5 Best Open Source API Testing Tools in 2025

APIs have become the backbone of communication between applications in today's software-oriented world. Digital systems are increasingly complex due to the proliferation of microservices and distributed systems, making API testing both efficient and crucial.

A recent study projects the API testing industry to grow from USD 4.92 billion in 2025 to USD 27.38 billion by 2034, reflecting a compound annual growth rate (CAGR) of 20.99% during the 2025-2034 timeline.

Source*:* Market Research Future.

Fortunately, a number of open source API testing tools are available to developers. These tools provide dependability, ease of integration, and flexibility without the expenses associated with proprietary software. In this blog, we will analyze the five best open source API testing tools of 2025. These tools are chosen based on their practical usability and performance in contemporary DevOps ecosystems. Be it speed, performance, or ease of collaboration, there's something here for every team.

Why Open Source API Testing Tools Matter

There are several reasons why open source API testing tools are gaining traction. Here's why:.

1. The contributors to the open source API testing tools have a motive, and that is to stand out in the existing market.

2. The motive of every contributor is to make their software stand out in the increasingly competitive market.

3. They often integrate seamlessly into common development and CI/CD ecosystems.

4. Versatility in different use cases is accompanied by support for both functional and performance testing.

Some tools even aid in performance analysis by acting as api performance testing tools, analyzing response times and system behavior under varying load conditions. Automated testing tools are even more crucial today as more teams transition to cloud-native and microservice architectures.

What to Look for in an Open Source API Test Tool

API test open source solutions should have the following features:

1. Automation, as well as the user-friendliness of the processes involved.

2. Improved testing performance capability.

3. Support for multiple programming languages as well as platforms.

4. Compatibility with CI/CD pipelines.

5. Ability to generate mocks and simulate mock environments.

The best tools should have a balance of flexibility and ease of use, which promotes productivity for developers and testers.

Top Tools Developers Trust in 2025

Selecting the right open source API testing tools from the hundreds available can be daunting. Based on our extensive research, we have curated a list of the top tools that developers trust in 2025. These tools incorporate the right mix of automation, performance, and usability for contemporary API testing. Let's have a deeper look at each tool in detail.

1. Keploy

Keploy is notable in 2025 for its automation-first approach to API testing. Rather than following the traditional frameworks, it records real API traffic to test-case and mock generation. Developers are afforded the luxury of spending less time writing test scripts and more time on the overall code quality and velocity.

Features:

1. Captures and replays API traffic to auto-generate tests and mocks (reduced manual intervention).

2. Proficient in Go, Java, Node.js, and Python.

3. Compatible with all the major CI/CD tools.

4. Deterministic testing reduces flaky tests.

5. Supports mock-based isolated service testing.

Keploy is best suited for agile engineering teams seeking workflow automation in testing. Teams moving from manual testing to automated workflows also benefit from Keploy.

2. Apache JMeter

In the area of testing, Apache JMeter is still among the most trusted open source tools for API testing. Although it initially started with the purpose of load testing, now it also provides functional testing for APIs. Thus, in terms of API performance testing tools, JMeter is a valuable one.

Features:

1. High-volume load testing simulation for distributed systems.

2. Beginners can use the GUI, and experts can use the CLI.

3. Support for HTTP, HTTPS, SOAP, and REST.

4. Custom extension and reporting plugins available.

5. Can be integrated into automation frameworks.

JMeter is invaluable for performance testing as it emulates real-world usage. Its scalability and configurability make it optimal for teams performing regular API load testing.

3. REST Assured

REST Assured is a Java framework that validates REST APIs. It is widely appreciated among developers because of its expressive and succinct syntax, especially in terms of coding test logic instead of using graphical interfaces.

Features:

1. Provides Fluent API with validation capabilities for JSON and XML.

2. Provides Integration with TestNG, JUnit, and Maven.

3. Provides Built-in matchers for validation of the responses.

4. Provides Support for multiple methods of authentication.

REST Assured is Designed for backend teams that are leveraging Java within their technological stack. While it may not prioritize speed, it is greatly effective for functional testing and is well-suited for test-driven development workflows.

4. Karate DSL

Karate DSL is a complete API test automation, performance testing, and mocking framework. Its domain language is programmer-friendly as well as tester-friendly and is straightforward enough so that substantive tests can be written with minimal programming knowledge, making it more accessible.

Features:

1. Provides easier to read and write syntax with BDD (Behavior Driven Development).

2. Includes Built-in capabilities for performance testing.

3. Provides Parallel test execution to speed up execution time of testing.

4. Native mock-server available for simulation.

5. Provides Integration testing capabilities out of the box.

For teams looking to implement performance and automation of testing, Karate DSL is their solution. It closes the gap that exists between QA and Development by fostering collaboration in testing.

5. Hoppscotch

Hoppscotch, previously called Postwoman, is an API testing platform accessible over the Web. Its new name is gaining traction since it allows the testing of one API at a time in a collaborative manner. Even though it does not support automation or performance testing, it is a great tool for manual exploratory testing.

Features:

1. Clickable in-browser interface; software installation is not needed.

2. REST, GraphQL, WebSocket and others are supported.

3. Workspaces and environments are sharable with other people.

4. Fast analysis of requests and responses.

5. Useful for frontend and DevOPs.

Hoppscotch is a great asset in a team's arsenal for API manual testing and supporting during the development stage. While not suitable for environments reliant on automations, it is extremely useful in the initial development and QA phases.

Comparison Table

Now, let's take a look at the core features of each of the tools that we mentioned above, along with a comparison of features to help you understand them better.

Use Case Scenarios

Your team's requirements are important factors to consider when choosing open source API testing tools.

• For ease in automation and a reduction in manual effort, Keploy is undoubtedly the best option.

• Whereas, for more intensive API performance testing, Apache JMeter and Karate DSL are solid contenders.

• When it comes to functional backend validation in Java, REST Assured is a good choice.

• On the other hand, for manual workflows, Hoppscotch's intuitive interface streamlines the process for advanced users.

Each of these tools has a distinct purpose. The testing process as a whole may benefit from a multi-tool approach, as no single tool is the best in every situation.

Why Keploy Leads the Pack in 2025

Of all the tools covered in this discussion, Keploy stands out because it changes the mindset surrounding API testing. It streamlines work by eliminating repetitive tasks through the automatic generation of tests and mocks from actual API calls, thus improving developer confidence.

Unlike other open-source API testing tools that demand complex configurations and custom scripts, Keploy's traffic capture feature means there's no setup time. Teams can start testing right away. This tool has a game-changing effect in regards to how testing and API usage is perceived.

It is ideal for teams moving to microservices or those adopting more automated deployment processes. As it helps teams enforce quality standards without compromising with the efficiency, which is vital in dynamic settings.

Conclusion

The evolution of open source API testing tools has been gradual but significant over the years. As we look forward to 2025 and beyond, we can be confident that the tools will be not only powerful but also more accessible than ever. Teams of all sizes will be better served by a wide range of high-quality tools available for functional testing, automation, or performance.

Keploy is an excellent choice if you're after innovation, automation, and speed. For benchmarking performance, JMeter and Karate DSL are still valuable. For REST validation or exploratory testing, REST Assured and Hoppscotch provide effective alternatives.

Selecting the proper instruments requires comprehension of the defined objectives, processes, and the maturity level of the team's testing capabilities. However, whatever your level, the right selection of open source API testing tools will allow you to enable better and faster deployments with greater confidence.

FAQs

1. How does functional API testing differ from performance API testing?

Ans: Functional testing verifies if the API operates accurately during standard scenarios – for example, issuing the correct status codes and responses. Performance testing looks at how well the API functions under stress, such as during heavy use, measuring things like response time, scalability, and latency.

2. Can Open Source Tools Be Used to Automate API Testing?

Ans: Absolutely. Test automation can be achieved using open-source tools such as Keploy, REST Assured, and Karate DSL. The aforementioned tools assist in minimizing manual efforts, expediting regression testing, and providing efficient integration with CI/CD pipelines for constant verification.

3. What makes Keploy stand apart from the other open source API testing tools?

Ans: Other tools do not have the capability to capture and replay real API traffic which Keploy does, enabling it to automate test case creation and mock. This spares users the burden of writing test scripts. Its primary objectives are to enhance coverage, improve testing speed, and reduce flakiness, especially in microservice environments. In comparison to other tools, I have come to appreciate the advantage of Keploy's isolated service test providing capability which employs deterministic mocks making it much more flexible.