REST clients help you send requests. API testing tools help you understand whether your API is actually good. Most people confuse these two.
Postman, Bruno, Insomnia are often called “API testing tools”. They are not.
Calling them testing tools is the same as saying that
curl is testing,
a browser is testing,
or an SQL client is database testing.
They are not.
These are tools you can use while testing, but that does not magically turn them into testing tools. You can test with anything — that does not mean everything is a test tool. Nobody likes saying this out loud, but it is still true.
What REST clients actually give you
REST clients let you send requests. They show you responses. They let you add assertions and expected results, if you want. They let you chain requests, group them into collections, run them locally, in CI, or from the cloud.
All of that is useful. None of that is testing.
REST clients do not test anything for you. They do not know what is wrong. They do not know what is dangerous. They do not know what is a regression. They do not know what is noise and what is not.
They give you a hammer and say: “Test.”
Where REST clients hit the limit
A REST client will never tell you whether your API behaves correctly with invalid input. You have to design that yourself.
It will not tell you whether a 500 response is acceptable or a serious bug. You have to know that.
It will not tell you what differences are real regressions and what is just environment noise.
If you want those answers, you end up writing scripts. And at that point, let’s be honest: you are building a small testing framework on top of a REST client.
What API testing actually is
API testing is not AI agents. It is not “prompt once, test everything”. It is not magic dashboards.
API testing is systematic verification of how a system behaves when things are not as expected.
That means invalid values, boundary conditions, enums, unexpected sizes, noise, regression, API structure and basic best practices.
This is where Rentgen comes in.
Where Rentgen fits
Rentgen does not try to build yet another API client. It starts exactly where API clients stop.
Rentgen takes one real request and automatically generates tests around it. It shows what is broken, what is strange, what is dangerous and what violates basic API hygiene.
No scripts. No cloud. No ceremonies.
Rentgen is not a replacement for Postman, Bruno or any other API client. It is the step before them.
Final thoughts
API clients help you talk to an API and help you build your own test framework.
API testing helps you quickly understand what is wrong with your API changes before you even start formal testing.