REST clients are not API testing tools. And pretending they are is lazy

#restapi #api #testing

Let’s stop being polite for a moment.

Postman is not an API testing tool. Bruno is not an API testing tool. Insomnia is not an API testing tool. Calling them testing tools is the same intellectual shortcut as calling a browser “end-to-end testing” or calling curl “automation”.

Yes, you can test with them. You can build testing tool with them. But you can also test with a terminal, a spreadsheet, or a notepad. That does not magically turn the tool into a testing tool.

REST clients exist to do one thing very well: send requests and show responses. They help you tweak headers, auth, payloads, environments. Some of them even let you add assertions if you really want to feel productive. All of that is useful. None of that is testing.

A REST client does not know whether something is wrong. It does not know whether a behavior is dangerous. It does not know whether a change is a regression or just noise. It does not know whether a 200 response is correct or quietly lying to you. It does not know whether a 500 is acceptable or a production-grade bug.

It shows you data and waits for you to decide what it means.

That is not testing. That is observation.

This is where people usually push back and say: “But we test our APIs in Postman.” What they actually mean is: “We send requests and nothing exploded.” Those two statements are not equivalent.

A REST client will never tell you whether your API behaves correctly with invalid input. It will never tell you whether your error handling leaks information. It will never tell you whether your response size is becoming a performance problem. It will never tell you whether an endpoint design will collapse once real data shows up. It will never tell you whether today’s behavior will become tomorrow’s incident.

If you want those answers using a REST client, you start writing scripts. Then helpers. Then shared logic. Then pre-request hacks. Then post-response checks. At some point you are no longer “using a REST client for testing”. You are building a small testing framework on top of a REST client and pretending this was the plan all along.

API testing is not magic dashboards, not AI agents, not “click once and verify everything”. API testing is systematic verification of how a system behaves when things are not as expected. Invalid values. Boundaries. Sizes. Noise. Design mistakes that don’t crash anything today but will absolutely hurt you later.

Most of these problems never show up when you send a happy-path request and see a green response.

REST clients help you talk to an API. They help you explore it. They help you debug it. They do not test it for you.

And confusing those two is how broken APIs get shipped with confidence.