like Assertible let you add team members and share test dashboards online.

Q: Do I need multiple tools for API testing?
A: Often, yes. Many teams use a combination: a GUI client (Postman/Hoppscotch) for manual exploratory testing, plus a library or CLI tool (REST Assured/Newman) for automation. They may also run JMeter for performance/load tests, and a security scanner (like OWASP ZAP) for security. There’s no single tool that does everything perfectly. Use what fits each purpose best. For example, use Postman for quick smoke tests, JMeter for stress tests, and an integration test suite (Karate or REST Assured) in your codebase. The key is covering all your testing needs, whether with one multipurpose tool or a set of specialized ones.

Q: How much do these API testing tools cost?
A: Several of the tools listed are free or open-source: Postman has a free tier, SoapUI has a free open-source version, JMeter/Karate/REST Assured/Hoppscotch are fully free, and Assertible has a free plan. Paid tools include Katalon Studio’s Enterprise edition, SmartBear ReadyAPI, and Google’s Apigee (cloud service). Costs vary: Postman’s Team/Enterprise plans, ReadyAPI licenses, and Apigee usage can add up for large teams. Always check the latest pricing on official sites. The investment can be worthwhile for advanced features and support, but many projects do well with the free or community editions of these tools.

Q: How do I start learning API testing?
A: Begin with a simple tool like Postman or Hoppscotch to manually send requests and inspect responses. Learn to create collections of tests and run them. Study HTTP methods, status codes, and basic assertions (e.g. checking JSON values). Then try automating those tests (e.g. using Postman’s Collection Runner or writing a small REST Assured test). Explore writing CI/CD scripts to run tests on each commit. There are many online tutorials and courses on API testing. As you grow, compare more tools (e.g. JMeter for performance, Karate for BDD-style tests) to find what suits your projects. The important thing is to test early and often – use these tools to validate every API before release.

Have more questions? Comment below or share this guide with your developer team! Your feedback and experiences with API testing tools help everyone build better software.