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Web / API

8 Free Online API & Web Tools

These 8 tools cover the loop you go round while debugging an HTTP API: send the request, read what came back, and work out why it is not what you expected. REST API Tester sends GET, POST, PUT, PATCH, and DELETE requests with custom headers and a JSON body, then reports the status, timing, and response — a small Postman that needs no install. cURL Command Builder turns the same request into a command you can paste into a terminal, a CI job, or a bug report. API Response Formatter and GraphQL Formatter make a raw response readable. HTTP Header Analyzer explains what each header in a response actually does. Mock Data Generator gives you realistic JSON to develop against before the endpoint exists, and HTTP Status Codes explains the number you just got back.

All 8 Web / API tools

What you can do with these API tools

Together they cover request, response, and the reference material you end up searching for mid-debug.

  • Send requests — any method, custom headers, JSON body, with the status code, response time, and body shown together.
  • Share requests — generate an equivalent cURL command from a form, so a colleague can reproduce the exact call.
  • Read responses — auto-detect and pretty-print JSON or XML, and reformat minified GraphQL queries, mutations, and schemas.
  • Inspect headers — paste raw request or response headers and get a table with plain-English explanations of each one.
  • Fake the backend — generate realistic users, products, addresses, and companies as JSON for prototyping and tests.
  • Look things up — a complete HTTP status code reference with causes and when each code is the right answer.

Why a browser-based API tester hits CORS

REST API Tester and CORS Request Tester send real requests from your browser, which means they obey the same-origin policy that a desktop client like Postman or curl does not. If an endpoint does not return an Access-Control-Allow-Origin header that covers this site, the browser blocks the response before any JavaScript sees it — the request usually did reach the server, but the answer is unreadable. That is not a bug in the tool; it is the browser security model, and it is exactly what a front-end app on another domain would hit. When it happens, either enable CORS on the API for the origins that need it, or build the request here as a cURL command and run it from your terminal instead.

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Frequently asked questions

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No. REST API Tester runs in the browser tab, so there is nothing to download, no account, and no workspace to configure — open it and send the request.

Because the browser blocks cross-origin responses unless the server sends permissive Access-Control-Allow-Origin headers. Use CORS Request Tester to see exactly which headers come back, or export the call as a cURL command and run it from a terminal, which is not subject to CORS.

The request is sent from your browser directly to the endpoint you entered, and nothing is proxied through or logged by this site. The formatters, header analyzer, and cURL builder do not make any network request at all.

Yes. cURL Command Builder is a pure form-to-command tool — fill in the method, URL, headers, and body and copy the result. It never contacts the endpoint.

Realistic fake users, products, addresses, and companies as ready-to-use JSON, which is what you want for seeding a database, building a UI before the API exists, or writing tests that need plausible values.

HTTP Status Codes lists every 1xx to 5xx code with a description, the common causes, and when to return it, colour-coded by class — useful when you need to decide between 400, 404, 409, and 422.