Best API testing tools in 2027: Postman, Bruno, Insomnia and more

API testing tools send requests to an API and check what comes back. Ten are covered here, checked on 11 November 2026: Postman, Bruno, Insomnia, Hoppscotch and Thunder Client as graphical clients, curl, HTTPie and REST Client for the terminal and editor, and Swagger UI with the OpenAPI Specification and k6 for documentation and load. They differ most in where requests are stored: a vendor cloud, plain files in Git, or the local machine.

Last updated: 11 November 2026.

Which graphical API clients are worth learning?

Five graphical clients cover most day-to-day API testing, each described below from its own documentation or repository.

Postman

Postman is described in its documentation as the API platform for building, testing, documenting, monitoring and governing APIs, with a Postman CLI for CI. A new account stores changes in Postman Cloud by default, and Native Git in the desktop app keeps collections as files in a local Git repository. A tester can save a login and a profile request in one collection and run both in order. The limit is the account: signed out, the desktop app falls back to a lightweight API Client with local storage, and the old Scratch Pad is deprecated.

Bruno

Bruno is an open-source API client that stores each collection as plain files in a folder, so Git can version it. Its README names the MIT licence and says there are no plans to add cloud sync. From Bruno 3.0.0 requests can also be saved as YAML in the OpenCollection format, which Bruno says it created. A team can keep the collection beside the code and review changes in a pull request. The limit is sharing: collaboration runs through Git, and paid versions exist for extra features.

Insomnia

Insomnia is an open-source API client from Kong for REST, GraphQL, gRPC, WebSockets and Server-Sent Events, with the Apache-2.0 licence on its GitHub repository. Its README lists three storage options: Local Vault, Git Sync and Cloud Sync. A tester can start in the local Scratch Pad without an account and move to a project later. The limit is that the README says most capabilities need an account, and which features sit behind a paid plan changes, so the plan page needs a check. Insomnia 13.4.0 was published on 29 October 2026.

Hoppscotch

Hoppscotch is an open-source API development ecosystem with web, desktop and CLI clients, and its documentation says the web app needs no installation. GitHub lists the MIT licence. Collections sync with cloud or local session storage, can be exported as a file and can be imported from Postman or OpenAPI. A learner can send a first request from the browser without installing anything. The limit is the browser: CORS can block requests to APIs that do not send the right headers, and the desktop app or a proxy is the fix.

Thunder Client

Thunder Client is a lightweight REST client that runs inside VS Code and JetBrains IDEs. Its documentation says requests and collections are stored locally and shared through the team's own Git repository, never a third-party cloud. A developer can test an endpoint next to the code without switching windows. The limit is licensing: its repository carries an end-user licence agreement from Thunder Client Limited rather than an open-source licence, and that agreement says a subscription is required to use its CLI in CI and CD pipelines.

Which command-line, editor and specification tools help with API testing?

Five more tools work from the terminal, the editor, a specification or a load script: curl, HTTPie, REST Client, Swagger UI and k6.

curl

curl is a command-line tool for transferring data from or to a server using URLs, and its man page lists HTTP and HTTPS among many protocols. It is open source under its own licence, inspired by MIT/X, and curl 8.23.0 was released on 14 October 2026. A tester can check a status code with one line, as the worked example below does. The limit is scope: curl is a transfer tool rather than a test framework, so repeated checks need a script around it.

HTTPie

HTTPie for Terminal is a command-line HTTP client installed as the http and https commands, with JSON support, colourised output, sessions and an offline mode that builds a request without sending it. PyPI lists a BSD licence and 3.2.4, uploaded on 1 November 2024, as the current release. In a local run on 11 November 2026, a POST with key=value items failed in a non-interactive shell until --ignore-stdin was added, as HTTPie's scripting documentation advises. The limit is release cadence: the latest release dates from November 2024.

REST Client

REST Client is a free Visual Studio Code extension by Huachao Mao that sends an HTTP request written in a .http or .rest file and shows the response beside it. Several requests can share one file, separated by ###, so the file can sit in the repository and be reviewed like code. The extension is MIT-licensed on GitHub, and the latest tagged release there is v0.25.0 from 21 June 2022. The limit is scope: its listing describes sending requests, variables and authentication but no test runner or report.

Swagger UI

Swagger UI is an open-source tool that turns an OpenAPI description into interactive documentation, where anyone can read and try each operation without the implementation in place. SmartBear supports it, and the repository carries the Apache-2.0 licence. A team can host it beside an API so testers can send requests from the browser. The limit is CORS: the Swagger documentation says browsers enforce it, so the API must allow the page unless Swagger UI is on the same host and port or behind a proxy that adds the headers.

k6

k6 is Grafana Labs' open-source performance testing tool; tests are JavaScript files run by k6's embedded engine. Its documentation lists load, spike, stress and soak tests, plus low-load synthetic monitoring, and GitHub lists AGPL-3.0 as the licence for v2.3.0, published on 21 September 2026. A team can schedule a light run to watch availability or raise the load to test a spike. The limit is a surprise default: a failed check does not fail the test unless paired with a threshold.

How do the ten tools compare?

The table sets each tool's best use, where requests are stored according to its own documentation, its licence as stated on the vendor page or repository, and one main limit.

How can curl test an API, and what does a faked write show?

curl tests an API by sending a request and printing the status code or body, and a faked write shows why a test must read the data back. JSONPlaceholder is a free fake REST API with no signup and no API key, so it is a safe target. The script below, run with curl 8.7.1 on 11 November 2026, reads one post, asks for a missing post, creates a post and then looks for the post it created. The REST API explainer covers the methods and status codes used here.

base=https://jsonplaceholder.typicode.com
curl -s -o /dev/null -w "GET  /posts/1 -> %{http_code}\n" $base/posts/1
curl -s -o /dev/null -w "GET  /posts/9999 -> %{http_code}\n" $base/posts/9999
curl -s $base/posts -H "Content-Type: application/json" \
     -d '{"title":"foo","body":"bar","userId":1}' -w "\nPOST /posts -> %{http_code}\n"
curl -s -o /dev/null -w "GET  /posts/101 after the POST -> %{http_code}\n" $base/posts/101

The output was:

GET  /posts/1 -> 200
GET  /posts/9999 -> 404
{
  "title": "foo",
  "body": "bar",
  "userId": 1,
  "id": 101
}
POST /posts -> 201
GET  /posts/101 after the POST -> 404

The POST returned 201 and an id of 101, yet a GET for post 101 returned 404. JSONPlaceholder's guide says a created resource is not really saved and the response is faked, so a test that only checks the write status would pass while the data never existed. Against a real API, a test should read the resource back.

What is OpenAPI, and how does it differ from Swagger?

The OpenAPI Specification is a standard, programming-language-agnostic way to describe an HTTP API so that people and programs can understand it without reading the source code. Version 3.2.1 is dated 10 September 2026. Swagger is the older name: Swagger 2.0 was donated to the OpenAPI Initiative on 31 December 2015, and the Swagger UI README calls OpenAPI "formerly known as Swagger".

A valid document needs the openapi and info fields plus at least one of paths, components or webhooks, and it can be JSON or YAML. This file describes the one operation tested above.

openapi: 3.1.0
info: { title: JSONPlaceholder posts (excerpt), version: 1.0.0 }
servers:
  - url: https://jsonplaceholder.typicode.com
paths:
  /posts/{id}:
    get:
      summary: Fetch one post
      parameters:
        - { name: id, in: path, required: true, schema: { type: integer } }
      responses:
        "200":
          description: The post
          content:
            application/json:
              schema:
                type: object
                required: [id, title]
                properties:
                  id: { type: integer }
                  title: { type: string }
        "404":
          description: No post with that id

A description becomes a test when a script checks the live API against it. This script, run with Python 3.15.0, openapi-spec-validator 0.9.0, jsonschema 4.26.0 and PyYAML 6.0.3, validates the file, fetches post 1 and checks the body against the 200 schema.

import json, urllib.request, urllib.error, yaml
from openapi_spec_validator import validate
from jsonschema import validate as check

spec = yaml.safe_load(open("openapi.yaml"))
validate(spec)                                    # raises if the spec is invalid
print("spec valid, openapi", spec["openapi"])

op = spec["paths"]["/posts/{id}"]["get"]
schema = op["responses"]["200"]["content"]["application/json"]["schema"]
base = spec["servers"][0]["url"]

with urllib.request.urlopen(base + "/posts/1") as r:
    check(json.load(r), schema)                   # raises if the body breaks the contract
    print("GET /posts/1 ->", r.status, "matches the schema")

try:
    urllib.request.urlopen(base + "/posts/9999")
except urllib.error.HTTPError as e:
    print("GET /posts/9999 ->", e.code, "documented:", str(e.code) in op["responses"])

spec valid, openapi 3.1.0
GET /posts/1 -> 200 matches the schema
GET /posts/9999 -> 404 documented: True

Version support is a trap. Changing the first line to 3.2.1 also validated on openapi-spec-validator 0.9.0, but 0.7.2 on Python 3.9.6 raised ValidatorDetectError for a 3.2.0 file, and PyPI lists 0.9.0 as requiring Python 3.10 or later. The Swagger UI README's compatibility table lists 3.2.0 as the newest version supported at its 5.32.0 release of 27 February 2026. A validator or viewer needs checking before a file moves to a newer version.

What is the difference between Postman and Bruno?

Postman and Bruno differ mainly in where collections live and whether an account is needed. Postman keeps collections in Postman Cloud by default and offers Native Git as an option in the desktop app. Bruno keeps collections as files in a folder, has no cloud sync by design and needs no account for the open-source app.

Licensing differs too. Bruno's repository names the MIT licence, while Postman's documentation describes a free account, paid plans and an open collection format but states no licence for the app. Bruno's documentation includes a guide to importing Postman collections exported as v2 or v2.1 JSON. The criterion is data location: a team that wants requests in the same Git history as the code leans towards Bruno, and one that wants a hosted workspace with a runner and CLI leans towards Postman.

Which API testing tool should a learner choose?

The right tool depends on the situation rather than a universal ranking. The lines below pair common situations with the tools whose documentation fits them.

• A learner with nothing installed: Hoppscotch in the browser, or curl.
• A team that wants requests in Git: Bruno, Postman with Native Git, Thunder Client or REST Client.
• A developer who works inside VS Code: REST Client for plain files, or Thunder Client for a graphical view.
• A team that documents an API: an OpenAPI file shown through Swagger UI.
• A script or CI job: curl, or HTTPie with --check-status and --ignore-stdin.
• A load or spike test: k6.

The automation testing syllabus guide covers the wider test automation skills that API checks sit beside.

What limits apply when choosing an API testing tool?

Free tiers and plan terms change, so each limit above needs rechecking on the vendor page. Collections also hold URLs, tokens and sample data. A tool that syncs to a vendor cloud, as Postman does by default, needs the employer's approval for company work, while Bruno, Thunder Client and Insomnia's Local Vault keep data on the device or in the team's Git repository.

A passing test against a fake API proves little, as the JSONPlaceholder example shows. Specification tooling also lags: viewers and validators support specific OpenAPI versions. Tools that test security rather than behaviour, such as Burp Suite and ZAP, are covered in the list of cybersecurity tools for beginners.

Where does the AI and machine learning programme fit?

Model APIs are tested like any other API. Certification in Artificial Intelligence and Machine Learning is an online programme from Vishlesan i-Hub, IIT Patna, delivered on Masai's online platform, that covers Python, machine learning, deep learning, generative AI and LLMs, agentic systems and deploying models to production. Its curriculum includes REST APIs for models and a module on Production Deployment, MLOps & Capstone.

Frequently asked questions

Which tool is best for testing APIs?

No single tool is best for testing APIs, because the answer depends on where requests should be stored and what must be automated. Postman and Bruno suit teams that keep collections, curl and HTTPie suit scripts, Swagger UI suits documentation from an OpenAPI file, and k6 suits load. The criterion is the team's storage, automation and approval needs.

Are there free Postman alternatives that work offline?

Bruno, Insomnia, Thunder Client and REST Client are Postman alternatives that can work without a cloud account. Bruno stores collections as files and has no cloud sync, Insomnia has a local Scratch Pad that needs no account, Thunder Client stores data locally, and REST Client keeps requests in plain .http files. Free tiers and limits change, so each vendor page needs a check.

What is the difference between OpenAPI and Swagger?

OpenAPI is the specification for describing HTTP APIs, and Swagger is its former name plus a family of tools. Swagger 2.0 was donated to the OpenAPI Initiative on 31 December 2015, and the standard is now the OpenAPI Specification. Swagger UI, a tool from SmartBear, still renders OpenAPI descriptions as interactive documentation.

Can curl replace Postman?

curl can replace Postman for one-off requests and scripted checks, because it sends any HTTP request from a terminal and prints the result. It does not organise requests into collections, manage environments or report test results, so repeated or shared testing needs shell scripts around it or a client such as Bruno, Postman or HTTPie.

Is Newman still used for Postman collections?

Newman is Postman's command-line collection runner, and the Postman documentation now recommends the Postman CLI instead. Newman runs collections in the v2.1 JSON format, so it cannot run the v3 collections that Postman v12 uses in Local View. The documentation includes a command mapping for moving Newman scripts to the Postman CLI.

Next step

A learner can start with one graphical client and curl, run the worked example against JSONPlaceholder, and keep the OpenAPI file in the same folder. For structured practice that includes REST APIs for models, Certification in Artificial Intelligence and Machine Learning is the programme to look at.