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About this tool
Regular formatted display of JSON, plus automatic formatting/conversion for API interface testing (GET, POST, POST-JSON).
This JSON formatter pretty-prints or minifies a document and also converts the two other shapes API parameters arrive in — a GET query string and an x-www-form-urlencoded POST body — into JSON objects, using the browser's own URLSearchParams to split them. Indentation is selectable at 2, 4 or 8 spaces or a real tab, and an editable key/value table stays in sync both ways, so editing a row rewrites the URL, the form body or the JSON depending on which mode you are in. Nothing is transmitted: the request preview that shows method, headers and body is a local illustration, not a live call.
Open JSON Formatting on AltFTool — it loads instantly in your browser.
Choose one of the four mode cards — Regular Formatter, API GET Test, API POST Form or API POST JSON — and fill the input panel it opens: Raw JSON Input, GET Target URL (placeholder https://api.example.com/v1/search?q=query...) or POST Form Payload (placeholder key1=val1&key2=val2). Load Sample drops a worked example into whichever panel is showing, and each mode also starts with one pre-filled.
There is no convert button — Formatted Result JSON updates as you type. Set Tab Size to 2 Spaces, 4 Spaces, 8 Spaces or Tab to change the indentation, and in the GET and POST modes edit the Parameters Editor rows (Key, Value, Add Row, or Remove on a row) to have the URL or form body rewritten to match. Input the parser rejects is replaced by Invalid JSON:, or Invalid JSON payload: in API POST JSON mode, followed by the browser's own message.
Use Copy above the result — it flips to Copied for two seconds — or Download, which saves the formatted JSON as api_payload_regular.json, api_payload_get.json, api_payload_post.json or api_payload_postJson.json depending on the mode you are in. The output header shows output.json with a live line and character count, and Clear empties every input field.
Raw JSON, a GET query string and a urlencoded form body all normalise into the same formatted JSON object.
Edit a key or value in the table and the URL, form body or JSON text is rewritten to match, so you never hand-edit percent-encoding.
Every parse produces both the indented version and the single-line minified version, so you can take whichever the next tool needs.
No. The GET, POST-form and POST-JSON panels build and display what the request would look like — method, target, headers and body — but nothing leaves your browser. Use it to prepare and check a payload, then paste it into your own HTTP client.
Four: 2 spaces, 4 spaces, 8 spaces, or a literal tab character. The same setting drives both the formatted output and anything the parameter table writes back into the JSON panel.
It splits the URL at the first `?` and hands the rest to URLSearchParams, which decodes percent-encoding and `+` for you, then builds one JSON key per parameter. A parameter that repeats keeps only its last value, since a plain JSON object cannot hold two entries with the same key.
Almost always because it is JavaScript rather than JSON: trailing commas, single-quoted strings, unquoted keys, comments and `undefined` are all valid in a JS object literal and all rejected by JSON. The panel shows the parser's own message, which names the character position where it stopped.