The usual move is to save both responses and run diff before.json after.json. That works until it doesn't.

Why text diffs lie about JSON

Take this response before and after a refactor:

{ "id": 42, "name": "Alice", "role": "admin",
  "tags": ["beta", "staff"],
  "plan": { "tier": "pro", "seats": 5 } }

{ "id": 42, "name": "Alice", "roles": ["admin", "editor"],
  "tags": ["staff", "beta"],
  "plan": { "tier": "pro", "seats": "5" } }

A line diff has three problems here:

1. Formatting noise. If one side is minified or the keys come out in a different order, every line "changed".
2. Reordered arrays look like edits. ["beta", "staff"] vs ["staff", "beta"] is usually the same set of tags, but it shows up as two changed lines.
3. Type changes are nearly invisible. 5 vs "5" is two characters. In a 400-line response you will scroll right past it, and it's the one that breaks clients.

Diff the structure, not the text

A structural diff parses both sides and compares values by path. Running the two responses above through one gives:

Whitespace and key order are gone from the picture. But the tags rows are still noise.

Turn on order-insensitive arrays

Compare arrays as sets instead of by index and the same diff becomes:

Three rows, and every one is real:

• role was renamed to roles and became an array. Breaking change for every client reading role.
• seats went from number to string. Breaking change for anything doing math on it.

For arrays of objects (say, a list of line items), matching elements by their id instead of by position gives the same benefit: inserting one item at the top doesn't mark every following item as changed.

Doing it in the browser

I built this into Formaty's JSON Diff: paste the old response on the left, the new one on the right, and toggle order-insensitive comparison (arrays of objects are then matched by id). It shows side-by-side or inline highlighting plus the path-level change list. Everything runs locally, so production payloads never leave your machine.

If you'd rather script it, the same idea works in a test: parse both sides, walk them by path, and treat arrays as multisets unless order matters for that field.

FAQ

Does key order matter? No. Objects are compared by key, so {"a":1,"b":2} equals {"b":2,"a":1}.

Can I export the result? Yes. Copy or download the change list as a JSON report to attach to a PR or ticket.

Formaty is free and open source (GitHub). 60-second demo.