API Testing Tools: 9 Popular Tools Compared and Which to Learn First

The most widely used API testing tools are Postman for manual and exploratory testing, REST Assured for Java automation, pytest with requests for Python automation, Karate for low-code automation, SoapUI for SOAP services, and JMeter or k6 for load testing. The right choice depends on what you test (REST, GraphQL or SOAP), which language your team uses and whether you need manual checks, automated regression or performance tests. This guide compares the main options so a fresher or manual tester can pick a sensible starting point.

What makes a good API testing tool?

• Support for the protocols you use: REST, GraphQL, SOAP or gRPC.
• Easy handling of authentication such as API keys, bearer tokens and OAuth 2.0.
• Assertions on status codes, response time, headers and JSON body, ideally with schema validation.
• Environments and variables, so the same tests run against dev, QA and staging.
• A command-line runner so tests can run in Jenkins, GitHub Actions or GitLab CI.
• Readable reports that developers can act on.

API testing tools compared

Licences and free plans change, so check each vendor website for current pricing before choosing a tool for a team.

The main API testing tools in detail

1. Postman

Postman is the usual first tool for API testing. You build requests, group them into collections, store values in environments and write checks in the Tests tab. It is ideal for learning and for exploratory testing, and collections are easy to share with developers.

pm.test('status is 200', function () {
  pm.response.to.have.status(200);
});
pm.test('user has an id', function () {
  const body = pm.response.json();
  pm.expect(body.id).to.be.a('number');
});

2. REST Assured

REST Assured is a Java library with a given, when, then style. It fits teams that already use Java, TestNG or JUnit, and Maven, and it is common in Indian service companies with Selenium Java frameworks.

given()
  .header('Accept', 'application/json')
.when()
  .get('https://api.example.com/users/1')
.then()
  .statusCode(200)
  .body('id', equalTo(1));

3. pytest with requests

For Python teams, the requests library plus pytest is simple and powerful. Fixtures handle setup such as tokens, and parametrize runs the same test with many inputs.

import requests

BASE = 'https://api.example.com'

def test_get_user():
    r = requests.get(BASE + '/users/1', timeout=10)
    assert r.status_code == 200
    assert r.json()['id'] == 1

4. Karate

Karate tests are written in feature files with readable steps. It has built-in JSON matching, data-driven tests and parallel runs. It suits testers who want automation without writing much Java.

5. Playwright API testing

Playwright, known for browser automation, also has an API request context. It is handy for setting up test data through the API before a UI test, or for testing APIs with the same framework and reports as your UI suite. Read our Selenium tutorial if you are comparing UI automation tools.

6. SoapUI and ReadyAPI

SoapUI is a long-standing tool for SOAP web services, with WSDL import and XML assertions. Many banking, insurance and telecom systems still expose SOAP services, so it remains relevant there.

7. JMeter and k6

These answer a different question: how does the API behave with many users? JMeter is a mature, GUI-based tool; k6 uses JavaScript scripts and fits developer workflows. Use them for load, stress and soak tests, not for checking business rules.

Which API testing tool should you learn first?

• Manual tester moving to API testing: Postman first, then Newman to run collections in CI.
• Java automation role: Postman, then REST Assured with TestNG.
• Python automation role: Postman, then pytest with requests.
• Little coding background: Postman, then Karate.
• Performance testing role: JMeter or k6 after you understand functional API testing.

Before any tool, make sure you understand HTTP methods, status codes, headers, JSON and authentication. If you are still deciding between manual and automation paths, read manual testing vs automation testing.

What to test in an API, whatever tool you use

• Correct status codes for valid and invalid requests (200, 201, 400, 401, 403, 404, 500).
• Response body values and the JSON schema.
• Authentication and authorisation: a user must not read another user's data.
• Input validation: missing fields, wrong types, very long strings and special characters.
• Response time within the agreed limit.
• Idempotency: repeating a PUT or DELETE should not cause extra side effects.

Frequently asked questions

Is Postman enough for API testing?

For learning, manual testing and small regression suites, yes. For large automated suites inside a code-based framework, most teams move to REST Assured, pytest or Karate and keep Postman for exploration.

Which is better, Postman or REST Assured?

They serve different needs. Postman is a GUI tool that is quick for manual work and easy to share. REST Assured is a Java library that fits into a maintainable automation framework with version control and CI.

Can Selenium be used for API testing?

No. Selenium automates browsers. For APIs, use a dedicated tool or library such as Postman, REST Assured or requests. You can combine them in one framework, using API calls to set up data for Selenium UI tests.

Is API testing a good skill for freshers?

Yes. API testing is in demand because most modern applications are built on APIs, and it is a natural next step for manual testers who want to move into automation.

See all our software testing guides.