The best API testing tools let you send requests, check the responses and turn those checks into repeatable tests that run every time your code changes. Developers usually want a fast client that lives next to their code, while QA teams want automation, reporting and security checks. Eleven options follow in no particular order.
Every app that talks to another app is leaning on an API, whether that's a payment step, a booking form or an AI agent pulling data from your CRM. When one of those connections breaks, customers notice long before anyone reads the error logs.
API testing is how you catch that early. The eleven tools below are listed alphabetically and in no particular order, from free open source clients to full test platforms, and every description comes from the vendor's own site, read this week. If you're new to the topic, my explainer on how API testing tools help developers covers the basics.
API testing tools side by side
In the free plan column, Yes means the tool's own site names a free plan, a free edition, a free account or an open source version. Trial only means a time-limited trial. Not stated means the pages I read named neither, even where there's a button inviting you to try it or book a demo.
| Tool | Type | Good for | Free plan |
|---|---|---|---|
| Apidog | All-in-one API design and testing platform | Small teams designing and testing together | Yes |
| Bruno | Open source, Git-native API client | Keeping API collections in your code repo | Yes |
| Hoppscotch | Open source API development platform | Fast browser-based API requests | Yes |
| Insomnia | API and MCP testing, debugging and mocking | Design-first teams working from OpenAPI specs | Yes |
| Karate | Open source API, UI and AI test automation | One syntax for API and UI tests | Yes |
| Katalon | Test automation platform with API testing | QA teams covering web, mobile and APIs | Trial only |
| Postman | API platform for building and testing APIs | Large teams sharing API collections | Not stated |
| ReadyAPI | Automated API functional and security testing | Testers who need functional and security checks | Trial only |
| REST Assured | Open source Java library for REST testing | Java teams testing REST services in code | Yes |
| Schemathesis | Open source property-based API testing | Finding edge-case bugs from an API schema | Yes |
| Testfully | API client for building, testing and monitoring | Offline or air-gapped API work | Yes |
Eleven API testing tools worth a look
Apidog
Best for: Small product teams that want API design, debugging, mocking, testing and documentation in one shared workspace instead of several separate tools.
Bruno
Best for: Developers who want their API collections stored as plain files in Git, next to the code, rather than locked inside a cloud account.
Hoppscotch
Best for: Developers who like a quick, lightweight way to build and test API requests in the browser, with the option to self-host.
Insomnia
Best for: API teams that start from an OpenAPI specification and want design, testing and mocking in one client that syncs with Git.
Karate
Best for: QA engineers who want one open source engine and one simple syntax for API, UI and performance tests running in any CI pipeline.
Katalon
Best for: QA teams that already test web and mobile apps and want API tests managed, run and reported from the same platform.
Postman
Best for: Larger engineering organisations that want a widely used home for API collections, tests and documentation, now with tools aimed at AI agents.
ReadyAPI
Best for: Dedicated test teams that want functional and security API testing at scale, with tests you can describe in plain language.
REST Assured
Best for: Java developers who want to write readable tests for REST services directly in their test suite, using a free open source library.
Schemathesis
Best for: Teams with an OpenAPI or GraphQL schema that want tests generated automatically to catch the odd inputs people never think to write.
Testfully
Best for: Teams that need an API client which works fully offline or in the cloud, with compliance needs such as SOC 2 already covered.
What the GOV.UK API standards say about testing
The GOV.UK API technical and data standards, written for teams building APIs across government, say you should try to offer a testing service, also known as a sandbox, for the people using your API. Their reasoning is simple: users may find an API harder to integrate without one. They also point out that sandboxes can be generated much more easily from an OpenAPI document.
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That's useful advice far beyond government. If partners, customers or your own mobile app depend on your API, a tool that can mock responses or spin up a sandbox from your OpenAPI file saves everyone time. A tool that only sends one request at a time won't do that for you.
Client, framework or full platform?
API testing tools fall into three rough groups. Clients are desktop or browser apps where you build and send requests by hand, then save them as collections. Code libraries let developers write tests in the same language as the product, so they run in the normal build. Platforms add automation, reporting, security checks and team features on top.
Small teams often start with a client, then add a code library once tests need to run automatically. Larger QA teams usually want a platform that ties API tests to web and mobile testing. There's no shame in mixing them; plenty of teams do.
Another close comparison is our Insomnia against Postman comparison.
Questions to ask before you choose
- Where are our requests and collections stored, and can they live in Git?
- Can tests run automatically in our build pipeline, not just by hand?
- Does it read our OpenAPI or GraphQL schema to create tests or mocks?
- How does it handle secrets such as API keys and tokens?
- Can non-developers in QA read and update the tests?
- What happens to our data if we stop using it?
Open-source challengers get a head-to-head in our Bruno versus Postman comparison.
Signs your API needs better testing
- Partners report bugs before you do. Breakages reach customers first.
- Nobody trusts the docs. The schema and the real responses don't match.
- Releases feel risky. Small changes break unrelated endpoints.
- Tests live on one laptop. Collections aren't shared or versioned.
Do you make API testing tools? Tool makers can ask to be considered for a featured place on this list.
My plain advice on choosing an API testing tool
Start with where your team already works. If your developers live in their code editor and Git, a client or library that stores tests as files will get used. If your testers need dashboards and sign-off reports, a platform will save them hours. The worst outcome is a tool that nobody opens after the first week.
Then think about the people on the other end of your API. A sandbox they can practise against is often worth more than another feature for you. And keep an eye on which apps call your API, especially newer AI tools; my guide to what the OpenAI API is shows how quickly those connections pile up.
Frequently asked questions
What is API testing?
API testing checks that an application programming interface returns the right data, status codes and errors when other software calls it, without needing to click through a user interface.
Are there free API testing tools?
Yes. Several open source clients and libraries are free to use, and some hosted tools have free plans for small teams. Check the free plan column in the table above.
What's the difference between an API client and an API testing framework?
A client lets you build and send requests by hand. A framework lets you write automated tests in code so they run every time your software changes.
Can I test APIs without writing code?
Yes. Most clients let you build requests and simple checks through a visual interface, and some platforms let you describe tests in plain language.
Should API tests run in a CI pipeline?
Ideally, yes. Running API tests automatically on every change catches broken endpoints before customers or partners find them.
What is contract testing?
Contract testing checks that an API still matches the agreed schema, so changes on one side don't quietly break the apps that depend on it.
