JSON is the format most APIs speak, and most of the trouble with it comes in a few shapes: a payload that won’t parse, two responses that differ somewhere, a value buried five levels deep, or data that needs to become something else, such as YAML for a config file, CSV for a spreadsheet, or types for your code.
These tools each take one of those jobs. The formatter finds the exact line and column where a document breaks. The diff compares by path, so reordered keys don’t count as changes, and gives you the JSON Patch between the two. The JSONPath tester shows every value an expression selects and where it lives. The converters keep what the formats can share, and say plainly what they can’t, like YAML anchors or numbers too large for JavaScript.
Everything runs in your browser, so pasting a production response or an API token never sends it anywhere. If you’d rather learn to slice JSON on the command line, the jq Trainer teaches jq filters level by level against the same kind of data.