CodePen
codepen.io
Write HTML, CSS and JavaScript in a tab and watch it render live
Free tierClassic
Loading page
hoppscotch.io
Sends HTTP, GraphQL and WebSocket requests from a browser tab.
An open-source API client that opens instantly rather than installing several hundred megabytes of desktop app: set method, headers, auth and body, fire it, read the response. It covers REST, GraphQL, WebSocket, SSE and MQTT, and history and collections persist in the browser. Because it runs inside a page, calls to servers without permissive CORS headers fail unless you add the browser extension or run their proxy.
AltFTool does this too
If you would rather not leave the site, API Tester covers the same job here — free, no account, and it runs in your browser.
Open API TesterQuestions
A genuinely usable free tier with paid plans for volume or team features. Requests to APIs without CORS headers are blocked unless you install the extension or self-host the proxy.
No. Hoppscotch does the work inside your browser, so the file never leaves your device — you can load the page, disconnect from the network, and it still works. That makes it a reasonable choice for material containing personal data.
Requests to APIs without CORS headers are blocked unless you install the extension or self-host the proxy.
Alternatives
Same category or same task, ordered by how closely they overlap.
codepen.io
Write HTML, CSS and JavaScript in a tab and watch it render live
diffchecker.com
Shows exactly what changed between two blocks of text, side by side
jsoncrack.com
Turns a wall of JSON into a node graph you can pan and search
regex101.com
Builds and explains a regular expression while you type it
stackblitz.com
Runs npm install and a Node dev server inside the browser itself
bundlephobia.com
Shows what an npm package costs your bundle before you install it